FEATURES SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM AVDD VIN+A1 VIN–A1 SERIAL LVDS SERIAL LVDS 14 DIGITAL SERIALIZER PIPELINE ADC SERIAL LVDS 14 VIN+A2 VIN–A2 PIPELINE ADC VIN+D1 VIN–D1 PIPELINE ADC VIN+D2 VIN–D2 PIPELINE ADC DIGITAL SERIALIZER SERIAL LVDS 14 SERIAL LVDS DIGITAL SERIALIZER SERIAL LVDS 14 DIGITAL SERIALIZER SERIAL LVDS SENSE VCM1, VCM2 REF SELECT SERIAL LVDS 1V D0+D1 D0–D1 D1+D1 D1–D1 D0+D2 D0–D2 D1+D2 D1–D2 FCO+1, FCO+2 FCO–1, FCO–2 GND DCO+1, DCO+2 DCO–1, DCO–2 11537-200 CLK– SYNC CLK+ CLOCK MANAGEMENT SCLK/DTP SERIAL PORT INTERFACE SDIO/OLM D0+A1 D0–A1 D1+A1 D1–A1 D0+A2 D0–A2 D1+A2 D1–A2 Figure 1. GENERAL DESCRIPTION The AD9681 is an octal, 14-bit, 125 MSPS analog-to-digital converter (ADC) with an on-chip sample-and-hold circuit that is designed for low cost, low power, small size, and ease of use. The device operates at a conversion rate of up to 125 MSPS and is optimized for outstanding dynamic performance and low power in applications where a small package size is critical. The ADC requires a single 1.8 V power supply and an LVPECL-/ CMOS-/LVDS-compatible sample rate clock for full performance operation. No external reference or driver components are required for many applications. The AD9681 automatically multiplies the sample rate clock for the appropriate LVDS serial data rate. Data clock outputs (DCO±1, DCO±2) for capturing data on the output and frame clock outputs (FCO±1, FCO±2) for signaling a new output byte are provided. Individual channel power-down is supported, and the device typically consumes less than 2 mW when all channels are disabled. Rev. A DRVDD VREF APPLICATIONS Medical imaging Communications receivers Multichannel data acquisition PDWN AD9681 RBIAS1, RBIAS2 Low power 8 ADC channels integrated into 1 package 110 mW per channel at 125 MSPS with scalable power options SNR: 74 dBFS (to Nyquist); SFDR: 90 dBc (to Nyquist) DNL: ±0.8 LSB (typical); INL: ±1.2 LSB (typical) Crosstalk, worst adjacent channel, 70 MHz, −1 dBFS: −83 dB typical Serial LVDS (ANSI-644, default) Low power, reduced signal option (similar to IEEE 1596.3) Data and frame clock outputs 650 MHz full power analog bandwidth 2 V p-p input voltage range 1.8 V supply operation Serial port control Flexible bit orientation Built-in and custom digital test pattern generation Programmable clock and data alignment Power-down and standby modes CSB1, CSB2 Data Sheet Octal, 14-Bit, 125 MSPS, Serial LVDS, 1.8 V Analog-to-Digital Converter AD9681 The ADC contains several features designed to maximize flexibility and minimize system cost, such as programmable clock and data alignment and programmable digital test pattern generation. The available digital test patterns include built-in deterministic and pseudorandom patterns, along with custom user-defined test patterns entered via the serial port interface (SPI). The AD9681 is available in an RoHS-compliant, 144-ball CSPBGA. It is specified over the industrial temperature range of −40°C to +85°C. This product is protected by a U.S. patent. PRODUCT HIGHLIGHTS 1. 2. 3. 4. Small Footprint. Eight ADCs are contained in a small, 10 mm × 10 mm package. Low Power. The device dissipates 110 mW per channel at 125 MSPS with scalable power options. Ease of Use. Data clock outputs (DCO±1, DCO±2) operate at frequencies of up to 500 MHz and support double data rate (DDR) operation. User Flexibility. SPI control offers a wide range of flexible features to meet specific system requirements. Document Feedback Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 ©2013 Analog Devices, Inc. All rights reserved. Technical Support www.analog.com AD9681 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Power Dissipation and Power-Down Mode ........................... 23 Applications ....................................................................................... 1 Digital Outputs and Timing ..................................................... 24 General Description ......................................................................... 1 Output Test Modes ..................................................................... 27 Simplified Functional Block Diagram ........................................... 1 Serial Port Interface (SPI) .............................................................. 28 Product Highlights ........................................................................... 1 Configuration Using the SPI ..................................................... 28 Revision History ............................................................................... 2 Hardware Interface..................................................................... 29 Functional Block Diagram .............................................................. 3 Configuration Without the SPI ................................................ 29 Specifications..................................................................................... 4 SPI Accessible Features .............................................................. 29 DC Specifications ......................................................................... 4 Memory Map .................................................................................. 30 AC Specifications.......................................................................... 5 Reading the Memory Map Register Table............................... 30 Digital Specifications ................................................................... 6 Memory Map Register Table ..................................................... 31 Switching Specifications .............................................................. 7 Memory Map Register Descriptions ........................................ 34 Timing Specifications .................................................................. 8 Applications Information .............................................................. 37 Absolute Maximum Ratings.......................................................... 12 Design Guidelines ...................................................................... 37 Thermal Characteristics ............................................................ 12 Power and Ground Recommendations ................................... 37 ESD Caution ................................................................................ 12 Board Layout Considerations ................................................... 37 Pin Configuration and Function Descriptions ........................... 13 Clock Stability Considerations ................................................. 38 Typical Performance Characteristics ........................................... 15 Reference Decoupling ................................................................ 38 Equivalent Circuits ......................................................................... 18 SPI Port ........................................................................................ 38 Theory of Operation ...................................................................... 19 Outline Dimensions ....................................................................... 39 Analog Input Considerations.................................................... 19 Ordering Guide .......................................................................... 39 Voltage Reference ....................................................................... 20 Clock Input Considerations ...................................................... 21 REVISION HISTORY 12/13—Rev. 0 to Rev. A Changes to Ordering Guide .......................................................... 39 11/13—Revision 0: Initial Version Rev. A | Page 2 of 40 Data Sheet AD9681 FUNCTIONAL BLOCK DIAGRAM AVDD PDWN DRVDD SERIAL LVDS AD9681 SERIAL LVDS 14 DIGITAL SERIALIZER PIPELINE ADC SERIAL LVDS 14 VIN+A2 VIN–A2 PIPELINE ADC VIN+B1 VIN–B1 PIPELINE ADC VIN+B2 VIN–B2 PIPELINE ADC DIGITAL SERIALIZER SERIAL LVDS 14 SERIAL LVDS DIGITAL SERIALIZER SERIAL LVDS 14 DIGITAL SERIALIZER SERIAL LVDS RBIAS1, RBIAS2 SERIAL LVDS VREF SENSE FCO+1, FCO+2 FCO–1, FCO–2 1V REF SELECT SERIAL LVDS GND VIN+C1 VIN–C1 VIN+C2 VIN–C2 SERIAL LVDS 14 DIGITAL SERIALIZER PIPELINE ADC SERIAL LVDS 14 DIGITAL SERIALIZER PIPELINE ADC SERIAL LVDS 14 VIN+D1 VIN–D1 PIPELINE ADC VIN+D2 VIN–D2 PIPELINE ADC SERIAL LVDS DIGITAL SERIALIZER SERIAL LVDS 14 DIGITAL SERIALIZER SERIAL LVDS SERIAL LVDS VCM1, VCM2 Figure 2. Rev. A | Page 3 of 40 CLK– CLK+ CLOCK MANAGEMENT SYNC SDIO/OLM SCLK/DTP CSB1, CSB2 SERIAL PORT INTERFACE D0+C1 D0–C1 D1+C1 D1–C1 D0+C2 D0–C2 D1+C2 D1–C2 D0+D1 D0–D1 D1+D1 D1–D1 D0+D2 D0–D2 D1+D2 D1–D2 DCO+1, DCO+2 DCO–1, DCO–2 11537-001 VIN+A1 VIN–A1 D0+A1 D0–A1 D1+A1 D1–A1 D0+A2 D0–A2 D1+A2 D1–A2 D0+B1 D0–B1 D1+B1 D1–B1 D0+B2 D0–B2 D1+B2 D1–B2 AD9681 Data Sheet SPECIFICATIONS DC SPECIFICATIONS AVDD = 1.8 V, DRVDD = 1.8 V, 2 V p-p differential input, 1.0 V internal reference, AIN = −1.0 dBFS, unless otherwise noted. Table 1. Parameter1 RESOLUTION ACCURACY No Missing Codes Offset Error Offset Matching Gain Error Gain Matching Differential Nonlinearity (DNL) Integral Nonlinearity (INL) TEMPERATURE DRIFT Offset Error Gain Error INTERNAL VOLTAGE REFERENCE Output Voltage (1 V Mode) Load Regulation at 1.0 mA (VREF = 1 V) Input Resistance INPUT-REFERRED NOISE VREF = 1.0 V ANALOG INPUTS Differential Input Voltage (VREF = 1 V) Common-Mode Voltage Differential Input Resistance Differential Input Capacitance POWER SUPPLY AVDD DRVDD IAVDD IDRVDD (ANSI-644 Mode) IDRVDD (Reduced Range Mode) TOTAL POWER CONSUMPTION Total Power Dissipation (Eight Channels, Including Output Drivers ANSI-644 Mode) Total Power Dissipation (Eight Channels, Including Output Drivers Reduced Range Mode) Power-Down Dissipation Standby Dissipation2 1 2 Temp Full Full Full Full Full Full Full Min 14 −0.23 0 −8.0 0 −0.92 −4.0 Full Full Full 25°C Full Full Full Full Full 25°C Max Unit Bits Guaranteed +0.21 0.24 −3.1 1.8 ±0.8 ±1.2 +0.62 0.7 +1.7 6.0 +1.75 +4.0 % FSR % FSR % FSR % FSR LSB LSB −4 38 0.98 25°C Full Full Full Full Typ 1.0 3 7.5 ppm/°C ppm/°C 1.02 0.99 0.5 1.7 1.7 2 0.9 5.2 3.5 V mV kΩ LSB rms 1.3 V p-p V kΩ pF 1.8 1.8 368 120 90 1.9 1.9 423 126 V V mA mA mA Full 879 988 mW 25°C 825 mW 25°C 25°C 2 485 mW mW See the AN-835 Application Note, Understanding High Speed ADC Testing and Evaluation, for definitions and for information about how these tests were completed. Controlled via the SPI. Rev. A | Page 4 of 40 Data Sheet AD9681 AC SPECIFICATIONS AVDD = 1.8 V, DRVDD = 1.8 V, 2 V p-p differential input, 1.0 V internal reference, AIN = −1.0 dBFS, unless otherwise noted. Table 2. Parameter1 SIGNAL-TO-NOISE RATIO (SNR) fIN = 9.7 MHz fIN = 19.7 MHz fIN = 69.5 MHz fIN = 139.5 MHz fIN = 201 MHz fIN = 301 MHz SIGNAL-TO-NOISE AND DISTORTION RATIO (SINAD) fIN = 9.7 MHz fIN = 19.7 MHz fIN = 69.5 MHz fIN = 139.5 MHz fIN = 201 MHz fIN = 301 MHz EFFECTIVE NUMBER OF BITS (ENOB) fIN = 9.7 MHz fIN = 19.7 MHz fIN = 69.5 MHz fIN = 139.5 MHz fIN = 201 MHz fIN = 301 MHz SPURIOUS-FREE DYNAMIC RANGE (SFDR) fIN = 9.7 MHz fIN = 19.7 MHz fIN = 69.5 MHz fIN = 139.5 MHz fIN = 201 MHz fIN = 301 MHz WORST HARMONIC (SECOND OR THIRD) fIN = 9.7 MHz fIN = 19.7 MHz fIN = 69.5 MHz fIN = 139.5 MHz fIN = 201 MHz fIN = 301 MHz WORST OTHER (EXCLUDING SECOND OR THIRD) fIN = 9.7 MHz fIN = 19.7 MHz fIN = 69.5 MHz fIN = 139.5 MHz fIN = 201 MHz fIN = 301 MHz TWO-TONE INTERMODULATION DISTORTION (IMD)—AIN1 AND AIN2 = −7.0 dBFS fIN1 = 70 MHz, fIN2 = 72.5 MHz CROSSTALK, WORST ADJACENT CHANNEL2 Crosstalk, Worst Adjacent Channel Overrange Condition3 ANALOG INPUT BANDWIDTH, FULL POWER Temp 25°C 25°C Full 25°C 25°C 25°C Min Typ Max Unit 74.8 74.7 73.9 71.5 69.6 66.6 dBFS dBFS dBFS dBFS dBFS dBFS 74.7 74.7 73.8 71.4 69.3 65.8 dBFS dBFS dBFS dBFS dBFS dBFS 12.1 12.1 12.0 11.6 11.2 10.6 Bits Bits Bits Bits Bits Bits 94 94 90 87 83 73 dBc dBc dBc dBc dBc dBc 25°C 25°C Full 25°C 25°C 25°C −94 −94 −90 −87 −83 −73 dBc dBc dBc dBc dBc dBc 25°C 25°C Full 25°C 25°C 25°C −98 −94 −96 −90 −85 −75 25°C 25°C 25°C 25°C 94 −83 −79 650 25°C 25°C Full 25°C 25°C 25°C 25°C 25°C Full 25°C 25°C 25°C 25°C 25°C Full 25°C 25°C 25°C 1 72.6 72.3 11.7 81 −81 −84 See the AN-835 Application Note, Understanding High Speed ADC Testing and Evaluation, for definitions and for details on how these tests were completed. Crosstalk is measured at 70 MHz, with −1.0 dBFS analog input on one channel and no input on the adjacent channel. 3 Overrange condition is defined as 3 dB above input full scale. 2 Rev. A | Page 5 of 40 dBc dBc dBc dBc dBc dBc dBc dB dB MHz AD9681 Data Sheet DIGITAL SPECIFICATIONS AVDD = 1.8 V, DRVDD = 1.8 V, 2 V p-p differential input, 1.0 V internal reference, AIN = −1.0 dBFS, unless otherwise noted. Table 3. Parameter1 CLOCK INPUTS (CLK+, CLK−) Logic Compliance Differential Input Voltage2 Input Voltage Range Input Common-Mode Voltage Input Resistance (Differential) Input Capacitance LOGIC INPUTS (PDWN, SYNC, SCLK) Logic 1 Voltage Logic 0 Voltage Input Resistance Input Capacitance LOGIC INPUTS (CSB1, CSB2) Logic 1 Voltage Logic 0 Voltage Input Resistance Input Capacitance LOGIC INPUT (SDIO) Logic 1 Voltage Logic 0 Voltage Input Resistance Input Capacitance LOGIC OUTPUT (SDIO)3 Logic 1 Voltage (IOH = 800 μA) Logic 0 Voltage (IOL = 50 μA) DIGITAL OUTPUTS (D0±xx, D1±xx), ANSI-644 Logic Compliance Differential Output Voltage (VOD) Output Offset Voltage (VOS) Output Coding (Default) DIGITAL OUTPUTS (D0±xx, D1±xx), LOW POWER, REDUCED SIGNAL OPTION Logic Compliance Differential Output Voltage (VOD) Output Offset Voltage (VOS) Output Coding (Default) 1 2 3 Temp Min Full Full Full 25°C 25°C 0.2 GND − 0.2 Full Full 25°C 25°C 1.2 0 Full Full 25°C 25°C 1.2 0 Full Full 25°C 25°C 1.2 0 Typ Max Unit 3.6 AVDD + 0.2 V p-p V V kΩ pF AVDD + 0.2 0.8 V V kΩ pF AVDD + 0.2 0.8 V V kΩ pF AVDD + 0.2 0.8 V V kΩ pF CMOS/LVDS/LVPECL 0.9 15 4 30 2 26 2 26 5 Full Full 1.79 0.05 V V Full Full 290 1.15 LVDS 345 400 1.25 1.35 Twos complement mV V Full Full 160 1.15 LVDS 200 230 1.25 1.35 Twos complement mV V See the AN-835 Application Note, Understanding High Speed ADC Testing and Evaluation, for definitions and for details on how these tests were completed. Specified for LVDS and LVPECL only. Specified for 13 SDIO/OLM pins sharing the same connection. Rev. A | Page 6 of 40 Data Sheet AD9681 SWITCHING SPECIFICATIONS AVDD = 1.8 V, DRVDD = 1.8 V, 2 V p-p differential input, 1.0 V internal reference, AIN = −1.0 dBFS, unless otherwise noted. Table 4. Parameter1, 2 CLOCK3 Input Clock Rate Conversion Rate Clock Pulse Width High Clock Pulse Width Low OUTPUT PARAMETERS3 Propagation Delay Rise Time (20% to 80%) Fall Time (20% to 80%) FCO±1, FCO±2 Propagation Delay DCO±1, DCO±2 Propagation Delay4 DCO±1, DCO±2 to Data Delay4 DCO±1, DCO±2 to FCO±1, FCO±2 Delay4 Lane Delay Data to Data Skew Wake-Up Time (Standby) Wake-Up Time (Power-Down)5 Pipeline Latency APERTURE Aperture Delay Aperture Uncertainty (Jitter) Out-of-Range Recovery Time Symbol Temp Min 10 10 tEH tEL Full Full Full Full Full Full Full Full Full Full Full 1.5 tPD tR tF tFCO tCPD tDATA tFRAME tLD tDATA-MAX − tDATA-MIN tA tJ Typ Max Unit 1000 125 MHz MSPS ns ns 3.1 ns ps ps ns ns ps ps ps ps ns μs 4.00 4.00 Full 25°C 25°C Full 2.3 300 300 2.3 tFCO + (tSAMPLE/16) (tSAMPLE/16) (tSAMPLE/16) 90 ±50 250 375 16 25°C 25°C 25°C 1 135 1 1.5 (tSAMPLE/16) − 300 (tSAMPLE/16) − 300 1 3.1 (tSAMPLE/16) + 300 (tSAMPLE/16) + 300 ±200 See the AN-835 Application Note, Understanding High Speed ADC Testing and Evaluation, for definitions and for details on how these tests were completed. Measured on standard FR-4 material. 3 Adjustable using the SPI. The conversion rate is the clock rate after the divider. 4 tSAMPLE/16 is based on the number of bits in two LVDS data lanes. tSAMPLE = 1/fSAMPLE. 5 Wake-up time is defined as the time required to return to normal operation from power-down mode. 2 Rev. A | Page 7 of 40 Clock cycles ns fs rms Clock cycles AD9681 Data Sheet TIMING SPECIFICATIONS Table 5. Parameter SYNC TIMING REQUIREMENTS tSSYNC tHSYNC SPI TIMING REQUIREMENTS tDS tDH tCLK tS tH tHIGH tLOW tEN_SDIO tDIS_SDIO Description Limit Unit SYNC to rising edge of CLK+ setup time SYNC to rising edge of CLK+ hold time See Figure 53 Setup time between the data and the rising edge of SCLK Hold time between the data and the rising edge of SCLK Period of the SCLK Setup time between CSB1/CSB2 and SCLK Hold time between CSB1/CSB2 and SCLK SCLK pulse width high SCLK pulse width low Time required for the SDIO pin to switch from an input to an output relative to the SCLK falling edge (not shown in Figure 53) Time required for the SDIO pin to switch from an output to an input relative to the SCLK rising edge (not shown in Figure 53) 0.24 0.40 ns typ ns typ 2 2 40 2 2 10 10 10 ns min ns min ns min ns min ns min ns min ns min ns min 10 ns min Timing Diagrams Refer to the Memory Map Register Descriptions section and Table 21 for SPI register setting of output modes. N–1 VIN±x1, VIN±x2 N tA CLK– tEH N+1 tEL CLK+ DCO–1, DCO–2 tCPD DDR DCO+1, DCO+2 DCO–1, DCO–2 SDR DCO+1, DCO+2 FCO–1, FCO–2 FCO+1, FCO+2 D0–A1 BITWISE MODE D0+A1 tFCO tFRAME tPD tDATA D12 N – 17 D10 N – 17 D08 N – 17 D06 N – 17 D04 N – 17 D02 N – 17 LSB N – 17 0 N – 17 MSB N – 17 D11 N – 17 D09 N – 17 D07 N – 17 D05 N – 17 D03 N – 17 D01 N – 17 0 N – 17 D10 N – 16 D08 N – 16 D06 N – 16 D04 N – 16 D02 N – 16 LSB N – 16 0 N – 16 MSB N – 16 D11 N – 16 D09 N – 16 D07 N – 16 D05 N – 16 D03 N – 16 D01 N – 16 0 N – 16 D05 N – 17 D04 N – 17 D03 N – 17 D02 N – 17 D01 N – 17 LSB N – 17 0 N – 17 0 N – 17 D05 N – 16 D04 N – 16 D03 N – 16 D02 N – 16 D01 N – 16 LSB N – 16 0 N – 16 0 N – 16 MSB N – 17 D12 N – 17 D11 N – 17 D10 N – 17 D09 N – 17 D08 N – 17 D07 N – 17 D06 N – 17 MSB N – 16 D12 N – 16 D11 N – 16 D10 N – 16 D09 N – 16 D08 N – 16 D07 N – 16 D06 N – 16 tLD D1–A1 D1+A1 D12 N – 16 FCO–1, FCO–2 BYTEWISE MODE D0–A1 D0+A1 D1–A1 D1+A1 Figure 3. 16-Bit DDR/SDR, Two-Lane, 1× Frame Mode (Default) Rev. A | Page 8 of 40 11537-003 FCO+1, FCO+2 Data Sheet AD9681 N–1 VIN±x1, VIN±x2 N+1 tA N tEH CLK– CLK+ tEL tCPD DCO–1, DCO–2 DDR DCO+1, DCO+2 DCO–1, DCO–2 SDR DCO+1, DCO+2 FCO+1, FCO+2 BITWISE MODE tFRAME tFCO FCO–1, FCO–2 tDATA tPD D0–A1 D0+A1 D10 N – 17 D08 N – 17 D06 N – 17 D04 N – 17 D02 N – 17 LSB N – 17 D10 N – 16 D08 N – 16 D06 N – 16 MSB N – 17 D09 N – 17 D07 N – 17 D05 N – 17 D03 N – 17 D01 N – 17 MSB N – 16 D09 N – 16 D07 N – 16 D05 N – 17 D04 N – 17 D03 N – 17 D02 N – 17 D01 N – 17 LSB N – 17 D05 N – 16 D04 N – 16 MSB N – 17 D10 N – 17 D09 N – 17 D08 N – 17 D07 N – 17 D06 N – 17 MSB N – 16 D10 N – 16 D04 N – 16 D02 N – 16 LSB N – 16 D05 N – 16 D03 N – 16 D01 N – 16 D03 N – 16 D02 N – 16 D01 N – 16 LSB N – 16 D09 N – 16 D08 N – 16 D07 N – 16 D06 N – 16 tLD D1–A1 D1+A1 FCO–1, FCO–2 FCO+1, FCO+2 BYTEWISE MODE D0–A1 D0+A1 11537-004 D1–A1 D1+A1 Figure 4. 12-Bit DDR/SDR, Two-Lane, 1× Frame Mode N–1 VIN±x1, VIN±x2 N tA CLK– tEH N+1 tEL CLK+ DCO–1, DCO–2 tCPD DDR DCO+1, DCO+2 DCO–1, DCO–2 SDR DCO+1, DCO+2 FCO–1, FCO–2 FCO+1, FCO+2 D0–A1 BITWISE MODE D0+A1 tFCO tFRAME tPD tDATA D12 N – 17 D10 N – 17 D08 N – 17 D06 N – 17 D04 N – 17 D02 N – 17 LSB N – 17 0 N – 17 MSB N – 17 D11 N – 17 D09 N – 17 D07 N – 17 D05 N – 17 D03 N – 17 D01 N – 17 0 N – 17 D10 N – 16 D08 N – 16 D06 N – 16 D04 N – 16 D02 N – 16 LSB N – 16 0 N – 16 MSB N – 16 D11 N – 16 D09 N – 16 D07 N – 16 D05 N – 16 D03 N – 16 D01 N – 16 0 N – 16 D05 N – 17 D04 N – 17 D03 N – 17 D02 N – 17 D01 N – 17 LSB N – 17 0 N – 17 0 N – 17 D05 N – 16 D04 N – 16 D03 N – 16 D02 N – 16 D01 N – 16 LSB N – 16 0 N – 16 0 N – 16 MSB N – 17 D12 N – 17 D11 N – 17 D10 N – 17 D09 N – 17 D08 N – 17 D07 N – 17 D06 N – 17 MSB N – 16 D12 N – 16 D11 N – 16 D10 N – 16 D09 N – 16 D08 N – 16 D07 N – 16 D06 N – 16 tLD D1–A1 D1+A1 D12 N – 16 FCO–1, FCO–2 D0–A1 D0+A1 D1–A1 D1+A1 Figure 5. 16-Bit DDR/SDR, Two-Lane, 2× Frame Mode Rev. A | Page 9 of 40 11537-005 FCO+1, FCO+2 BYTEWISE MODE AD9681 Data Sheet N–1 VIN±x1, VIN±x2 N+1 tA N tEH CLK– CLK+ tEL tCPD DCO–1, DCO–2 DDR DCO+1, DCO+2 DCO–1, DCO–2 SDR DCO+1, DCO+2 tFCO FCO–1, FCO–2 FCO+1, FCO+2 tDATA tPD D0–A1 BITWISE MODE tFRAME D0+A1 D10 N – 17 D08 N – 17 D06 N – 17 D04 N – 17 D02 N – 17 LSB N – 17 D10 N – 16 D08 N – 16 D06 N – 16 MSB N – 17 D09 N – 17 D07 N – 17 D05 N – 17 D03 N – 17 D01 N – 17 MSB N – 16 D09 N – 16 D07 N – 16 D05 N – 17 D04 N – 17 D03 N – 17 D02 N – 17 D01 N – 17 LSB N – 17 D05 N – 16 D04 N – 16 MSB N – 17 D10 N – 17 D09 N – 17 D08 N – 17 D07 N – 17 D06 N – 17 MSB N – 16 D10 N – 16 D04 N – 16 D02 N – 16 LSB N – 16 D05 N – 16 D03 N – 16 D01 N – 16 D03 N – 16 D02 N – 16 D01 N – 16 LSB N – 16 D09 N – 16 D08 N – 16 D07 N – 16 D06 N – 16 tLD D1–A1 D1+A1 FCO–1, FCO–2 FCO+1, FCO+2 D0–A1 BYTEWISE MODE D1–A1 D1+A1 11537-006 D0+A1 Figure 6. 12-Bit DDR/SDR, Two-Lane, 2× Frame Mode N–1 VIN±x1, VIN±x2 tA N tEH CLK– tEL CLK+ DCO–1, DCO–2 tCPD DCO+1, DCO+2 FCO–1, FCO–2 tFCO tFRAME FCO+1, FCO+2 D0+x tPD tDATA MSB D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 LSB 0 0 MSB D14 D13 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 16 N – 16 N – 16 Figure 7. Wordwise DDR, One-Lane, 1× Frame, 16-Bit Output Mode Rev. A | Page 10 of 40 11537-002 D0–x Data Sheet AD9681 N–1 VIN±x1, VIN±x2 tA N tEH CLK– tEL CLK+ tCPD DCO–1, DCO–2 DCO+1, DCO+2 tFCO FCO–1, FCO–2 tFRAME FCO+1, FCO+2 MSB D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 MSB N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 17 N – 16 D0+x Figure 8. Wordwise DDR, One-Lane, 1× Frame, 12-Bit Output Mode CLK+ tHSYNC 11537-079 tSSYNC SYNC Figure 9. SYNC Input Timing Requirements Rev. A | Page 11 of 40 D10 N – 16 11537-082 tDATA tPD D0–x AD9681 Data Sheet ABSOLUTE MAXIMUM RATINGS Table 6. Parameter Electrical AVDD to GND DRVDD to GND Digital Outputs (D0±xx, D1±xx, DCO±1, DCO±2, FCO±1, FCO±2) to GND CLK+, CLK− to GND VIN±x1, VIN±x2 to GND SCLK/DTP, SDIO/OLM, CSB1, CSB2 to GND SYNC, PDWN to GND RBIAS1, RBIAS2 to GND VREF, VCM1, VCM2, SENSE to GND Environmental Operating Temperature Range (Ambient) Maximum Junction Temperature Lead Temperature (Soldering, 10 sec) Storage Temperature Range (Ambient) THERMAL CHARACTERISTICS Rating −0.3 V to +2.0 V −0.3 V to +2.0 V −0.3 V to +2.0 V Typical θJA is specified for a 4-layer PCB with a solid ground plane. Airflow improves heat dissipation, which reduces θJA. In addition, metal in direct contact with the package leads from metal traces, through holes, ground, and power planes reduces θJA. Table 7. Thermal Resistance (Simulated) −0.3 V to +2.0 V −0.3 V to +2.0 V −0.3 V to +2.0 V −0.3 V to +2.0 V −0.3 V to +2.0 V −0.3 V to +2.0 V Package Type 144-Ball, 10 mm × 10 mm CSP-BGA 1 2 −40°C to +85°C 150°C 300°C −65°C to +150°C Airflow Velocity (m/sec) 0 θJA1, 2 30.2 JT1, 2 0.13 Per JEDEC 51-7, plus JEDEC 51-5 2S2P test board. Per JEDEC JESD51-2 (still air) or JEDEC JESD51-6 (moving air). ESD CAUTION Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Rev. A | Page 12 of 40 Unit °C/W Data Sheet AD9681 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS AD9681 TOP VIEW (Not to Scale) 1 2 3 4 5 6 7 8 9 10 11 12 A VIN–D1 VIN+D1 NC VIN–C2 NC VIN–C1 NC NC VIN–B2 NC VIN+B1 VIN–B1 B NC NC NC VIN+C2 NC VIN+C1 NC NC VIN+B2 NC NC NC C VIN–D2 VIN+D2 SYNC VCM1 VCM2 VREF SENSE RBIAS1 RBIAS2 GND VIN+A2 VIN–A2 D GND GND GND AVDD AVDD AVDD AVDD AVDD AVDD GND NC NC E CLK– CLK+ GND AVDD GND GND GND GND AVDD CSB1 VIN+A1 VIN–A1 F GND GND GND AVDD GND GND GND GND AVDD CSB2 G D1–D2 D1+D2 GND AVDD GND GND GND GND AVDD PDWN D0+A1 D0–A1 H D0–D2 D0+D2 GND AVDD GND GND GND GND AVDD GND D1+A1 D1–A1 J D1–D1 D1+D1 GND AVDD AVDD AVDD AVDD AVDD AVDD GND D0+A2 D0–A2 K D0–D1 D0+D1 DRVDD DRVDD GND GND GND GND DRVDD DRVDD D1+A2 D1–A2 L D1–C2 D1+C2 D1+C1 D0+C1 FCO+1 DCO+1 DCO+2 FCO+2 D1+B2 D0+B2 D0+B1 D0–B1 M D0–C2 D0+C2 D1–C1 D0–C1 FCO–1 DCO–1 DCO–2 FCO–2 D1–B2 D0–B2 D1+B1 D1–B1 NOTES 1. NC = NO CONNECT. THESE PINS ARE NOT ELECTRICALLY CONNECTED TO THE DEVICE. HOWEVER, CONNECT THESE PINS TO BOARD GROUND WHERE POSSIBLE. 11537-009 SDIO/OLM SCLK/DTP Figure 10. Pin Configuration Table 8. Pin Function Descriptions Pin No. A3, A5, A7, A8, A10, B1 to B3, B5, B7, B8, B10 to B12, D11, D12 C10, D1 to D3, D10, E3, E5 to E8, F1 to F3, F5 to F8, G3, G5 to G8, H3, H5 to H8, H10, J3, J10, K5 to K8 D4 to D9, E4, E9, F4, F9, G4, G9, H4, H9, J4 to J9 K3, K4, K9, K10 E1, E2 Mnemonic NC Description No Connect. These pins are not electrically connected to the device. However, connect these pins to board ground where possible. GND Ground. AVDD 1.8 V Analog Supply. DRVDD CLK−, CLK+ 1.8 V Digital Output Driver Supply. Input Clock Complement, Input Clock True. Rev. A | Page 13 of 40 AD9681 Data Sheet Pin No. G12, G11 H12, H11 J12, J11 K12, K11 L12, L11 M12, M11 M10, L10 M9, L9 M4, L4 M3, L3 M1, M2 L1, L2 K1, K2 J1, J2 H1, H2 G1, G2 M6, L6; M7, L7 Mnemonic D0−A1, D0+A1 D1−A1, D1+A1 D0−A2, D0+A2 D1−A2, D1+A2 D0−B1, D0+B1 D1−B1, D1+B1 D0−B2, D0+B2 D1−B2, D1+B2 D0−C1, D0+C1 D1−C1, D1+C1 D0−C2, D0+C2 D1−C2, D1+C2 D0−D1, D0+D1 D1−D1, D1+D1 D0−D2, D0+D2 D1−D2, D1+D2 DCO−1, DCO+1; DCO−2, DCO+2 M5, L5; M8, L8 F12 F11 E10, F10 FCO−1, FCO+1; FCO−2, FCO+2 SCLK/DTP SDIO/OLM CSB1, CSB2 G10 E12, E11 C12, C11 A12, A11 A9, B9 A6, B6 A4, B4 A1, A2 C8, C9 C7 C6 C4, C5 PDWN VIN−A1, VIN+A1 VIN−A2, VIN+A2 VIN−B1, VIN+B1 VIN−B2, VIN+B2 VIN−C1, VIN+C1 VIN−C2, VIN+C2 VIN−D1, VIN+D1 VIN−D2, VIN+D2 RBIAS1, RBIAS2 SENSE VREF VCM1, VCM2 C3 SYNC C1, C2 Description Lane 0 Bank 1 Digital Output Complement, Lane 0 Bank 1 Digital Output True. Lane 1 Bank 1 Digital Output Complement, Lane 1 Bank 1 Digital Output True. Lane 0 Bank 2 Digital Output Complement, Lane 0 Bank 2 Digital Output True. Lane 1 Bank 2 Digital Output Complement, Lane 1 Bank 2 Digital Output True. Lane 0 Bank 1 Digital Output Complement, Lane 0 Bank 1 Digital Output True. Lane 1 Bank 1 Digital Output Complement, Lane 1 Bank 1 Digital Output True. Lane 0 Bank 2 Digital Output Complement, Lane 0 Bank 2 Digital Output True. Lane 1 Bank 2 Digital Output Complement, Lane 1 Bank 2 Digital Output True. Lane 0 Bank 1 Digital Output Complement, Lane 0 Bank 1 Digital Output True. Lane 1 Bank 1 Digital Output Complement, Lane 1 Bank 1 Digital Output True. Lane 0 Bank 2 Digital Output Complement, Lane 0 Bank 2 Digital Output True. Lane 1 Bank 2 Digital Output Complement, Lane 1 Bank 2 Digital Output True. Lane 0 Bank 1 Digital Output Complement, Lane 0 Bank 1 Digital Output True. Lane 1 Bank 1 Digital Output Complement, Lane 1 Bank 1 Digital Output True. Lane 0 Bank 2 Digital Output Complement, Lane 0 Bank 2 Digital Output True. Lane 1 Bank 2 Digital Output Complement, Lane 1 Bank 2 Digital Output True. Data Clock Digital Output Complement, Data Clock Digital Output True. DCO±1 is used to capture D0±x1/D1±x1 digital output data, and DCO±2 is used to capture D0±x2/D1±x2 digital output data. Frame Clock Digital Output Complement, Frame Clock Digital Output True. FCO±1 frames D0±x1/D1±x1 digital output data, and FCO±2 frames D0±x2/D1±x2 digital output data. Serial Clock/Digital Test Pattern. Serial Data Input/Output/Output Lane Mode. Chip Select Bar. CSB1 enables/disables the SPI for four channels in Bank 1; CSB2 enables/ disables the SPI for four channels in Bank 2. Power-Down. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Analog Input Complement, Analog Input True. Sets analog current bias. Connect each RBIASx pin to a 10 kΩ (1% tolerance) resistor to ground. Reference Mode Selection. Voltage Reference Input/Output. Analog Output Voltage at Midsupply. Sets the common mode of the analog inputs, external to the ADC, as shown in Figure 38 and Figure 39. Digital Input; Synchronizing Input to Clock Divider. This pin is internally pulled to ground by a 30 kΩ resistor. Rev. A | Page 14 of 40 Data Sheet AD9681 TYPICAL PERFORMANCE CHARACTERISTICS 0 –20 –40 –60 –80 –100 20 30 40 50 60 Figure 11. Single-Tone 32k FFT with fIN = 9.7 MHz; fSAMPLE = 125 MSPS 0 0 0 –60 –80 –100 20 30 40 50 60 AIN = –1dBFS fIN = 201MHz SNR = 69.71dBFS SINAD = 68.56dBc SFDR = 84.1dBc –20 –40 10 Figure 14. Single-Tone 32k FFT with fIN = 139.5 MHz; fSAMPLE = 125 MSPS AMPLITUDE (dBFS) –40 –60 –80 –100 10 20 30 40 50 60 FREQUENCY (MHz) Figure 12. Single-Tone 32k FFT with fIN = 19.7 MHz; fSAMPLE = 125 MSPS 0 –140 11537-111 0 AMPLITUDE (dBFS) –80 –100 30 40 50 60 AIN = –1dBFS fIN = 301MHz SNR = 66.97dBFS SINAD = 65.22dBc SFDR = 75.8dBc –20 –60 20 Figure 15. Single-Tone 32k FFT with fIN = 201 MHz; fSAMPLE = 125 MSPS 0 –40 10 FREQUENCY (MHz) AIN = –1dBFS fIN = 69.5MHz SNR = 73.85dBFS SINAD = 72.76dBc SFDR = 90dBc –20 0 11537-115 –120 –40 –60 –80 –100 –120 –120 10 20 30 40 FREQUENCY (MHz) 50 60 –140 11537-112 0 Figure 13. Single-Tone 32k FFT with fIN = 69.5 MHz; fSAMPLE = 125 MSPS 0 10 20 30 40 FREQUENCY (MHz) 50 60 11537-116 AMPLITUDE (dBFS) –100 FREQUENCY (MHz) –120 AMPLITUDE (dBFS) –80 –140 AIN = –1dBFS fIN = 19.7MHz SNR = 74.77dBFS SINAD = 73.7dBc SFDR = 94.2dBc –20 –140 –60 11537-113 10 11537-110 0 FREQUENCY (MHz) –140 –40 –120 –120 –140 AIN = –1dBFS fIN = 139.5MHz SNR = 71.73dBFS SINAD = 70.63dBc SFDR = 87.9dBc –20 AMPLITUDE (dBFS) AMPLITUDE (dBFS) 0 AIN = –1dBFS fIN = 9.7MHz SNR = 74.99dBFS SINAD = 73.96dBc SFDR = 96.4dBc Figure 16. Single-Tone 32k FFT with fIN = 301 MHz; fSAMPLE = 125 MSPS Rev. A | Page 15 of 40 AD9681 Data Sheet 100 120 SFDR (dBFS) 90 100 SFDR (dBc) SNRFS (dBFS) 80 SNR/SFDR (dBFS/dBc) SNR/SFDR (dBFS/dBc) 80 60 SFDR (dBc) 40 SNR (dB) 20 70 SNRFS (dBFS) 60 50 40 30 20 0 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 INPUT AMPLITUDE (dBFS) 0 11537-117 –20 –100 0 SNR/SFDR (dBFS/dBc) AMPLITUDE (dBFS) 250 300 350 400 450 500 95 –60 2F1–F2 2F2–F1 –80 F1+F2 200 100 –40 F2–F1 150 Figure 20. SNR/SFDR vs. fIN; fSAMPLE = 125 MSPS AIN = –7dBFS fIN = 70MHz, 72.5MHz IMD2 = –100dBc IMD3 = –99.5dBc SFDR = 97.5dBc –20 100 INPUT FREQUENCY (MHz) Figure 17. SNR/SFDR vs. Input Amplitude (AIN); fIN = 9.7 MHz; fSAMPLE = 125 MSPS 0 50 11537-120 10 F1+2F2 2F1+F2 –100 SFDR (dBc) 90 85 80 SNRFS (dBFS) 0 10 20 30 40 50 60 FREQUENCY (MHz) 70 –40 11537-118 –140 Figure 18. Two-Tone 32k FFT with fIN1 = 70.5 MHz and fIN2 = 72.5 MHz; fSAMPLE = 125 MSPS IMD3 (dBc) –60 –80 IMD3 (dBFS) –80 –70 –60 –50 –40 –30 –20 –10 INPUT AMPLITUDE (dBFS) 11537-119 SFDR/IMD3 (dBc/dBFS) SFDR (dBc) –40 –120 –90 35 60 85 Figure 21. SNR/SFDR vs. Temperature; fIN = 9.7 MHz, fSAMPLE = 125 MSPS –20 SFDR (dBFS) 10 TEMPERATURE (°C) 0 –100 –15 11537-121 75 –120 Figure 19. Two-Tone SFDR/IMD3 vs. Input Amplitude (AIN) with fIN1 = 70.0 MHz and fIN2 = 72.5 MHz; fSAMPLE = 125 MSPS Rev. A | Page 16 of 40 Data Sheet AD9681 1.0 110 0.8 105 0.6 100 INL (LSB) 0.4 0.2 0 –0.2 –0.4 95 90 85 80 –0.6 70 –0.8 65 1 2000 4000 6000 8000 10000 12000 14000 16000 OUTPUT CODE 60 20 11537-122 –1.0 SNRFS (dBFS) 75 30 40 50 60 70 80 90 100 110 120 130 SAMPLE RATE (MSPS) Figure 22. INL; fIN = 9.7 MHz, fSAMPLE = 125 MSPS 11537-126 SNR/SFDR (dBFS/dBc) SFDR (dBc) Figure 25. SNR/SFDR vs. Sample Rate; fIN = 9.7 MHz, fSAMPLE = 125 MSPS 110 0.8 105 0.6 100 SNR/SFDR (dBFS/dBc) 0.2 0 –0.2 95 90 SFDR (dBc) 85 80 75 SNRFS (dBFS) 70 –0.4 65 1 2000 4000 6000 8000 10000 12000 14000 16000 OUTPUT CODE 60 20 11537-123 –0.6 0.99 LSB RMS 700000 600000 500000 400000 300000 200000 OUTPUT CODE 11537-124 N+9 N + 10 N+8 N+7 N+6 N+5 N+4 N+3 N+2 N N+1 N–1 N–2 N–3 N–4 N–5 N–6 N–7 N–8 N–9 100000 N – 10 NUMBER OF HITS 50 60 70 80 90 100 110 120 130 Figure 26. SNR/SFDR vs. Sample Rate; fIN = 70 MHz, fSAMPLE = 125 MSPS 800000 0 40 SAMPLE RATE (MSPS) Figure 23. DNL; fIN = 9.7 MHz, fSAMPLE = 125 MSPS 900000 30 11537-127 DNL (LSB) 0.4 Figure 24. Input Referred Noise Histogram; fSAMPLE = 125 MSPS Rev. A | Page 17 of 40 AD9681 Data Sheet EQUIVALENT CIRCUITS AVDD AVDD 350Ω SCLK/DTP, SYNC, AND PDWN VIN±y1, VIN±y2 11537-012 11537-008 30kΩ Figure 27. Equivalent Analog Input Circuit Figure 31. Equivalent SCLK/DTP, SYNC, and PDWN Input Circuit AVDD 10Ω CLK+ AVDD 15kΩ 0.9V AVDD 15kΩ 10Ω 11537-013 11537-037 CLK– 375Ω RBIAS1, RBIAS2 AND VCM1, VCM2 Figure 32. Equivalent RBIASx and VCMx Circuit Figure 28. Equivalent Clock Input Circuit AVDD AVDD 30kΩ 400Ω SDIO/OLM 350Ω 31kΩ 11537-014 11537-036 CSB1, CSB2 Figure 29. Equivalent SDIO/OLM Input Circuit Figure 33. Equivalent CSBx Input Circuit DRVDD AVDD V D0–x1, D1–x1, D0–x2, D1–x2 V V D0+x1, D1+x1, D0+x2, D1+x2 V 375Ω VREF 11537-046 11537-015 7.5kΩ Figure 34. Equivalent VREF Circuit Figure 30. Equivalent Digital Output Circuit Rev. A | Page 18 of 40 Data Sheet AD9681 THEORY OF OPERATION The AD9681 is a multistage, pipelined ADC. Each stage provides sufficient overlap to correct for flash errors in the preceding stage. The quantized outputs from each stage are combined into a final 14-bit result in the digital correction logic. The serializer transmits this converted data in a 16-bit output. The pipelined architecture permits the first stage to operate with a new input sample while the remaining stages operate with preceding samples. Sampling occurs on the rising edge of the clock. Each stage of the pipeline, excluding the last, consists of a low resolution flash ADC connected to a switched-capacitor DAC and an interstage residue amplifier (for example, a multiplying digital-to-analog converter (MDAC)). The residue amplifier magnifies the difference between the reconstructed DAC output and the flash input for the next stage in the pipeline. One bit of redundancy is used in each stage to facilitate digital correction of flash errors. The last stage simply consists of a flash ADC. The output staging block aligns the data, corrects errors, and passes the data to the output buffers. The data is then serialized and aligned to the frame and data clocks. ANALOG INPUT CONSIDERATIONS The analog input to the AD9681 is a differential switched capacitor circuit designed for processing differential input signals. This circuit can support a wide common-mode range while maintaining excellent performance. By using an input commonmode voltage of midsupply, users can minimize signal dependent errors and achieve optimum performance. the output stage of the driving source. In addition, low Q inductors or ferrite beads can be placed on each leg of the input to reduce high differential capacitance at the analog inputs and, therefore, achieve the maximum bandwidth of the ADC. Such use of low Q inductors or ferrite beads is required when driving the converter front end at high IF frequencies. Place either a differential capacitor or two single-ended capacitors on the inputs to provide a matching passive network. This configuration ultimately creates a low-pass filter at the input to limit unwanted broadband noise. See the AN-742 Application Note, Frequency Domain Response of Switched-Capacitor ADCs; the AN-827 Application Note, A Resonant Approach to Interfacing Amplifiers to SwitchedCapacitor ADCs; and the Analog Dialogue article “TransformerCoupled Front-End for Wideband A/D Converters” (Volume 39, April 2005) for more information. In general, the precise values vary, depending on the application. Input Common Mode The analog inputs of the AD9681 are not internally dc biased. Therefore, in ac-coupled applications, the user must provide this bias externally. For optimum performance, set the device so that VCM = AVDD/2. However, the device can function over a wider range with reasonable performance, as shown in Figure 36. An on-chip, common-mode voltage reference is included in the design and is available at the VCMx pin. Decouple the VCMx pin to ground using a 0.1 μF capacitor, as described in the Applications Information section. Maximum SNR performance is achieved by setting the ADC to the largest span in a differential configuration. In the case of the AD9681, the largest available input span is 2 V p-p. 100 H H 80 CSAMPLE S S S CSAMPLE H CPAR H Figure 35. Switched Capacitor Input Circuit SNR (dBFS) 70 60 50 40 30 The clock signal alternately switches the input circuit between sample mode and hold mode (see Figure 35). When the input circuit is switched to sample mode, the signal source must be capable of charging the sample capacitors and settling within one-half of a clock cycle. A small resistor, in series with each input, can help reduce the peak transient current injected from Rev. A | Page 19 of 40 20 0.5 0.7 0.9 1.1 VCM (V) Figure 36. SNR/SFDR vs. Common-Mode Voltage; fIN = 9.7 MHz, fSAMPLE = 125 MSPS 1.3 11537-052 S SNR/SFDR (dBFS/dBc) VIN–x1, VIN–x2 SFDR (dBc) 90 CPAR 11537-051 VIN+x1, VIN+x2 AD9681 Data Sheet Differential Input Configurations Internal Reference Connection There are several ways to drive the AD9681, either actively or passively. However, optimum performance is achieved by driving the analog inputs differentially. Using a differential double balun configuration to drive the AD9681 provides excellent performance and a flexible interface to the ADC (see Figure 38) for baseband applications. Similarly, differential transformer coupling also provides excellent performance (see Figure 39). Because the noise performance of most amplifiers is not adequate to achieve the true performance of the AD9681, use of these passive configurations is recommended wherever possible. A comparator within the AD9681 detects the potential at the SENSE pin and configures the reference into two possible modes, which are summarized in Table 9. If SENSE is grounded, the reference amplifier switch is connected to the internal resistor divider (see Figure 37), setting VREF to 1.0 V. Table 9. Reference Configuration Summary Selected Mode Fixed Internal Reference Fixed External Reference Regardless of the configuration, the value of the shunt capacitor, C, is dependent on the input frequency and may need to be reduced or removed. It is recommended that the AD9681 inputs not be driven singleended. SENSE Voltage (V) GND to 0.2 Resulting VREF (V) 1.0 internal AVDD 1.0 applied to external VREF pin Resulting Differential Span (V p-p) 2.0 2.0 VIN+A/VIN+B VOLTAGE REFERENCE VIN–A/VIN–B A stable and accurate 1.0 V voltage reference is built into the AD9681. Configure VREF using either the internal 1.0 V reference or an externally applied 1.0 V reference voltage. The various reference modes are summarized in the Internal Reference Connection section and the External Reference Operation section. Bypass the VREF pin to ground externally, using a low ESR, 1.0 μF capacitor in parallel with a low ESR, 0.1 μF ceramic capacitor. ADC CORE VREF 1.0µF 0.1µF SELECT LOGIC SENSE ADC Figure 37. Internal Reference Configuration 0.1µF 0.1µF R 33Ω C 2V p-p *C1 C ADC 5pF 33Ω 0.1µF ET1-1-I3 VIN+x1, VIN+x2 33Ω R VIN–x1, VIN–x2 33Ω C VCM *C1 200Ω 0.1µF C 0.1µF *C1 IS OPTIONAL Figure 38. Differential Double Balun Input Configuration for Baseband Applications ADT1-1WT 1:1 Z RATIO R *C1 VIN+x1, VIN+x2 33Ω 2V p-p 49.9Ω C R 33Ω ADC 5pF VIN–x1, VIN–x2 VCM *C1 0.1µF 0.1μF *C1 IS OPTIONAL 11537-056 200Ω Figure 39. Differential Transformer Coupled Configuration for Baseband Applications Rev. A | Page 20 of 40 11537-059 R 11537-060 0.5V Data Sheet AD9681 If the internal reference of the AD9681 is used to drive multiple converters to improve gain matching, the loading of the reference by the other converters must be considered. Figure 40 shows how the internal reference voltage is affected by loading. 0 –0.5 –1.0 –2.5 –3.0 –3.5 –4.0 0 0.5 1.0 1.5 2.0 2.5 3.0 LOAD CURRENT (mA) 11537-061 –4.5 Figure 40. VREF Error vs. Load Current External Reference Operation The use of an external reference may be necessary to enhance the gain accuracy of the ADC or improve thermal drift characteristics. Figure 41 shows the typical drift characteristics of the internal reference in 1.0 V mode. 4 VREF ERROR (mV) 2 0 Figure 42 and Figure 43 show two preferred methods for clocking the AD9681 (at clock rates of up to 1 GHz prior to the internal clock divider). A low jitter clock source is converted from a singleended signal to a differential signal using either an RF transformer or an RF balun. The RF balun configuration is recommended for clock frequencies from 125 MHz to 1 GHz, and the RF transformer is recommended for clock frequencies from 10 MHz to 200 MHz. The antiparallel Schottky diodes across the transformer/balun secondary winding limit clock excursions into the AD9681 to approximately 0.8 V p-p differential. This limit helps prevent the large voltage swings of the clock from feeding through to other portions of the AD9681 while preserving the fast rise and fall times of the signal that are critical to achieving a low jitter performance. However, the diode capacitance comes into play at frequencies above 500 MHz. Take care when choosing the appropriate signal limiting diode. –2 Mini-Circuits® ADT1-1WT, 1:1 Z –4 CLOCK INPUT 0.1µF XFMR 0.1µF CLK+ 100Ω 50Ω CLK– SCHOTTKY DIODES: HSMS2822 –15 10 35 TEMPERATURE (°C) 60 85 11537-062 0.1µF –8 –40 ADC 0.1µF –6 11537-064 VREF ERROR (%) The AD9681 has a flexible clock input structure. The clock input can be a CMOS, LVDS, LVPECL, or sine wave signal. Regardless of the type of signal being used, clock source jitter is of the utmost concern, as described in the Jitter Considerations section. –2.0 –5.0 For optimum performance, clock the AD9681 sample clock inputs, CLK+ and CLK−, with a differential signal. The signal is typically ac-coupled into the CLK+ and CLK− pins via a transformer or capacitors. These pins are biased internally (see Figure 28) and require no external bias. Clock Input Options INTERNAL VREF = 1V –1.5 CLOCK INPUT CONSIDERATIONS Figure 42. Transformer Coupled Differential Clock (Up to 200 MHz) Figure 41. Typical VREF Drift 0.1µF CLOCK INPUT Do not leave the SENSE pin floating. 0.1µF CLK+ 50Ω ADC 0.1µF 0.1µF CLK– SCHOTTKY DIODES: HSMS2822 Figure 43. Balun Coupled Differential Clock (Up to 1 GHz) Rev. A | Page 21 of 40 11537-065 When the SENSE pin is tied to AVDD, the internal reference is disabled, allowing the use of an external reference. An internal reference buffer loads the external reference with an equivalent 7.5 kΩ load (see Figure 34). The internal buffer generates the positive and negative full-scale references for the ADC core. Therefore, limit the external reference to a maximum of 1.0 V. AD9681 Data Sheet Input Clock Divider If a low jitter clock source is not available, another option is to ac couple a differential PECL signal to the sample clock input pins, as shown in Figure 44. The AD9510/AD9511/AD9512/ AD9513/AD9514/AD9515-x/AD9516-x/AD9517-x clock drivers offer excellent jitter performance. 0.1µF CLOCK INPUT The AD9681 clock divider can be synchronized using the external SYNC input. Bit 0 and Bit 1 of Register 0x109 allow the clock divider to be resynchronized on every SYNC signal or only on the first SYNC signal after the register is written. A valid SYNC causes the clock divider to reset to its initial state. This synchronization feature allows the clock dividers of multiple devices to be aligned to guarantee simultaneous input sampling. 0.1µF CLK+ 0.1µF 50kΩ AD951x PECL DRIVER 240Ω 50kΩ ADC 100Ω 0.1µF CLK– 11537-066 CLOCK INPUT The AD9681 contains an input clock divider with the ability to divide the input clock by integer values from 1 to 8. 240Ω Figure 44. Differential PECL Sample Clock (Up to 1 GHz) A third option is to ac couple a differential LVDS signal to the sample clock input pins, as shown in Figure 45. The AD9510/ AD9511/AD9512/AD9513/AD9514/AD9515-x/AD9516-x/ AD9517-x clock drivers offer excellent jitter performance. 0.1µF CLOCK INPUT 0.1µF 50kΩ 0.1µF ADC CLK– 11537-067 CLOCK INPUT CLK+ 100Ω Typical high speed ADCs use both clock edges to generate a variety of internal timing signals and, as a result, may be sensitive to clock duty cycle. Commonly, a ±5% tolerance is required on the clock duty cycle to maintain dynamic performance characteristics. The AD9681 contains a duty cycle stabilizer (DCS) that retimes the nonsampling (falling) edge, providing an internal clock signal with a nominal 50% duty cycle. This allows the user to provide a wide range of clock input duty cycles without affecting the performance of the AD9681. Noise and distortion performance are nearly flat for a wide range of duty cycles with the DCS turned on. 0.1µF AD951x LVDS DRIVER Clock Duty Cycle 50kΩ Figure 45. Differential LVDS Sample Clock (Up to 1 GHz) In some applications, it may be acceptable to drive the sample clock inputs with a single-ended 1.8 V CMOS signal. In such applications, drive the CLK+ pin directly from a CMOS gate, and bypass the CLK− pin to ground with a 0.1 μF capacitor (see Figure 46). Jitter on the rising edge of the input is still of concern and is not easily reduced by the internal stabilization circuit. The duty cycle control loop does not function for clock rates of less than 20 MHz, nominally. The loop has a time constant associated with it that must be considered in applications in which the clock rate can change dynamically. A wait time of 1.5 μs to 5 μs is required after a dynamic clock frequency increase or decrease before the DCS loop is relocked to the input signal. VCC 0.1µF CLOCK INPUT 50Ω1 1kΩ AD951x CMOS DRIVER OPTIONAL 0.1µF 100Ω 1kΩ CLK+ ADC 150Ω RESISTOR IS OPTIONAL. 11537-068 CLK– 0.1µF Figure 46. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz) Rev. A | Page 22 of 40 Data Sheet AD9681 Jitter Considerations POWER DISSIPATION AND POWER-DOWN MODE High speed, high resolution ADCs are sensitive to the quality of the clock input. The degradation in SNR at a given input frequency (fA) that is due only to aperture jitter (tJ) is expressed by As shown in Figure 48, the power dissipated by the AD9681 is proportional to its sample rate and can be set to one of several power saving modes using Register 0x100, Bits[2:0]. 0.9 0.8 In this equation, the rms aperture jitter represents the root sum square of all jitter sources, including the clock input, analog input signal, and ADC aperture jitter specifications. IF undersampling applications are particularly sensitive to jitter (see Figure 47). RMS CLOCK JITTER REQUIREMENT 125MSPS SETTING 0.7 105MSPS SETTING 80MSPS SETTING 65MSPS SETTING 50MSPS SETTING 40MSPS SETTING 0.6 0.5 120 110 0.4 90 0.3 10 14 BITS 80 12 BITS 10 BITS 8 BITS 50 40 30 1 0.125ps 0.25ps 0.5ps 1.0ps 2.0ps 10 100 ANALOG INPUT FREQUENCY (MHz) 1000 50 70 90 110 130 Figure 48. Total Power vs. fSAMPLE for fIN = 9.7 MHz 70 60 30 SAMPLE RATE (MSPS) 11537-070 SNR (dB) 20MSPS SETTING 16 BITS 100 11537-071 130 TOTAL POWER (W) 1 SNR Degradation = 20 log10 2 f A t J Figure 47. Ideal SNR vs. Input Frequency and Jitter Treat the clock input as an analog signal in cases where aperture jitter may affect the dynamic range of the AD9681. Separate the clock driver power supplies from the ADC output driver supplies to avoid modulating the clock signal with digital noise. Low jitter, crystal controlled oscillators are excellent clock sources. If another type of source generates the clock (by gating, dividing, or another method), ensure that it is retimed by the original clock at the last step. See the AN-501 Application Note, Aperture Uncertainty and ADC System Performance, and the AN-756 Application Note, Sampled Systems and the Effects of Clock Phase Noise and Jitter, for more in depth information about jitter performance as it relates to ADCs. The AD9681 is placed in power-down mode either by the SPI port or by asserting the PDWN pin high. In this state, the ADC typically dissipates 2 mW. During power-down, the output drivers are placed in a high impedance state. Asserting the PDWN pin low returns the AD9681 to its normal operating mode. Note that PDWN is referenced to the digital output driver supply (DRVDD) and should not exceed that supply voltage. Low power dissipation in power-down mode is achieved by shutting down the reference, reference buffer, biasing networks, and clock. The internal capacitors are discharged when the device enters power-down mode and then must be recharged when returning to normal operation. As a result, wake-up time is related to the time spent in power-down mode, and shorter power-down cycles result in proportionally shorter wake-up times. When using the SPI port interface, the user can place the ADC in power-down mode or standby mode. Standby mode allows the user to keep the internal reference circuitry powered when faster wake-up times are required. See the Memory Map section for more details on using these features. Rev. A | Page 23 of 40 AD9681 Data Sheet DIGITAL OUTPUTS AND TIMING The AD9681 LVDS outputs facilitate interfacing with LVDS receivers in custom ASICs and FPGAs for superior switching performance in noisy environments. Single point-to-point net topologies are recommended with a 100 Ω termination resistor placed as near to the receiver as possible. If there is no far end receiver termination or there is poor differential trace routing, timing errors may result. To avoid such timing errors, it is recommended that the trace length be less than 24 inches, with all traces the same length. Place the differential output traces as near to each other as possible. An example of the FCO and data stream with proper trace length and position is shown in Figure 49. Figure 50 shows an LVDS output timing example in reduced range mode. D0 500mV/DIV D1 500mV/DIV DCO 500mV/DIV FCO 500mV/DIV 4ns/DIV Figure 49. LVDS Output Timing Example in ANSI-644 Mode (Default) D0 400mV/DIV D1 400mV/DIV DCO 400mV/DIV FCO 400mV/DIV 4ns/DIV 11537-083 When operating in reduced range mode, the output current reduces to 2 mA. This results in a 200 mV swing (or 400 mV p-p differential) across a 100 Ω termination at the receiver. 11537-074 The AD9681 differential outputs conform to the ANSI-644 LVDS standard on default power-up. This can be changed to a low power, reduced signal option (similar to the IEEE 1596.3 standard) via the SPI. The LVDS driver current is derived on chip and sets the output current at each output equal to a nominal 3.5 mA. A 100 Ω differential termination resistor placed at the LVDS receiver inputs results in a nominal 350 mV swing (or 700 mV p-p differential) at the receiver. Figure 50. LVDS Output Timing Example in Reduced Range Mode Rev. A | Page 24 of 40 Data Sheet AD9681 Figure 51 shows an example of the LVDS output using the ANSI-644 standard (default) data eye and a time interval error (TIE) jitter histogram with trace lengths of less than 24 inches on standard FR-4 material. Figure 52 shows an example of trace lengths exceeding 24 inches on standard FR-4 material. Note that the TIE jitter histogram reflects the decrease of the data eye opening as the edge deviates from the ideal position. 500 EYE: ALL BITS It is the responsibility of the user to determine if the waveforms meet the timing budget of the design when the trace lengths exceed 24 inches. Additional SPI options allow the user to further increase the internal termination (increasing the current) of all eight outputs to drive longer trace lengths, which can be achieved by programming Register 0x15. Although this option produces sharper rise and fall times on the data edges and is less prone to bit errors, it also increases the power dissipation of the DRVDD supply. 500 ULS: 7000/400354 300 200 100 0 –100 –200 –300 ULS: 8000/414024 300 200 100 0 –100 –200 –300 –400 –400 –500 –500 –0.8ns –0.4ns 0ns 0.4ns –0.8ns 0.8ns 7k 12k 6k 10k TIE JITTER HISTOGRAM (Hits) 5k 4k 3k 2k 0ns 0.4ns –0.8ns 8k 6k 4k 2k 250ps 300ps 350ps 400ps 450ps 500ps 11537-075 1k 0 200ps –0.4ns Figure 51. Data Eye for LVDS Outputs in ANSI-644 Mode with Trace Lengths of Less Than 24 Inches on Standard FR-4 Material, External 100 Ω Far End Termination Only 0k –800ps –600ps –400ps –200ps 0ps 200ps 400ps 600ps 11537-076 TIE JITTER HISTOGRAM (Hits) EYE: ALL BITS 400 EYE DIAGRAM VOLTAGE (mV) EYE DIAGRAM VOLTAGE (mV) 400 Figure 52. Data Eye for LVDS Outputs in ANSI-644 Mode with Trace Lengths of Greater Than 24 Inches on Standard FR-4 Material, External 100 Ω Far End Termination Only The default format of the output data is twos complement. Table 10 shows an example of the output coding format. To change the output data format to offset binary, see the Memory Map section, Register 0x14, Bit[0]. Table 10. Digital Output Coding Input (V) VIN+ − VIN− VIN+ − VIN− VIN+ − VIN− VIN+ − VIN− VIN+ − VIN− Condition (V) <−VREF − 0.5 LSB −VREF 0V +VREF − 1.0 LSB >+VREF − 0.5 LSB Offset Binary Output Mode 0000 0000 0000 0000 0000 0000 0000 0000 1000 0000 0000 0000 1111 1111 1111 1100 1111 1111 1111 1100 Rev. A | Page 25 of 40 Twos Complement Mode 1000 0000 0000 0000 1000 0000 0000 0000 0000 0000 0000 0000 0111 1111 1111 1100 0111 1111 1111 1100 AD9681 Data Sheet Data from each ADC is serialized and provided on a separate channel in two lanes in DDR mode. The data rate for each serial stream is equal to 16 bits times the sample clock rate, with a maximum of 500 Mbps/lane [(16 bits × 125 MSPS)/(2 × 2) = 500 Mbps/lane)]. The lowest typical conversion rate is 10 MSPS. See the Memory Map section for details on enabling this feature. Two output clock types are provided to assist in capturing data from the AD9681. DCO±1 and DCO±2 are used to clock the output data and their frequency is equal to 4× the sample clock (CLK±) rate for the default mode of operation. Data is clocked out of the AD9681 and must be captured on the rising and falling edges of the DCO that supports double data rate (DDR) capturing. DCO±1 is used to capture the D0±x1/D1±x1 (Bank 1) data; DCO±2 is used to capture the D0±x2/D1±x2 (Bank 2) data. FCO±1 and FCO±2 signal the start of a new output byte and toggle at a rate equal to the sample clock rate in 1× frame mode. FCO±1 frames the D0±x1/D1±x1 (Bank 1) data and FCO±2 frames the D0±x2/ D1±x2 (Bank 2) data. See the Timing Diagrams section for more information. When the SPI is used, the DCO phase can be adjusted in 60° increments relative to one data cycle (30° relative to one DCO cycle). This enables the user to refine system timing margins if required. The default DCO±1 and DCO±2 to output data edge timing, as shown in Figure 3, is 180° relative to one data cycle (90° relative to one DCO cycle). A 12-bit serial stream can also be initiated from the SPI. This allows the user to implement and test compatibility to lower resolution systems. When changing the resolution to a 12-bit serial stream, the data stream is shortened. See Figure 4 for the 12-bit example. However, in the default option with the serial output number of bits at 16, the data stream stuffs two 0s at the end of the 14-bit serial data. In default mode, as shown in Figure 3, the MSB is first in the data output serial stream. This can be inverted so that the LSB is first in the data output serial stream by using the SPI. There are 12 digital output test pattern options available that can be initiated through the SPI. This is a useful feature when validating receiver capture and timing (see Table 11 for the output bit sequencing options that are available). Some test patterns have two serial sequential words and can alternate in various ways, depending on the test pattern chosen. Note that some patterns do not adhere to the data format select option. In addition, custom user defined test patterns can be assigned in Register 0x19, Register 0x1A, Register 0x1B, and Register 0x1C. Table 11. Flexible Output Test Modes Digital Output Word 21 N/A N/A Subject to Data Format Select1 N/A Yes N/A Yes N/A Yes 0101 0101 0101 (12-bit) 0101 0101 0101 0100 (16-bit) N/A No PN sequence long2 Digital Output Word 11 N/A 1000 0000 0000 (12-bit) 1000 0000 0000 0000 (16-bit) 1111 1111 1111 (12-bit) 0000 0000 0000 0000 (16-bit) 0000 0000 0000 (12-bit) 0000 0000 0000 0000 (16-bit) 1010 1010 1010 (12-bit) 1010 1010 1010 1010 (16-bit) N/A 0110 PN sequence short2 N/A N/A Yes 0111 One-/zero-word toggle 1111 1111 1111 (12-bit) 111 1111 1111 1100 (16-bit) No 1000 1001 User input 1-/0-bit toggle No No 1010 1× sync N/A No 1011 One bit high Register 0x19 to Register 0x1A 1010 1010 1010 (12-bit) 1010 1010 1010 1000 (16-bit) 0000 0011 1111 (12-bit) 0000 0001 1111 1100 (16-bit) 1000 0000 0000 (12-bit) 1000 0000 0000 0000 (16-bit) 0000 0000 0000 (12-bit) 0000 0000 0000 0000 (16bit) Register 0x1B to Register 0x1C N/A N/A No 1100 Mixed bit frequency N/A No Output Test Mode Bit Sequence (Reg. 0x0D) 0000 0001 Pattern Name Off (default) Midscale short 0010 +Full-scale short 0011 −Full-scale short 0100 Checkerboard 0101 1 2 1010 0011 0011 (12-bit) 1010 0001 1001 1100 (16-bit) Yes N/A means not applicable. All test mode options except PN sequence short and PN sequence long can support 12-bit to 16-bit word lengths to verify data capture to the receiver. Rev. A | Page 26 of 40 Notes Offset binary code shown Offset binary code shown Offset binary code shown PN23 ITU 0.150 X23 + X18 + 1 PN9 ITU 0.150 X9 + X5 + 1 Pattern associated with the external pin Data Sheet AD9681 The PN sequence short pattern produces a pseudorandom bit sequence that repeats itself every 29 − 1 or 511 bits. Refer to Section 5.1 of the ITU-T 0.150 (05/96) standard for a description of the PN sequence and how it is generated. The seed value is all 1s (see Table 12 for the initial values). The output is a parallel representation of the serial PN9 sequence in MSB-first format. The first output word is the first 14 bits of the PN9 sequence in MSB aligned form. Table 12. PN Sequence Sequence PN Sequence Short PN Sequence Long Initial Value 0x7F80 0x7FFC Next Three Output Samples (MSB First) Twos Complement 0x77C4, 0xF320, 0xA538 0x7F80, 0x8004, 0x7000 The PN sequence long pattern produces a pseudorandom bit sequence that repeats itself every 223 − 1 or 8,388,607 bits. Refer to Section 5.6 of the ITU-T 0.150 (05/96) standard for a description of the PN sequence and how it is generated. The seed value is all 1s (see Table 12 for the initial values), and the AD9681 inverts the bit stream with relation to the ITU standard. The output is a parallel representation of the serial PN23 sequence in MSB-first format. The first output word is the first 14 bits of the PN23 sequence in MSB aligned format. This pattern allows the user to perform timing alignment adjustments among the FCO±1, FCO±2, DCO±1, DCO±2, and output data. The SCLK/DTP pin has an internal 30 kΩ resistor to GND and can be left unconnected for normal operation. Table 14. Digital Test Pattern Pin Settings Selected Digital Test Pattern Normal Operation DTP DTP Voltage No connect AVDD Resulting D0±xx and D1±xx Normal operation 1000 0000 0000 0000 Additional and custom test patterns can also be observed when commanded from the SPI port. Consult the Memory Map section for information about the options available. CSB1 and CSB2 Pins Tie the CSB1 and CSB2 pins to AVDD for applications that do not require SPI mode operation. Tying CSB1 and CSB2 high causes all SCLK and SDIO SPI communication information to be ignored. CSB1 selects/deselects SPI circuitry affecting the D0±x1/D1±x1 outputs (Bank 1). CSB2 selects/deselects SPI circuitry affecting the D0±x2/D1±x2 (Bank 2) outputs. Consult the Memory Map section for information on how to change these additional digital output timing features through the SPI. It is recommended that CSB1 and CSB2 be controlled with the same signal; that is, tie them together. In this way, whether tying them to AVDD or selecting SPI functionality, both banks of ADCs are controlled identically and are always in the same state. SDIO/OLM Pin RBIAS1 and RBIAS2 Pins For applications that do not require SPI mode operation, the CSB1 and CSB2 pins are tied to AVDD, and the SDIO/OLM pin controls the output lane mode according to Table 13. To set the internal core bias current of the ADC, place a 10.0 kΩ, 1% tolerance resistor to ground at each of the RBIAS1 and RBIAS2 pins. For applications where the SDIO/OLM pin is not used, tie CSB1 and CSB2 to AVDD. When using the one-lane mode, use an encode rate of ≤62.5 MSPS to meet the maximum output rate of 1 Gbps. OUTPUT TEST MODES Table 13. Output Lane Mode Pin Settings Output Lane Mode Voltage (SDIO/OLM Pin) AVDD (Default) GND Output Mode Two-lane. 1× frame, 16-bit serial output. One-lane. 1× frame, 16-bit serial output. SCLK/DTP Pin The SCLK/DTP pin can enable a single digital test pattern if it and the CSB1 and CSB2 pins are held high during device power-up. When SCLK/DTP is tied to AVDD, the ADC channel outputs shift out the following pattern: 1000 0000 0000 0000. The FCO±1, FCO±2, DCO±1, and DCO±2 pins function normally while all channels shift out the repeatable test pattern. The AD9681 includes a built-in test feature designed to enable verification of the integrity of each data output channel, as well as to facilitate board level debugging. Various output test modes are provided to place predictable values on the outputs of the AD9681. The output test modes are described in Table 11 and controlled by the output test mode bits at Address 0x0D. When an output test mode is enabled, the analog section of the ADC is disconnected from the digital back-end blocks and the test pattern is run through the output formatting block. Some of the test patterns are subject to output formatting, and some are not. The PN generators from the PN sequence tests can be reset by setting Bit 4 or Bit 5 of Register 0x0D. These tests can be performed with or without an analog signal (if present, the analog signal is ignored), but they do require an encode clock. For more information, see the AN-877 Application Note, Interfacing to High Speed ADCs via SPI. Rev. A | Page 27 of 40 AD9681 Data Sheet SERIAL PORT INTERFACE (SPI) Other modes involving the CSB1 and CSB2 pins are available. To permanently enable the device, hold CSB1 and CSB2 low indefinitely; this is called streaming. CSB1 and CSB2 can stall high between bytes to allow additional external timing. Tie CSB1 and CSB2 high to place SPI functions in high impedance mode. This mode turns on any SPI secondary pin functions. The AD9681 serial port interface (SPI) allows the user to configure the converter for specific functions or operations through a structured register space provided inside the ADC. The SPI offers the user added flexibility and customization, depending on the application. Addresses are accessed via the serial port and can be written to or read from via the port. Memory is organized into bytes that can be further divided into fields, which are documented in the Memory Map section. For general operational information, see the AN-877 Application Note, Interfacing to High Speed ADCs via SPI. SPI information specific to the AD9681 is found in the AD9681 datasheet and takes precedence over the general information found in the AN-877 Application Note. It is recommended that CSB1 and CSB2 be controlled with the same signal by tying them together. In this way, whether tying them to AVDD or selecting SPI functionality, both banks of ADCs are controlled identically and are always in the same state. CONFIGURATION USING THE SPI During an instruction phase, a 16-bit instruction is transmitted. Data follows the instruction phase, and its length is determined by the W0 and W1 bits. Four pins define the SPI of this ADC: the SCLK/DTP pin (SCLK functionality), the SDIO/OLM pin (SDIO functionality) and the CSB1 and CSB2 pins (see Table 15). SCLK (a serial clock) is used to synchronize the read and write data presented from and to the ADC. SDIO (serial data input/output) serves a dual function, allowing data to be sent to and read from the internal ADC memory map registers. CSB1 and CSB2 (chip select bar) are active low controls that enable or disable the read and write cycles. In addition to word length, the instruction phase determines whether the serial frame is a read or write operation, allowing the serial port to both program the chip and read the contents of the on-chip memory. The first bit of the first byte in a multibyte serial data transfer frame indicates whether a read command or a write command is issued. If the instruction is a readback operation, performing a readback causes the serial data input/output (SDIO) pin to change direction from an input to an output at the appropriate point in the serial frame. Table 15. Serial Port Interface Pins Input data registers on the rising edge of SCLK, and output data transmits on the falling edge. After the address information passes to the converter requesting a read, the SDIO line transitions from an input to an output within 1/2 of a clock cycle. This timing ensures that when the falling edge of the next clock cycle occurs, data can be safely placed on this serial line for the controller to read. Pin SCLK (SCLK/DTP) SDIO (SDIO/OLM) CSB1, CSB2 Function Serial clock. The serial shift clock input, which is used to synchronize serial interface reads and writes. Serial data input/output. A dual-purpose pin that serves as an input or an output, depending on the instruction being sent and the relative position in the timing frame. Chip select bar. An active low control that gates the read and write cycles. CSB1 enables/disables the SPI for four channels in Bank 1; CSB2 enables/ disables the SPI for four channels in Bank 2. All data is composed of 8-bit words. Data can be sent in MSBfirst mode or in LSB-first mode. MSB-first mode is the default on power-up and can be changed via the SPI port configuration register. For more information about this and other features, see the AN-877 Application Note, Interfacing to High Speed ADCs via SPI. The falling edge of CSB1 and/or CSB2, in conjunction with the rising edge of SCLK, determines the start of the framing. For an example of the serial timing and its definitions, see Figure 53 and Table 5. tHIGH tDS tS tDH tCLK tH tLOW CSBx SDIO DON’T CARE DON’T CARE R/W W1 W0 A12 A11 A10 A9 A8 A7 D5 Figure 53. Serial Port Interface Timing Diagram Rev. A | Page 28 of 40 D4 D3 D2 D1 D0 DON’T CARE 11537-078 SCLK DON’T CARE Data Sheet AD9681 HARDWARE INTERFACE CONFIGURATION WITHOUT THE SPI The pins described in Table 15 comprise the physical interface between the user programming device and the serial port of the AD9681. The SCLK/DTP pin (SCLK functionality) and the CSB1 and CSB2 pins function as inputs when using the SPI interface. The SDIO/OLM pin (SDIO functionality) is bidirectional, functioning as an input during write phases and as an output during readback. In applications that do not interface to the SPI control registers, the SDIO/OLM pin, the SCLK/DTP pin, and the PDWN pin serve as standalone CMOS-compatible control pins. When the device is powered up, it is assumed that the user intends to use the pins as static control lines for output lane mode control, digital test pattern control, and power-down feature control. In this mode, connect CSB1 and CSB2 to AVDD, which disables the serial port interface. The SPI interface is flexible enough to be controlled by either FPGAs or microcontrollers. One method for SPI configuration is described in detail in the AN-812 Application Note, Microcontroller-Based Serial Port Interface (SPI) Boot Circuit. The SPI port should not be active during periods when the full dynamic performance of the converter is required. Because the SCLK signal, the CSB1 and CSB2 signals, and the SDIO signal are typically asynchronous to the ADC clock, noise from these signals can degrade converter performance. If the on-board SPI bus is used for other devices, it may be necessary to provide buffers between this bus and the AD9681 to prevent these signals from transitioning at the converter inputs during critical sampling periods. Some pins serve a dual function when the SPI interface is not being used. When the pins are strapped to DRVDD or ground during device power-on, they serve a specific function. Table 13 and Table 14 describe the strappable functions that are supported on the AD9681. When the device is in SPI mode, the PDWN pin (if enabled) remains active. For SPI control of power-down, set the PDWN pin to its inactive state (low). SPI ACCESSIBLE FEATURES Table 16 provides a brief description of the general features that are accessible via the SPI. These features are described further in the AN-877 Application Note, Interfacing to High Speed ADCs via SPI. The AD9681 device-specific features are described in detail in the Memory Map Register Descriptions section following Table 17, the external memory map register table. Table 16. Features Accessible Using the SPI Feature Name Power Mode Clock Offset Test I/O Output Mode Output Phase ADC Resolution Rev. A | Page 29 of 40 Description Allows the user to set either power-down mode or standby mode Allows the user to access the DCS, set the clock divider, set the clock divider phase, and enable the sync function Allows the user to digitally adjust the converter offset Allows the user to set test modes to have known data on output bits Allows the user to set the output mode Allows the user to set the output clock polarity Allows scalable power consumption options with respect to the sample rate AD9681 Data Sheet MEMORY MAP READING THE MEMORY MAP REGISTER TABLE Default Values Each row in the memory map register table has eight bit locations. The memory map is divided into three sections: the chip configuration registers (Address 0x00 to Address 0x02); the device index and transfer registers (Address 0x05 and Address 0xFF); and the global ADC function registers, including setup, control, and test (Address 0x08 to Address 0x109). After the AD9681 is reset (via Bit 5 and Bit 2 of Address 0x00), the registers are loaded with default values. The default values for the registers are listed in the Default Value (Hex) column of Table 17, the memory map register table. The memory map register table (see Table 17) lists the default hexadecimal value for each hexadecimal address shown. The column with the Bit 7 (MSB) heading is the start of the default hexadecimal value given. For example, Address 0x05, the device index register, has a hexadecimal default value of 0x3F. This means that in Address 0x05, Bits[7:6] = 0, and the remaining bits, Bits[5:0], = 1. This setting is the default channel index setting. The default value results in all specified ADC channels receiving the next write command. For more information on this function and others, see the AN-877 Application Note, Interfacing to High Speed ADCs via SPI. This application note details the functions controlled by Register 0x00 to Register 0xFF. The remaining registers are documented in the Memory Map Register Descriptions section. Open Locations All address and bit locations that are not listed in Table 17 are not currently supported for this device. Write the unused bits of a valid address location with 0s. Writing to these locations is required only when some of the bits of an address location are valid (for example, Address 0x05). Do not write to an address location if the entire address location is open or if the address is not listed in Table 17 (for example, Address 0x13). Logic Levels An explanation of logic level terminology follows: “Bit is set” is synonymous with “bit is set to Logic 1” or “writing Logic 1 for the bit.” “Clear a bit” is synonymous with “bit is set to Logic 0” or “writing Logic 0 for the bit.” Channel Specific Registers Some channel setup functions can be programmed independently for each channel. In such cases, channel address locations are internally duplicated for each channel; that is, each channel has its own set of registers. These registers and bits are designated in Table 17 as local. Access these local registers and bits by setting the appropriate data channel bits (A1, A2 through D1, D2) and the clock channel bits (DCO±1, DCO±2 and FCO±1, FCO±2), found in Register 0x05. If all the valid bits are set in Register 0x05, the subsequent write to a local register affects the registers of all the data channels and the DCO±x/FCO±x clock channels. In a read cycle, set only one channel (A1, A2 through D1, D2) to read one local register. If all the bits are set during a SPI read cycle, the device returns the value for Channel A1. Registers and bits that are designated as global in Table 17 are applicable to the channel features for which independent settings are not allowed; thus, they affect the entire device. The settings in Register 0x05 do not affect the global registers and bits. Rev. A | Page 30 of 40 Data Sheet AD9681 MEMORY MAP The AD9681 uses a 3-wire (bidirectional SDIO) interface and 16-bit addressing. Therefore, Bit 0 and Bit 7 in Register 0x00 are set to 0, and Bit 3 and Bit 4 are set to 1. When Bit 5 in Register 0x00 is set high, the SPI enters a soft reset where all of the user registers revert to their default values and Bit 2 is automatically cleared. Table 17. Memory Map Register Table Reg. Addr. Register Name (Hex) Chip Configuration Registers 0x00 SPI port configuration 0x01 Chip ID (global) 0x02 Chip grade (global) Bit 7 (MSB) 0= SDIO active Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB first Soft reset 1= 16-bit address 1= 16-bit address Soft reset LSB first Bit 0 (LSB) 0 = SDIO active 8-bit chip ID, Bits[7:0]; 0x8F = the AD9681, an octal, 14-bit, 125 MSPS serial LVDS Open Speed grade ID, Bits[6:4]; 110 = 125 MSPS Default Value (Hex) 0x18 0x8F Open Open Open Open Read only Comments Nibbles are mirrored such that a given register value yields the same function for either LSB-first mode or MSBfirst mode. The default for ADCs is 16-bit mode. Unique chip ID used to differentiate devices. Read only. Unique speed grade ID used to differentiate graded devices. Read only. Device Index and Transfer Registers 0x05 Device index Open Open DCO±1, DCO±2 clock channels FCO±1, FCO±2 clock channels D1, D2 data channels C1, C2 data channels B1, B2 data channels A1, A2 data channels 0x3F 0xFF Open Open Initiate override 0x00 Internal power-down mode, Bits[1:0]; 00 = chip run 01 = full power-down 10 = standby 11 = digital reset 0x00 Determines various generic modes of chip operation. Open 0x01 Turns duty cycle stabilizer on or off. 0x00 Divide ratio is the value plus 1. Transfer Open Open Open Open Open Global ADC Function Registers 0x08 Power modes (global) Open Open Open Open 0x09 Clock (global) Open Open External powerdown pin function; 0 = full powerdown, 1= standby Open Open Open 0x0B Clock divide (global) Open Open Open Open Open Rev. A | Page 31 of 40 Open Open Duty cycle stabilizer; 0 = off 1 = on Clock divide ratio, Bits[2:0]; 000 = divide by 1 001 = divide by 2 010 = divide by 3 011 = divide by 4 100 = divide by 5 101 = divide by 6 110 = divide by 7 111 = divide by 8 Bits are set to determine which device on chip receives the next write command. The default is all devices on chip. Sets resolution/ sample rate override. AD9681 Reg. Addr. (Hex) 0x0C Register Name Enhancement control Data Sheet Bit 7 (MSB) Open Bit 6 Open Bit 5 Open Bit 4 Open Bit 3 Open Bit 2 Bit 1 Open Chop mode; 0 = off 1 = on Output test mode, Bits[3:0] (local); User input test mode, Reset PN Reset PN Bits[7:6]; long gen short gen 0000 = off (default) 00 = single 0001 = midscale short 01 = alternate 0010 = positive FS 10 = single once 0011 = negative FS 11 = alternate once 0100 = alternating checkerboard (affects user input test 0101 = PN23 sequence mode only; 0110 = PN9 sequence Register 0x0D, 0111 = one-/zero-word toggle Bits[3:0] = 1000) 1000 = user input 1001 = 1-/0-bit toggle 1010 = 1× sync 1011 = one bit high 1100 = mixed bit frequency 8-bit device offset adjustment, Bits[7:0] (local); offset adjust in LSBs from +127 to −128 (twos complement format) Open Open Open Open Open LVDS-ANSI/ Output Output LVDS-IEEE invert; format; option; 0= 0 = not 0 = LVDSoffset inverted ANSI binary 1= 1 = LVDS1 = twos inverted IEEE comple(local) reduced ment range link (default) (global); (global) see Table 18 Open Open Open Open Open Output driver FCO±x, termination, Bits[5:4]; DCO±x output 00 = none drive 01 = 200 Ω (local); 10 = 100 Ω 0 = 1× 11 = 100 Ω drive 1 = 2× drive Open Input clock phase adjust, Bits[6:4]; Output clock phase adjust, Bits[3:0]; (value is number of input clock cycles (0000 to 1011; see Table 20) of phase delay; see Table 19) 0x0D Test mode (local except for PN sequence resets) 0x10 Offset adjust (local) 0x14 Output mode 0x15 Output adjust 0x16 Output phase 0x18 VREF Open Open Open Open Open 0x19 USER_PATT1_LSB (global) USER_PATT1_MSB (global) USER_PATT2_LSB (global) USER_PATT2_MSB (global) B7 B6 B5 B4 B15 B14 B13 B7 B6 B15 B14 0x1A 0x1B 0x1C Bit 0 (LSB) Open Default Value (Hex) 0x00 0x00 When set, test data is placed on the output pins in place of normal data. 0x00 Device offset trim. Configures outputs and format of the data. 0x01 0x00 Determines LVDS or other output properties. 0x03 On devices that use global clock divide, determines which phase of the divider output supplies the output clock. Internal latching is unaffected. Digital adjustment of input full-scale voltage. Does not affect analog voltage reference. B3 Input full-scale adjustment; digital scheme, Bits[2:0]; 000 = 1.0 V p-p 001 = 1.14 V p-p 010 = 1.33 V p-p 011 = 1.6 V p-p 100 = 2.0 V p-p B2 B1 B0 0x00 B12 B11 B10 B9 B8 0x00 B5 B4 B3 B2 B1 B0 0x00 B13 B12 B11 B10 B9 B8 0x00 Rev. A | Page 32 of 40 Comments Enables/ disables chop mode. 0x04 User Defined Pattern 1 LSB. User Defined Pattern 1 MSB. User Defined Pattern 2 LSB. User Defined Pattern 2 MSB. Data Sheet Reg. Addr. (Hex) 0x21 Register Name Serial output data control (global) AD9681 Bit 7 (MSB) LVDS output LSB first Bit 6 Bit 5 Bit 4 SDR/DDR one-lane/two-lane, wordwise/bitwise/bytewise, Bits[6:4]; 000 = SDR two-lane, bitwise 001 = SDR two-lane, bytewise 010 = DDR two-lane, bitwise 011 = DDR two-lane, bytewise 100 = DDR one-lane, wordwise Open Open Open Default Value (Hex) 0x30 Bit 3 PLL low encode rate mode Bit 2 Select 2× frame Open Open Open Sample rate, Bits[2:0]; 000 = 20 MSPS 001 = 40 MSPS 010 = 50 MSPS 011 = 65 MSPS 100 = 80 MSPS 101 = 105 MSPS 110 = 125 MSPS Open Open SDIO pulldown Open Open Open 0x00 Disables SDIO pull-down. 0x00 VCM control. Open 0x00 0x22 Serial channel status (local) Open 0x100 Resolution/sample rate override Open Resolution/ sample rate override enable 0x101 User I/O Control 2 Open Open Open Open Open 0x102 User I/O Control 3 Open Open Open Open 0x109 Sync Open Open Open Open VCM powerdown Open Resolution, Bits[5:4]; 01 = 14 bits 10 = 12 bits Bit 0 Bit 1 (LSB) Serial output number of bits, Bits[1:0]; 00 = 16 bits 10 = 12 bits Rev. A | Page 33 of 40 Channel output reset Sync next only Channel powerdown Enable sync 0x00 0x00 Comments Serial stream control. Default causes MSB first and the native bit stream. Powers down individual sections of a converter. Resolution/ sample rate override (requires transfer register, Register 0xFF). AD9681 Data Sheet MEMORY MAP REGISTER DESCRIPTIONS Output Mode (Register 0x14) For additional information about functions controlled in Register 0x00 to Register 0xFF, see the AN-877 Application Note, Interfacing to High Speed ADCs via SPI. Bit 7—Open Device Index (Register 0x05) There are certain features in the map that can be set independently for each channel, whereas other features apply globally to all channels (depending on context), regardless of which are selected. Bits[3:0] in Register 0x05 select which individual data channels are affected. The output clock channels are selected in Register 0x05 as well. A smaller subset of the independent feature list can be applied to those devices. Transfer (Register 0xFF) All registers except Register 0x100 are updated the moment they are written. Setting Bit 0 = 1 in the transfer register initializes the settings in the ADC resolution/sample rate override register (Address 0x100). Bit 6—LVDS-ANSI/LVDS-IEEE Option Setting Bit 6 = 1 chooses the LVDS-IEEE (reduced range) option. (The default setting is LVDS-ANSI.) As described in Table 18, when either LVDS-ANSI mode or the LVDS-IEEE reduced range link is selected, the user can select the driver termination resistor in Register 0x15, Bits[5:4]. The driver current is automatically selected to give the proper output swing. Table 18. LVDS-ANSI/LVDS-IEEE Options LVDS-ANSI/ LVDS-IEEE Option, Bit 6 0 1 Power Modes (Register 0x08) Output Mode LVDS-ANSI Output Driver Termination User selectable LVDS-IEEE reduced range link User selectable Bits[7:6]—Open Bits[5:3]—Open Bit 5—External Power-Down Pin Function Bit 2—Output Invert When set (Bit 5 = 1), the external PDWN pin initiates standby mode. When cleared (Bit 5 = 0), the external PDWN pin initiates full power-down mode. Setting Bit 2 = 1 inverts the output bit stream. Output Driver Current Automatically selected to give proper swing Automatically selected to give proper swing Bit 1—Open Bit 0—Output Format Bits[4:2]—Open Bits[1:0]—Internal Power-Down Mode In normal operation (Bits[1:0] = 00), all ADC channels are active. In full power-down mode (Bits[1:0] = 01), the digital datapath clocks are disabled and the digital datapath is reset. Outputs are disabled. In standby mode (Bits[1:0] = 10), the digital datapath clocks and the outputs are disabled. During a digital reset (Bits[1:0] = 11), all the digital datapath clocks and the outputs (where applicable) on the chip are reset, except the SPI port. Note that the SPI is always left under control of the user; that is, it is never automatically disabled or in reset (except by power-on reset). Enhancement Control (Register 0x0C) Bits[7:3]—Open Bit 2—Chop Mode For applications that are sensitive to offset voltages and other low frequency noise, such as homodyne or direct conversion receivers, chopping in the first stage of the AD9681 is a feature that can be enabled by setting Bit 2 = 1. In the frequency domain, chopping translates offsets and other low frequency noise to fCLK/2, where they can be filtered. By default, setting Bit 0 = 1 sends the data output in twos complement format. Clearing this bit (Bit 0 = 0) changes the output mode to offset binary. Output Adjust (Register 0x15) Bits[7:6]—Open Bits[5:4]—Output Driver Termination These bits allow the user to select the internal output driver termination resistor. Bits[3:1]—Open Bit 0—FCO±x, DCO±x Output Drive Bit 0 of the output adjust register controls the drive strength on the LVDS driver of the FCO±1, FCO±2, DCO±1, and DCO±2 outputs only. The default value (Bit 0 = 0) sets the drive to 1×. Increase the drive to 2× by setting the appropriate channel bit in Register 0x05 and then setting Bit 0 = 1. These features cannot be used with the output driver termination selected. The termination selection takes precedence over the 2× driver strength on FCO±1, FCO±2, DCO±1, and DCO±2 when both the output driver termination and output drive are selected. Bits[1:0]—Open Rev. A | Page 34 of 40 Data Sheet AD9681 Output Phase (Register 0x16) Serial Output Data Control (Register 0x21) Bit 7—Open The serial output data control register programs the AD9681 in various output data modes, depending on the data capture solution. Table 21 describes the various serialization options available in the AD9681. Bits[6:4]—Input Clock Phase Adjust When the clock divider (Register 0x0B) is used, the applied clock is at a higher frequency than the internal sampling clock. Bits[6:4] determine at which phase of the external clock the sampling occurs. This is applicable only when the clock divider is used. It is prohibited to select a value for Bits[6:4] that is greater than the value of Bits[2:0], Register 0x0B. See Table 19 for more information. Table 19. Input Clock Phase Adjust Options Input Clock Phase Adjust, Bits[6:4] 000 (Default) 001 010 011 100 101 110 111 Number of Input Clock Cycles of Phase Delay 0 1 2 3 4 5 6 7 Resolution/Sample Rate Override (Register 0x100) This register is designed to allow the user to downgrade the device (that is, establish lower power) for applications that do not require full sample rate. Settings in this register are not initialized until Bit 0 of the transfer register (Register 0xFF) is set to 1. This function does not affect the sample rate; it affects the maximum sample rate capability of the ADC, as well as the resolution. User I/O Control 2 (Register 0x101) Bits[7:1]—Open Bit 0—SDIO Pull-Down Set Bit 0 = 1 to disable the internal 30 kΩ pull-down on the SDIO/OLM pin. This feature limits loading when many devices are connected to the SPI bus. User I/O Control 3 (Register 0x102) Bits[3:0]—Output Clock Phase Adjust Bits[7:4]—Open See Table 20 for more information. Bit 3—VCM Power-Down Table 20. Output Clock Phase Adjust Options Set Bit 3 = 1 to power down the internal VCM generator. This feature is used when applying an external reference. Output Clock Phase Adjust, Bits[3:0] 0000 0001 0010 0011 (Default) 0100 0101 0110 0111 1000 1001 1010 1011 DCO Phase Adjustment (Degrees Relative to D0±x/D1±x Edge) 0 60 120 180 240 300 360 420 480 540 600 660 Bits[2:0]—Open Rev. A | Page 35 of 40 AD9681 Data Sheet Table 21. SPI Register Options Serialization Options Selected Register 0x21 Contents 0x30 0x20 0x10 0x00 0x34 0x24 0x14 0x04 0x40 0x32 0x22 0x12 0x02 0x36 0x26 0x16 0x06 0x42 Serial Output Number of Bits (SONB) 16-bit 16-bit 16-bit 16-bit 16-bit 16-bit 16-bit 16-bit 16-bit 12-bit 12-bit 12-bit 12-bit 12-bit 12-bit 12-bit 12-bit 12-bit Frame Mode 1× 1× 1× 1× 2× 2× 2× 2× 1× 1× 1× 1× 1× 2× 2× 2× 2× 1× Serial Data Mode DDR two-lane, bytewise DDR two-lane, bitwise SDR two-lane, bytewise SDR two-lane, bitwise DDR two-lane, bytewise DDR two-lane, bitwise SDR two-lane, bytewise SDR two-lane, bitwise DDR one-lane, wordwise DDR two-lane, bytewise DDR two-lane, bitwise SDR two-lane, bytewise SDR two-lane, bitwise DDR two-lane, bytewise DDR two-lane, bitwise SDR two-lane, bytewise SDR two-lane, bitwise DDR one-lane, wordwise Rev. A | Page 36 of 40 DCO Multiplier 4 × fS 4 × fS 8 × fS 8 × fS 4 × fS 4 × fS 8 × fS 8 × fS 8 × fS 3 × fS 3 × fS 6 × fS 6 × fS 3 × fS 3 × fS 6 × fS 6 × fS 6 × fS Timing Diagram Figure 3 (default setting) Figure 3 Figure 3 Figure 3 Figure 5 Figure 5 Figure 5 Figure 5 Figure 7 Figure 4 Figure 4 Figure 4 Figure 4 Figure 6 Figure 6 Figure 6 Figure 6 Figure 8 Data Sheet AD9681 APPLICATIONS INFORMATION DESIGN GUIDELINES Crosstalk Between Inputs Before starting the design and layout of the AD9681 as a system, it is recommended that the designer become familiar with these guidelines, which describe the special circuit connections and layout requirements that are needed for certain pins. To avoid crosstalk between inputs, consider the following guidelines: POWER AND GROUND RECOMMENDATIONS When connecting power to the AD9681, it is recommended that two separate 1.8 V supplies be used. Use one supply for analog (AVDD); use a separate supply for the digital outputs (DRVDD). For both AVDD and DRVDD, use several different decoupling capacitors for both high and low frequencies. Place these capacitors near the point of entry at the PCB level and near the pins of the device, with minimal trace length. A single PCB ground plane is typically sufficient when using the AD9681. With proper decoupling and smart partitioning of the PCB analog, digital, and clock sections, optimum performance is easily achieved. BOARD LAYOUT CONSIDERATIONS For optimal performance, give special consideration to the AD9681 board layout. The high channel count and small footprint of the AD9681 create a dense configuration that must be managed for matters relating to crosstalk and switching noise. Sources of Coupling Coupling of Digital Output Switching Noise to Analog Inputs and Clock To avoid the coupling of digital output switching noise to the analog inputs and the clock, use the following guidelines: Trace pairs interfere with each other by inductive coupling and capacitive coupling. Use the following guidelines: Inductive coupling is current induced in a trace by a changing magnetic field from an adjacent trace, caused by its changing current flow. Mitigate this effect by making traces orthogonal to each other whenever possible and by increasing the distance between them. Capacitive coupling is charge induced in a trace by the changing electric field of an adjacent trace. This effect can be mitigated by minimizing facing areas, increasing the distance between traces, or changing dielectric properties. Through-vias are particularly good conduits for both types of coupling and must be used carefully. Adjacent trace runs on the same layer may cause unbalanced coupling between channels. Traces on one layer should be separated by a plane (ac ground) from the traces on another layer. Significant coupling occurs through gaps in that plane, such as the setback around through-vias. When routing inputs, sequentially alternate input channels on the top and bottom (or other layer) of the board. Ensure that the top channels have no vias within 5 mm of any other input channel via. For bottom channels, use a via-in-pad to minimize topmetal coupling between channels. Avoid running input traces parallel with each other that are nearer than 2 mm apart. When possible, lay out traces orthogonal to each other and to any other traces that are not dc. Secondhand or indirect coupling may occur through nonrelated dc traces that bridge the distance between two traces or vias. Rev. A | Page 37 of 40 Vias on the outputs are a main conduit of noise to the vias on the inputs. Maintain 5 mm of separation between any output via and any input via. Place the encode clock traces on the top surface. Vias are not recommended in the clock traces. However, if they are required, ensure that there are no clock trace vias within 5 mm of any input via or output via. Place output surface traces (not imbedded between planes) orthogonal to one another as much as possible. Avoid parallel output to input traces within 2 mm. Route digital output traces away from the analog input side of the board. Coupling among outputs is not a critical issue, but separation between these high speed output pairs increases the noise margin of the signals and is good practice. AD9681 Data Sheet CLOCK STABILITY CONSIDERATIONS VCM When powered on, the AD9681 goes into an initialization phase where an internal state machine sets up the biases and the registers for proper operation. During the initialization process, the AD9681 needs a stable clock. If the ADC clock source is not present or not stable during ADC power-up, the state machine is disrupted and the ADC starts up in an unknown state. To correct this, reinvoke an initialization sequence after the ADC clock is stable by issuing a digital reset using Register 0x08. In the default configuration (internal VREF, ac-coupled input) where VREF and VCM are supplied by the ADC itself, a stable clock during power-up is sufficient. When VREF or VCM is supplied by an external source, it, too, must be stable at power-up. Otherwise, a subsequent digital reset, using Register 0x08, is needed. The pseudocode sequence for a digital reset follows: Decouple the VCMx pin to ground with a 0.1 μF capacitor. SPI_Write (0x08, 0x03); # digital reset REFERENCE DECOUPLING Decouple the VREF pin externally to ground with a low ESR, 1.0 μF capacitor in parallel with a low ESR, 0.1 μF ceramic capacitor. SPI PORT Ensure that the SPI port is inactive during periods when the full dynamic performance of the converter is required. Because the SCLK, CSB1, CSB2, and SDIO signals are typically asynchronous to the ADC clock, noise from these signals can degrade converter performance. If the on-board SPI bus is used for other devices, it may be necessary to provide buffers between this bus and the AD9681 to prevent these signals from transitioning at the converter inputs during critical sampling periods. SPI_Write (0x08, 0x00); # normal operation Rev. A | Page 38 of 40 Data Sheet AD9681 OUTLINE DIMENSIONS A1 BALL CORNER 10.10 10.00 SQ 9.90 A1 BALL CORNER 12 11 10 9 8 7 6 5 4 3 2 1 A B C D 8.80 SQ E F G H 0.80 J K L M TOP VIEW 0.60 REF BOTTOM VIEW DETAIL A 1.70 MAX DETAIL A 1.00 MIN 0.32 MIN 0.50 COPLANARITY 0.45 0.12 0.40 BALL DIAMETER COMPLIANT TO JEDEC STANDARDS MO-275-EEAB-1. 11-18-2011-A SEATING PLANE Figure 54. 144-Ball Chip Scale Package Ball Grid Array [CSP_BGA] (BC-144-7) Dimensions shown in millimeters ORDERING GUIDE Model1 AD9681BBCZ-125 AD9681BBCZRL7-125 AD9681-125EBZ 1 Temperature Range −40°C to +85°C −40°C to +85°C Package Description 144-Ball Chip Scale Package Ball Grid Array [CSP_BGA] 144-Ball Chip Scale Package Ball Grid Array [CSP_BGA] Evaluation Board Z = RoHS Compliant Part. Rev. A | Page 39 of 40 Package Option BC-144-7 BC-144-7 AD9681 Data Sheet NOTES ©2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D11537-0-12/13(A) Rev. A | Page 40 of 40