24-Bit, 8.5 mW, 109 dB, 128 kSPS/64 kSPS/32 kSPS ADCs AD7767 FEATURES GENERAL DESCRIPTION Oversampled successive approximation (SAR) architecture High performance ac and dc accuracy, low power 115.5 dB dynamic range, 32 kSPS (AD7767-2) 112.5 dB dynamic range, 64 kSPS (AD7767-1) 109.5 dB dynamic range, 128 kSPS (AD7767) −118 dB THD Exceptionally low power 8.5 mW, 32 kSPS (AD7767-2) 10.5 mW, 64 kSPS (AD7767-1) 15 mW, 128 kSPS (AD7767) High dc accuracy 24 bits, no missing codes (NMC) INL: ±3 ppm (typical), ±7.6 ppm (maximum) Low temperature drift Zero error drift: 15 nV/°C Gain error drift: 0.4 ppm/°C On-chip low-pass FIR filter Linear phase response Pass-band ripple: ±0.005 dB Stop-band attenuation: 100 dB 2.5 V supply with 1.8 V/2.5 V/3 V/3.6 V logic interface options Flexible interfacing options Synchronization of multiple devices Daisy-chain capability Power-down function Temperature range: −40°C to +105°C The AD7767/AD7767-1/AD7767-2 are high performance, 24-bit, oversampled SAR analog-to-digital converters (ADCs). The AD7767/AD7767-1/AD7767-2 combine the benefits of a large dynamic range and input bandwidth, consuming 15 mW, 10.5 mW, and 8.5 mW power, respectively, and are contained in a 16-lead TSSOP package. Ideal for ultralow power data acquisition (such as PCI- and USB-based systems), the AD7767/AD7767-1/AD7767-2 provide 24-bit resolution. The combination of exceptional SNR, wide dynamic range, and outstanding dc accuracy make the AD7767/AD7767-1/AD7767-2 ideally suited for measuring small signal changes over a wide dynamic range. This is particularly suitable for applications where small changes on the input are measured on larger ac or dc signals. In such an application, the AD7767/AD7767-1/AD7767-2 accurately gather both ac and dc information. The AD7767/AD7767-1/AD7767-2 include an on-board digital filter (complete with linear phase response) that acts to eliminate out-of-band noise by filtering the oversampled input voltage. The oversampled architecture also reduces front-end antialias requirements. Other features of the AD7767 include a SYNC/PD (synchronization/power-down) pin, allowing the synchronization of multiple AD7767 devices. The addition of an SDI pin provides the option of daisy chaining multiple AD7767 devices. The AD7767/AD7767-1/AD7767-2 operate from a 2.5 V supply using a 5 V reference. The devices operate from −40°C to +105°C. APPLICATIONS Low power PCI/USB data acquisition systems Low power wireless acquisition systems Vibration analysis Instrumentation High precision medical acquisition RELATED DEVICES Table 1. 24-Bit ADCs Part No. AD7760 FUNCTIONAL BLOCK DIAGRAM AVDD AGND MCLK AD7762/ AD7763 DVDD VDRIVE DGND VREF+ AD7764 VIN+ DIGITAL FIR FILTER SUCCESSIVE APPROXIMATION ADC AD7765 SYNC/PD REFGND AD7767/ AD7767-1/ AD7767-2 SERIAL INTERFACE AND CONTROL LOGIC SCLK DRDY SDO SDI CS 06859-001 VIN– AD7766 AD7766-1 AD7766-2 1 Description 2.5 MSPS, 100 dB dynamic range, 1 on-board differential amp and reference buffer, parallel, variable decimation 625 kSPS, 109 dB dynamic range,1 on-board differential amp and reference buffer, parallel/serial, variable decimation 312 kSPS, 109 dB dynamic range,1 on-board differential amp and reference buffer, variable decimation (pin) 156 kSPS, 112 dB dynamic range,1 on-board differential amp and reference buffer, variable decimation (pin) 128 kSPS, 109.5 dB,1 15 mW, 16-bit INL, serial interface 64 kSPS 112.5 dB,110.5 mW, 16-bit INL, serial interface 32 kSPS, 115.5 dB,1 8.5 mW, 16-bit INL, serial interface Dynamic range at maximum output data rate. Figure 1. Rev. C 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 www.analog.com Fax: 781.461.3113 ©2007–2010 Analog Devices, Inc. All rights reserved. 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AD7767 TABLE OF CONTENTS Features .............................................................................................. 1 Supply and Reference Voltages ................................................. 16 Applications ....................................................................................... 1 AD7767 Interface ........................................................................... 17 Functional Block Diagram .............................................................. 1 Initial Power-Up ......................................................................... 17 General Description ......................................................................... 1 Reading Data............................................................................... 17 Related Devices ................................................................................. 1 Power-Down, Reset, and Synchronization ............................. 17 Revision History ............................................................................... 2 Daisy Chaining ............................................................................... 18 Specifications..................................................................................... 3 Reading Data in Daisy-Chain Mode ....................................... 18 Timing Specifications .................................................................. 5 Choosing the SCLK Frequency ................................................ 18 Timing Diagrams.......................................................................... 6 Daisy-Chain Mode Configuration and Timing Diagrams ... 19 Absolute Maximum Ratings............................................................ 8 Driving the AD7767 ....................................................................... 20 ESD Caution .................................................................................. 8 Differential Signal Source ......................................................... 20 Pin Configuration and Function Descriptions ............................. 9 Single-Ended Signal Source ...................................................... 20 Typical Performance Characteristics ........................................... 10 Antialiasing ................................................................................. 21 Terminology .................................................................................... 14 Power Dissipation....................................................................... 21 Theory of Operation ...................................................................... 15 VREF+ Input Signal ....................................................................... 22 AD7767/AD7767-1/AD7767-2 Transfer Function ................ 15 Multiplexing Analog Input Channels ...................................... 22 Converter Operation .................................................................. 15 Outline Dimensions ....................................................................... 23 Analog Input Structure .............................................................. 16 Ordering Guide .......................................................................... 23 REVISION HISTORY 5/10—Rev. B to Rev. C Changes to Pin 8 Description ......................................................... 9 Changes to Table 8 .......................................................................... 20 3/09—Rev. A to Rev. B Changes to tSETTLING Parameter, Table 3 .......................................... 5 Changes to Table 7 .......................................................................... 17 1/09—Rev. 0 to Rev. A Changes to Features Section............................................................ 1 Change to Intermodulation Distortion (IMD) Parameter and Integral Nonlinearity Parameter, Table 2 ................................. 3 Changes to Supply and Reference Voltages Section ................... 16 Changes to Choosing the SCLK Frequency Section .................. 18 Changes to Figure 24 ...................................................................... 12 Changes to Driving the AD7767 Section .................................... 20 Changes to Single-Ended Signal Source Section ........................ 20 Added Figure 41; Renumbered Sequentially .............................. 20 Change to Figure 42 ....................................................................... 21 Added Table 8; Renumbered Sequentially .................................. 20 Replaced VREF+ Input Signal Section ............................................ 22 Replaced Figure 46 ......................................................................... 22 8/07—Revision 0: Initial Version Rev. C | Page 2 of 24 AD7767 SPECIFICATIONS AVDD = DVDD = 2.5 V ± 5%, VDRIVE = 1.8 V to 3.6 V, VREF = 5 V, MCLK = 1 MHz, common-mode input = VREF/2, TA = −40°C to +105°C, unless otherwise noted. Table 2. Parameter OUTPUT DATA RATE (ODR) AD7767 AD7767-1 AD7767-2 ANALOG INPUT 1 Differential Input Voltage Absolute Input Voltage Test Conditions/Comments Typ Decimate by 8 Decimate by 16 Decimate by 32 VIN+ − VIN− VIN+ −0.1 VIN− Common-Mode Input Voltage Input Capacitance DYNAMIC PERFORMANCE AD7767 Dynamic Range 2 Signal-to-Noise Ratio (SNR)2 Spurious-Free Dynamic Range (SFDR)2 Total Harmonic Distortion (THD)2 Intermodulation Distortion (IMD)2 Second-Order Terms Third-Order Terms AD7767-1 Dynamic Range2 Signal-to-Noise Ratio (SNR)2 Spurious-Free Dynamic Range (SFDR)2 Total Harmonic Distortion (THD)2 Intermodulation Distortion (IMD)2 Second-Order Terms Third-Order Terms AD7767-2 Dynamic Range2 Signal-to-Noise Ratio (SNR)2 Spurious-Free Dynamic Range (SFDR)2 Total Harmonic Distortion (THD)2 Intermodulation Distortion (IMD)2 Second-Order Terms Third-Order Terms DC ACCURACY1 Resolution Differential Nonlinearity2 Integral Nonlinearity2 Zero Error2 Gain Error2 Zero Error Drift2 Gain Error Drift2 Common-Mode Rejection Ratio2 Min −0.1 VREF/2 − 5% Decimate by 8, ODR = 128 kHz Shorted inputs Full-scale input amplitude, 1 kHz tone Full-scale input amplitude, 1 kHz tone Full-scale input amplitude, 1 kHz tone Tone A = 49.7 kHz, Tone B = 50.3 kHz Decimate by 16, ODR = 64 kHz Shorted inputs Full-scale input amplitude, 1 kHz tone Full-scale input amplitude, 1 kHz tone Full-scale input amplitude, 1 kHz tone Tone A = 24.7 kHz, Tone B = 25.3 kHz 108 107 111 110 VREF/2 22 109.5 108.5 −128 −118 Max Unit 128 64 32 kHz kHz kHz ±VREF +VREF + 0.1 V p-p V +VREF + 0.1 VREF/2 + 5% V V pF −116 −105 −133 −109 dB dB 112.5 111.5 −128 −118 dB dB dB dB dB dB dB −116 −105 −133 −108 Decimate by 32, ODR = 32 kHz Shorted inputs Full-scale input amplitude, 1 kHz tone Full-scale input amplitude, 1 kHz tone Full-scale input amplitude, 1 kHz tone Tone A = 11.7 kHz, Tone B = 12.3 kHz 114 112 115.5 113.5 −128 −118 −116 −105 −137 −108 For all devices No missing codes Guaranteed monotonic to 24 bits 18-bit linearity 50 Hz tone Rev. C | Page 3 of 24 dB dB dB dB 24 dB dB dB dB dB dB dB Bits ±3 20 0.0075 15 0.4 −110 ±7.6 0.075 ppm μV % FS nV/°C ppm/°C dB AD7767 Parameter DIGITAL FILTER RESPONSE1 Group Delay Settling Time (Latency) Pass-Band Ripple Pass Band −3 dB Bandwidth Stop-Band Frequency Stop-Band Attenuation REFERENCE INPUT1 VREF+ Input Voltage DIGITAL INPUTS (Logic Levels)1 VIL VIH Input Leakage Current Input Capacitance Master Clock Rate Serial Clock Rate DIGITAL OUTPUTS1 Data Format VOL VOH POWER REQUIREMENTS1 AVDD DVDD VDRIVE CURRENT SPECIFICATIONS AD7767 Operational Current AIDD DIDD IREF AD7767-1 Operational Current AIDD DIDD IREF AD7767-2 Operational Current AIDD DIDD IREF Static Current with MCLK Stopped AIDD DIDD Power-Down Mode Current AIDD DIDD POWER DISSIPATION AD7767 Operational Power AD7767-1 Operational Power AD7767-2 Operational Power 1 2 Test Conditions/Comments Min Typ Max 37/ODR 74/ODR Complete settling ±0.005 0.453 × ODR 0.49 × ODR 0.547 × ODR 100 μs μs dB Hz Hz Hz dB 2.4 2 × AVDD V −0.3 0.7 × VDRIVE +0.3 × VDRIVE VDRIVE + 0.3 ±1 V V μA/pin pF MHz Hz 5 1.024 1/t8 Serial 24 bits, twos complement (MSB first) ISINK = +500 μA ISOURCE = −500 μA Unit 0.4 VDRIVE – 0.3 ± 5% ± 5% 1.7 V V 2.5 2.5 2.5 3.6 V V V 1.3 3.9 0.35 1.5 4.8 0.425 mA mA mA 1.3 2.2 0.35 1.5 2.85 0.425 mA mA mA 1.3 1.37 0.35 1.5 1.86 0.425 mA mA mA 0.9 1 1 93 mA μA 0.1 1 6 93 μA μA 15 10.5 8.5 18 13 10.5 mW mW mW MCLK = 1.024 MHz 128 kHz output data rate 64 kHz output data rate 32 kHz output data rate For all devices For all devices MCLK = 1.024 MHz 128 kHz output data rate 64 kHz output data rate 32 kHz output data rate Specifications are for all devices, AD7767, AD7767-1, and AD7767-2. See the Terminology section. Rev. C | Page 4 of 24 AD7767 TIMING SPECIFICATIONS AVDD = DVDD = 2.5 V ± 5%, VDRIVE = 1.7 V to 3.6 V, VREF = 5 V, common-mode input = VREF/2, TA = −40°C (TMIN) to +105°C (TMAX), unless otherwise noted. 1 Table 3. Parameter DRDY OPERATION t1 t2 2 t32 t4 Limit at tMIN, tMAX Unit Description tREAD 3 510 100 900 265 128 71 294 435 492 tDRDY − t5 ns typ ns min ns max ns typ ns typ ns typ ns typ ns typ ns typ ns typ MCLK rising edge to DRDY falling edge MCLK high pulse width MCLK low pulse width MCLK rising edge to DRDY rising edge (AD7767) MCLK rising edge to DRDY rising edge (AD7767-1) MCLK rising edge to DRDY rising edge (AD7767-2) DRDY pulse width (AD7767) DRDY pulse width (AD7767-1) DRDY pulse width (AD7767-2) DRDY low period, read data during this period tDRDY3 n × 8 × tMCLK ns typ DRDY period 0 6 60 50 25 24 10 10 10 1/t8 6 0 ns min ns max ns max ns max ns max ns max ns min ns min ns min sec min ns max ns min DRDY falling edge to CS setup time CS falling edge to SDO tristate disabled Data access time after SCLK falling edge (VDRIVE = 1.7 V) Data access time after SCLK falling edge (VDRIVE = 2.3 V) Data access time after SCLK falling edge (VDRIVE = 2.7 V) Data access time after SCLK falling edge (VDRIVE = 3.0 V) SCLK falling edge to data valid hold time (VDRIVE = 3.6 V) SCLK high pulse width SCLK low pulse width Minimum SCLK period Bus relinquish time after CS rising edge CS rising edge to DRDY rising edge 0 0 ns min ns max DRDY falling edge to data valid setup time DRDY rising edge to data valid hold time 1 2 ns min ns max SDI valid to SCLK falling edge setup time SCLK falling edge to SDI valid hold time 1 20 1 510 (592 × n) + 2 ns typ ns typ ns min ns typ tMCLK SYNC/PD falling edge to MCLK rising edge MCLK rising edge to DRDY rising edge going into SYNC/PD mode SYNC/PD rising edge to MCLK rising edge MCLK rising edge to DRDY falling edge coming out of SYNC/PD mode Filter settling time after a reset or power-down t5 Read OPERATION t6 t7 t8 t9 t10 t11 tSCLK t12 t13 Read OPERATION WITH CS LOW t14 t15 DAISY-CHAIN OPERATION t16 t17 SYNC/PD OPERATION t18 t19 t20 t21 tSETTLING3 1 Sample tested during initial release to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of DVDD) and timed from a voltage level of 1.7 V. t2 and t3 allow a ~90% to 10% duty cycle to be used for the MCLK input, where the minimum is 10% for the clock high time and 90% for MCLK low time. The maximum MCLK frequency is 1.024 MHz. 3 n = 1 for AD7767, n = 2 for the AD7767-1, n = 4 for the AD7767-2. 2 Rev. C | Page 5 of 24 AD7767 TIMING DIAGRAMS t2 8×n 1 MCLK t3 1 8×n t4 t5 t1 t5 06859-002 tREAD DRDY tDRDY Figure 2. DRDY vs. MCLK Timing Diagram, n = 1 for AD7767 (Decimate by 8), n = 2 for AD7767-1 (Decimate by 16), n = 4 for AD7767-2 (Decimate by 32) tDRDY tREAD DRDY t13 t6 CS t10 1 t8 t7 SDO 23 t11 t9 t12 MSB D22 D21 D20 D1 06859-003 SCLK LSB Figure 3. Serial Timing Diagram, Reading Data Using CS CS = 0 tDRDY tREAD DRDY t14 t10 1 SCLK 23 t8 DATA INVALID MSB D22 t11 t9 D21 D20 t15 D1 LSB DATA INVALID 06859-004 SDO 24 Figure 4. Serial Timing Diagram, Reading Data Setting CS Logic Low Rev. C | Page 6 of 24 AD7767 PART OUT OF POWER-DOWN FILTER RESET BEGINS SAMPLING PART IN POWER-DOWN MCLK (I) A B t18 C D t20 SYNC/PD (I) t21 t19 DOUT (O) VALID DATA INVALID DATA VALID DATA Figure 5. Reset, Synchronization, and Power-Down Timing (For More Information, See the Power-Down, Reset, and Synchronization Section) Rev. C | Page 7 of 24 06859-005 tSETTLING DRDY (O) AD7767 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 4. Parameter AVDD to AGND DVDD to DGND AVDD to DVDD VREF+ to REFGND REFGND to AGND VDRIVE to DGND VIN+, VIN– to AGND Digital Inputs to DGND Digital Outputs to DGND AGND to DGND Input Current to Any Pin Except Supplies 1 Operating Temperature Range Storage Temperature Range Junction Temperature TSSOP Package θJA Thermal Impedance θJC Thermal Impedance Lead Temperature, Soldering Vapor Phase (60 sec) Infrared (15 sec) ESD 1 Rating −0.3 V to +3 V −0.3 V to +3 V −0.3 V to +0.3 V −0.3 V to +7 V −0.3 V to +0.3 V −0.3 V to +6 V −0.3 V to VREF+ + 0.3 V −0.3 V to VDRIVE + 0.3 V −0.3 V to VDRIVE + 0.3 V −0.3 V to +0.3 V ±10 mA 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. ESD CAUTION −40°C to +105°C −65°C to +150°C 150°C 150.4°C/W 27.6°C/W 215°C 220°C 1 kV Transient currents of up to 100 mA do not cause SCR latch-up. Rev. C | Page 8 of 24 AD7767 AVDD 1 16 CS 2 15 SDI 14 MCLK 13 SCLK 12 DRDY AGND 6 11 DGND SYNC/PD 7 10 SDO DVDD 8 9 VDRIVE VREF+ REFGND 3 VIN+ 4 VIN– 5 AD7767/ AD767-1/ AD7767-2 TOP VIEW (Not to Scale) 06859-006 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS Figure 6. 16-Lead TSSOP Pin Configuration Table 5. Pin Function Descriptions Pin No. 1 2 Mnemonic AVDD VREF+ 3 REFGND 4 5 6 7 VIN+ VIN− AGND SYNC/PD 8 DVDD 9 VDRIVE 10 SDO 11 12 DGND DRDY 13 SCLK 14 15 16 MCLK SDI CS Description +2.5 V Analog Power Supply. Reference Input for the AD7767. An external reference must be applied to this input pin. The VREF+ input can range from 2.4 V to 5 V. The reference voltage input is independent of the voltage magnitude applied to the AVDD pin. Reference Ground. Ground connection for the reference voltage. The input reference voltage (VREF+) should be decoupled to this pin. Positive Input of the Differential Analog Input. Negative Input of the Differential Analog Input. Power Supply Ground for Analog Circuitry. Synchronization and Power-Down Input Pin. This pin has dual functionality. It can be used to synchronize multiple AD7767 devices and/or to put the AD7767 device into power-down mode. See the Power-Down, Reset, and Synchronization section for further details. 2.5 V Digital Power Supply Input. In cases where an logic voltage of 2.5 V is used for interfacing (2.5 V applied to VDRIVE pin), the DVDD and VDRIVE pin may be connected to the same voltage supply rail. Logic Power Supply Input, 1.8 V to 3.6 V. The voltage supplied at this pin determines the operating voltage of the digital logic interface. Serial Data Output. The conversion result from the AD7767 is output on the SDO pin as a 24-bit, twos complement, MSB first, serial data stream. Digital Logic Power Supply Ground. Data Ready Output. A falling edge on the DRDY signal indicates that a new conversion data result is available in the output register of the AD7767. See the AD7767 Interface section for further details. Serial Clock Input. The SCLK input provides the serial clock for all serial data transfers with the AD7767 device. See the AD7767 Interface section for further details. Master Clock Input. The AD7767 sampling frequency is equal to the MCLK frequency. Serial Data Input. This is the daisy-chain input of the AD7767. See the Daisy Chaining section for further details. Chip Select Input. The CS input selects the AD7767 device and acts as an enable on the SDO pin. In cases where CS is used, the MSB of the conversion result is clocked onto the SDO line on the CS falling edge. The CS input allows multiple AD7767 devices to share the same SDO line. This allows the user to select the appropriate device by supplying it with a logic low CS signal, which enables the SDO pin of the device concerned. See the AD7767 Interface section for further details. Rev. C | Page 9 of 24 AD7767 TYPICAL PERFORMANCE CHARACTERISTICS 0 –20 –20 –40 –40 –60 –60 –80 –100 –120 –80 –100 –120 –140 –140 –160 –160 –180 0 8k 16k 24k 32k 40k 48k 56k 64k FREQUENCY (Hz) –180 0 24k 32k 40k 48k 56k 64k Figure 10. AD7767 FFT, 1 kHz, −6 dB Input Tone 0 0 –20 –20 –40 AMPLITUDE (dB) –40 –60 –80 –100 –120 –60 –80 –100 –120 –140 –140 –180 0 4k 8k 12k 16k 20k 24k 28k 32k FREQUENCY (Hz) 0 06859-102 –180 –20 –20 –40 –40 –60 –60 AMPLITUDE (dB) 0 –80 –100 –120 24k 28k 32k –120 –160 –180 16k 06859-103 –160 FREQUENCY (Hz) 20k –100 –140 12k 16k –80 –140 8k 12k Figure 11. AD7767-1 FFT, 1 kHz, −6 dB Input Tone 0 4k 8k FREQUENCY (Hz) Figure 8. AD7767-1 FFT, 1 kHz, −0.5 dB Input Tone 0 4k 06859-105 –160 –160 –180 0 4k 8k 12k FREQUENCY (Hz) Figure 12. AD7767-2 FFT, 1 kHz, −6 dB Input Tone Figure 9. AD7767-2 FFT, 1 kHz, −0.5 dB Input Tone Rev. C | Page 10 of 24 16k 06859-106 AMPLITUDE (dB) 16k FREQUENCY (Hz) Figure 7. AD7767 FFT, 1 kHz, −0.5 dB Input Tone AMPLITUDE (dB) 8k 06859-104 AMPLITUDE (dB) 0 06859-101 AMPLITUDE (dB) AVDD = DVDD = 2.5 V ± 5%, VDRIVE = 1.8 V to 3.6 V, VREF = 5 V, MCLK = 1 MHz, common-mode input = VREF/2. TA = 25°C, unless otherwise noted. All FFTs were generated using 8192 samples using a four-term Blackman-Harris window. 0 –20 –20 –40 –40 –60 –60 –80 –100 –120 –80 –100 –120 –140 –140 –160 –160 –180 8k 16k 24k 32k 40k 48k 56k 64k FREQUENCY (Hz) –180 0 –20 –40 –40 –60 –60 AMPLITUDE (dB) –20 –80 –100 –120 –160 –180 20k 24k 28k 32k FREQUENCY (Hz) 56k 64k TONE A = 24.7kHz TONE B = 25.3kHz SECOND-ORDER IMD = –133.33dB THIRD-ORDER IMD = –108.15dB –180 0 4k 8k 12k 16k 20k 24k 28k 32k FREQUENCY (Hz) Figure 14. AD7767-1 FFT, 1 kHz, −60 dB Input Tone Figure 17. AD7767-1 IMD FFT, 25 kHz Center Frequency 0 –40 TONE A = 11.7kHz TONE B = 12.3kHz –20 SECOND-ORDER IMD = –137.96dB THIRD-ORDER IMD = –108.1dB –40 –60 –60 AMPLITUDE (dB) –20 –80 –100 –120 –80 –100 –120 –140 –160 –160 –180 0 4k 8k 12k FREQUENCY (Hz) 16k 06859-109 –140 Figure 15. AD7767-2 FFT, 1 kHz, −60 dB Input Tone –180 0 4k 8k 12k FREQUENCY (Hz) Figure 18. AD7767-2 IMD FFT, 12 kHz Center Frequency Rev. C | Page 11 of 24 16k 06859-112 0 AMPLITUDE (dB) 48k –120 –160 16k 40k –100 –140 12k 32k –80 –140 06859-108 AMPLITUDE (dB) 0 8k 24k Figure 16. AD7767 IMD FFT, 50 kHz Center Frequency 0 4k 16k FREQUENCY (Hz) Figure 13. AD7767 FFT, 1 kHz, −60 dB Input Tone 0 8k 06859-111 0 TONE A = 49.7kHz TONE B = 50.3kHz SECOND-ORDER IMD = –133.71dB THIRD-ORDER IMD = –109.05dB 06859-110 AMPLITUDE (dB) 0 06859-107 AMPLITUDE (dB) AD7767 AD7767 –110 120 –112 115 –114 DYNAMIC RANGE –116 110 –118 CMRR (dB) –120 –122 AD7767-1 AD7767 105 OPEN INPUTS 100 –124 –126 95 FULL-SCALE 921Hz –128 0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1M MCLK FREQUENCY (Hz) 90 06859-113 –130 0 10k 20k 30k 40k 50k 60k fNOISE (Hz) 06859-116 THD (dB) AD7767-2 Figure 22. AD7767 CMRR vs. Common-Mode Ripple Frequency (fNOISE) Figure 19. AD7767/AD7767-1/AD7767-2 THD vs. MCLK Frequency 115 200 114 180 MAX = 8388637 MIN = 8388493 SPREAD = 145 AD7767-2 160 113 140 111 OCCURRENCE SNR (dB) 112 AD7767-1 110 120 100 80 60 109 AD7767 40 108 20 1M MCLK FREQUENCY (Hz) 0 CODES Figure 20. AD7767/AD7767-1/AD7767-2 SNR vs. MCLK Frequency Figure 23. AD7767 24-Bit Histogram 150 250 AVDD DVDD MAX = 8388608 MIN = 8388507 SPREAD = 102 CODES 140 200 OCCURRENCE 130 VDRIVE 120 110 150 100 0 10k 20k 30k fNOISE (Hz) 40k 50k 60k 0 CODES Figure 21. AD7767 Power Supply Sensitivity vs. Supply Ripple Frequency (fNOISE) with Decoupling Capacitors Rev. C | Page 12 of 24 Figure 24. AD7767-1 24-Bit Histogram 06859-119 100 06859-117 50 8388508 8388512 8388516 8388520 8388524 8388528 8388532 8388536 8388540 8388544 8388548 8388552 8388556 8388560 8388564 8388568 8388572 8388576 8388580 8388584 8388588 8388592 8388596 8388600 8388604 8388608 PSRR (dB) 06859-118 100k 200k 300k 400k 500k 600k 700k 800k 900k 8388493 8388497 8388501 8388505 8388509 8388513 8388517 8388521 8388525 8388529 8388533 8388537 8388541 8388545 8388549 8388553 8388557 8388561 8388565 8388569 8388573 8388577 8388581 8388585 8388589 8388593 8388597 8388601 8388605 8388609 8388613 8388617 8388621 8388625 8388629 8388633 8388637 0 06859-114 107 AD7767 LOW TEMPERATURE NOMINAL TEMPERATURE HIGH TEMPERATURE 3.04 300 2.28 250 1.52 INL (ppm) OCCURRENCE 3.80 MAX = 8388593 MIN = 8388526 SPREAD = 69 CODES 350 200 150 0.76 0 –0.76 –1.52 100 –2.28 50 –3.04 0 Figure 25. AD7767-2 24-Bit Histogram Figure 27. AD7767/AD7767-1/AD7767-2 24-Bit INL 1.0 0.8 0.6 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 4194304 8388608 12582912 16777216 2097152 6291456 10485760 14680064 24-BIT CODES 06859-121 DNL (LSBs) 0.4 0 4194304 8388608 12582912 16777216 2097152 6291456 10485760 14680064 24-BIT CODES Figure 26. AD7767/AD7767-1/AD7767-2 24-Bit DNL Rev. C | Page 13 of 24 06859-122 8388592 –3.80 06859-120 CODES 8388589 8388586 8388583 8388580 8388577 8388574 8388571 8388568 8388565 8388562 8388559 8388556 8388553 8388550 8388547 8388544 8388541 8388538 8388535 8388532 8388529 8388526 0 AD7767 TERMINOLOGY Signal-to-Noise Ratio (SNR) SNR is the ratio of the actual input signal’s rms value to the rms sum of all other spectral components below the Nyquist frequency, excluding harmonics and dc. The value for SNR is expressed in decibels. Total Harmonic Distortion (THD) THD is the ratio of the rms sum of harmonics to the fundamental. For the AD7767, it is defined as THD (dB ) = 20 log V 22 + V32 + V 42 + V52 + V62 V1 where: V1 is the rms amplitude of the fundamental. V2, V3, V4, V5, and V6 are the rms amplitudes of the second to the sixth harmonics. Nonharmonic Spurious-Free Dynamic Range (SFDR) SFDR is the ratio of the rms signal amplitude to the rms value of the peak spurious spectral component, excluding harmonics. Dynamic Range Dynamic range is the ratio of the rms value of the full scale to the rms noise measured with the inputs shorted together. The value for the dynamic range is expressed in decibels. Intermodulation Distortion (IMD) With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products at sum and difference frequencies of mfa ± nfb, where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are those for which neither m nor n are equal to 0. For example, the second-order terms include (fa + fb) and (fa − fb), and the third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). The AD7767 is tested using the CCIF standard, where two input frequencies near the top end of the input bandwidth are used. In this case, the second-order terms are usually distanced in frequency from the original sine waves, and the third-order terms are usually at a frequency close to the input frequencies. As a result, the second- and third-order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification, where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in decibels. Integral Nonlinearity (INL) INL is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Differential Nonlinearity (DNL) DNL is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Zero Error Zero error is the difference between the ideal midscale input voltage (when both inputs are shorted together) and the actual voltage producing the midscale output code. Zero Error Drift Zero error drift is the change in the actual zero error value due to a temperature change of 1°C. It is expressed as a percentage of full scale at room temperature. Gain Error The first transition (from 100 … 000 to 100 … 001) should occur ½ LSB above the nominal negative full scale for an analog voltage. The last transition (from 011 … 110 to 011 … 111) should occur 1½ LSB below the nominal full scale for an analog voltage. The gain error is the deviation of the difference between the actual level of the last transition and the actual level of the first transition, from the difference between the ideal levels. Gain Error Drift Gain error drift is the change in the actual gain error value due to a temperature change of 1°C. It is expressed as a percentage of full scale at room temperature. Common-Mode Rejection Ratio (CMRR) CMRR is defined as the ratio of the power in the ADC output at full-scale frequency f to the power of a 100 mV sine wave applied to the common-mode voltage of the VIN+ and VIN− inputs at frequency fS. CMRR (dB) = 10 log(Pf/PfS) where Pf is the power at the frequency f in the ADC output, and PfS is the power at the frequency fS in the ADC output. Rev. C | Page 14 of 24 AD7767 THEORY OF OPERATION The AD7767/AD7767-1/AD7767-2 operate using a fully differential analog input applied to a successive approximation (SAR) core. The output of the oversampled SAR is filtered using a linear-phase digital FIR filter. The fully filtered data is output in a serial format, with the MSB being clocked out first. The digital filtering that follows the converter output acts to remove the out-of-band quantization noise (see Figure 30). This also has the effect of reducing the data rate from fMCLK at the input of the filter to fMCLK/8, fMCLK/16, or fMCLK/32 at the digital output, depending on which model of the device is being used. AD7767/AD7767-1/AD7767-2 TRANSFER FUNCTION The digital filter consists of three separate filter blocks. Figure 31 shows the three constituent blocks of the filter. The order of decimation of the first filter block is set as 2, 4, or 8. The remaining sections each operate with a decimation of 2. The conversion results of the AD7767/AD7767-1/AD7767-2 are output in a twos complement, 24-bit serial format. The fully differential inputs VIN+ and VIN− are scaled by the AD7767/ AD7767-1/AD7767-2 relative to the reference voltage input (VREF+) as shown in Figure 28. DIGITAL FILTER STAGE 2 STAGE 3 SINC FILTER FIR FILTER FIR FILTER DEC × (2 × n) DEC × 2 DEC × 2 SDO 24-BIT OUTPUT 011 ... 111 06859-019 DATA STREAM 24 BITS TWOS COMPLEMENT STAGE 1 011 ... 110 Figure 31. FIR Filter Stages (n = 1 for AD7767, n = 2 for AD7767-1, n = 4 for AD7767-2) 000 ... 010 Table 6 shows the three available models of the AD7767, listing the change in output data rate relative to the order of decimation rate implemented. This brings into focus the trade-off that exists between extra filtering and reduction in bandwidth, whereby using a filter option with a larger decimation rate increases the noise performance while decreasing the usable input bandwidth. 000 ... 001 000 ... 000 111 ... 111 111 ... 110 100 ... 001 Table 6. AD7767 Models 100 ... 000 VIN– = VREF – 1LSB VREF VIN+ = 2 VREF VIN– = 2 VIN+ = VREF – 1LSB VIN– = 0V 06859-012 VIN+ = 0V Figure 28. AD7767/AD7767-1/AD7767-2 Transfer Function CONVERTER OPERATION Internally, the input waveform applied to the SAR core is converted and an equivalent digital word is output to the digital filter at a rate equal to MCLK. By employing oversampling, the quantization noise of the converter is spread across a wide bandwidth from 0 to fMCLK. This means that the noise energy contained in the signal band of interest is reduced (see Figure 29). BAND OF INTEREST fMCLK/2 06859-213 QUANTIZATION NOISE Figure 29. Quantization Noise Model AD7767 AD7767-1 AD7767-2 The settling time of the filter implemented on the AD7767, AD7767-1, and AD7767-2 is related to the length of the filter employed. The response of the filter in the time domain sets the filter settling time. Table 7 shows the filter settling times of the AD7767/AD7767-1/AD7767-2. The frequency responses of the digital filters on the AD7767, AD7767-1, and AD7767-2 are shown in Figure 32, Figure 33, and Figure 34, respectively. At the Nyquist frequency (output data rate/2), the digital filter provides 6 dB of attenuation. In each case, the filter provides stop-band attenuation of 100 dB and pass-band ripple of ±0.005 dB. 06859-214 fMCLK/2 Output Data Rate (ODR) 128 kHz 64 kHz 32 kHz Note that the output data rates shown in Table 6 are realized when using the maximum MCLK input frequency of 1.024 MHz. The output data rate scales linearly with the MCLK frequency, as does the digital power dissipated in the device. DIGITAL FILTER CUTOFF FREQUENCY BAND OF INTEREST Decimation Rate 8 16 32 Figure 30. Digital Filter Cutoff Frequency Rev. C | Page 15 of 24 AD7767 ANALOG INPUT STRUCTURE 0 The AD7767/AD7767-1/AD7767-2 are configured as a differential input structure. A true differential signal is sampled between the analog inputs VIN+ and VIN−, Pin 4 and Pin 5, respectively. Using differential inputs provides rejection of signals that are common to both the VIN+ and VIN− pins. –20 AMPLITUDE (dB) –40 –60 –80 Figure 35 shows the equivalent analog input circuit of the AD7767/AD7767-1/AD7767-2. The two diodes on each of the differential inputs provide ESD protection for the analog inputs. –100 –120 VREF+ –140 0 16k 32k 48k 64k 80k 96k 112k 128k FREQUENCY (Hz) D 06859-216 –160 VIN+ C1 RIN C2 RIN C2 D Figure 32. AD7767 Digital Filter Frequency Response GND AGND VREF+ D VIN– –20 C1 D AMPLITUDE (dB) –40 GND –60 Figure 35. Equivalent Analog Input Structure –80 Take care to ensure that the analog input signal does not exceed the reference supply voltage (VREF+) by more than 0.3 V, as specified in the Absolute Maximum Ratings section. If the input voltage exceeds this limit, the diodes become forward biased and start to conduct current. The diodes can handle 130 mA maximum. –100 –120 –160 0 8k 16k 24k 32k 40k 48k 56k 64k FREQUENCY (Hz) 06859-217 –140 Figure 33. AD7767-1 Digital Filter Frequency Response 0 The impedance of the analog inputs can be modeled as a parallel combination of C1 and the network formed by the series connection of RIN, C1, and C2. The value of C1 is dominated by the pin capacitance. RIN is typically 1.4 kΩ, the lumped component of serial resistors and the RON of the switches. C2 is typically 22 pF, and its value is dominated by the sampling capacitor. SUPPLY AND REFERENCE VOLTAGES –20 –40 –60 –80 –100 –120 –140 –160 0 4k 8k 12k 16k 20k 24k 28k FREQUENCY (Hz) Figure 34. AD7767-2 Digital Filter Frequency Response 32k 06859-218 AMPLITUDE (dB) AGND 06859-219 0 The AD7767/AD7767-1/AD7767-2 operate from a 2.5 V supply applied to the DVDD and AVDD pins. The interface is specified to operate between 1.7 V and 3.6 V. The AD7767/AD7767-1/ AD7767-2 operate from a reference input in the range of 2.2 V to 2 × AVDD applied to the VREF+ pin. The nominal reference supply voltage is 5 V, but a 2.5 V supply can also be used. When using a 5 V reference, the recommended reference devices are the ADR445, ADR435, or ADR425; when using 2.5 V, the ADR441, ADR431, or ADR421 are recommended. The voltage applied to the reference input (VREF+) operates both as a reference supply and as a power supply to the AD7767/AD7767-1/AD7767-2 device. Therefore, when using a 5 V reference input, the full-scale differential input range of the AD7767/AD7767-1/AD7767-2 is 10 V. See the Driving the AD7767 section for details on the maximum input voltage. Rev. C | Page 16 of 24 AD7767 AD7767 INTERFACE The AD7767 provides the user with a flexible serial interface, enabling the user to implement the most desirable interfacing scheme for their application. The AD7767 interface comprises seven different signals. Five of these signals are inputs: MCLK, CS, SYNC/PD, SCLK, and SDI. The other two signals are outputs: DRDY and SDO. The AD7767 offers the option of using a chip select input signal (CS) in its data read cycle. The CS signal is a gate for the SDO pin and allows many AD7767 devices to share the same serial bus. It acts as an instruction signal to each of these devices indicating permission to use the bus. When CS is logic high, the SDO line of the AD7767 is tristated. INITIAL POWER-UP There are two distinct patterns that can be initiated to read data from the AD7767 device: a pattern for when the CS falling edge occurs after the DRDY falling edge and a pattern for when the CS falling edge occurs before the DRDY falling edge (when CS is set to logic low). On initial power-up, apply a continuous MCLK signal. It is recommended that the user reset the AD7767 to clear the filters and ensure correct operation. The reset is completed as shown in Figure 5, with all events occurring relative to the rising edge of MCLK. A negative pulse on the SYNC/PD input initiates the reset, and the DRDY output switches to logic high and remains high until valid data is available. Following the power-up of the AD7767 by transitioning the SYNC/PD pin to logic high, a settling time is required before valid data is output by the device. This settling time, tSETTLING, is a function of the MCLK frequency and the decimation rate. Table 7 lists the settling time of each AD7767 model and should be referenced when reviewing Figure 5. Table 7. Filter Settling Time After SYNC/PD Model AD7767 AD7767-1 AD7767-2 1 Decimation Rate 8 16 32 tSETTLING1 (594 × tMCLK) + t21 (1186 × tMCLK) + t21 (2370 × tMCLK) + t21 When the CS falling edge occurs after the DRDY falling edge, the MSB of the conversion result is available on the SDO line on the CS falling edge. The remaining bits of the conversion result (MSB − 1, MSB − 2, and so on) are clocked onto the SDO line by the falling edges of SCLK that follow the CS falling edge. Figure 3 details this interfacing scheme. When CS is tied low, the AD7767 serial interface can operate in 3-wire mode as shown in Figure 4. In this case, the MSB of the conversion result is available on the SDO line on the falling edge of DRDY. The remaining bits of the data conversion result (MSB − 1, MSB − 2, and so on) are clocked onto the SDO line by the subsequent SCLK falling edges. POWER-DOWN, RESET, AND SYNCHRONIZATION tSETTLING is measured from the first MCLK rising edge after the rising edge of SYNC/PD to the falling edge of DRDY. READING DATA The AD7767 outputs its data conversion results in an MSB-first, twos complement, 24-bit format on the serial data output pin (SDO). MCLK is the master clock, which controls all the AD7767 conversions. The SCLK is the serial clock input for the device. All data transfers take place with respect to the SCLK signal. The DRDY line is used as a status signal to indicate when the data is available to be read from the AD7767. The falling edge of DRDY indicates that a new data-word is available in the output register of the device. DRDY stays low during the period that output data is permitted to be read from the SDO pin. The DRDY signal returns to logic high to indicate when not to read from the device. Ensure that a data read is not attempted during this period while the output register is being updated. The AD7767 SYNC/PD pin allows the user to synchronize multiple AD7767 devices. This pin also allows the user to reset and power down the AD7767 device. These features are implemented relative to the rising edges of MCLK and are shown in Figure 5, marked as A, B, C, and D. To power down, reset, or synchronize a device, the AD7767 SYNC/PD pin should be taken low. On the first rising edge of MCLK, the AD7767 is powered down. The DRDY pin transitions to logic high, indicating that the data in the output register is no longer valid. The status of the SYNC/PD pin is checked on each subsequent rising edge of MCLK. On the first rising edge of MCLK after the SYNC/PD pin is taken high, the AD7767 is taken out of power-down. On the next rising edge, the filter of the AD7767 is reset. On the following rising edge, the first new sample is taken. A settling time, tSETTLING, from the filter reset must elapse before valid data is output by the device (see Table 7). The DRDY output goes logic low after tSETTLING to indicate when valid data is available on SDO for readback. Rev. C | Page 17 of 24 AD7767 DAISY CHAINING Daisy chaining devices allows numerous devices to use the same digital interface lines by cascading the outputs of multiple ADCs on a single data line. This feature is especially useful for reducing component count and wiring connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. Data readback is analogous to clocking a shift register where data is clocked on the falling edge of SCLK. The block diagram in Figure 36 shows how devices must be connected to achieve daisy-chain functionality. The scheme shown operates by passing the output data of the SDO pin of an AD7767 device to the SDI input of the next AD7767 device in the chain. The data then continues through the chain until it is clocked onto the SDO pin of the first device in the chain. READING DATA IN DAISY-CHAIN MODE An example of a daisy chain of four AD7767 devices is shown in Figure 36 and Figure 37. In the case illustrated in Figure 36, the output of the AD7767 labeled A is the output of the full daisy chain. The last device in the chain (the AD7767 labeled D) has its serial data input (SDI) pin connected to ground. All the devices in the chain must use common MCLK, SCLK, CS, and SYNC/PD signals. To enable the daisy-chain conversion process, apply a common SYNC/PD pulse to all devices, synchronizing all the devices in the chain (see the Power-Down, Reset, and Synchronization section). After applying a SYNC/PD pulse to all the devices, there is a delay (as listed in Table 7) before valid conversion data appears at the output of the chain of devices. As shown in Figure 37, the first conversion result is output from the AD7767 device labeled A. This 24-bit conversion result is followed by the conversion results from the devices labeled B, C, and D, respectively, with all conversion results output in an MSB-first sequence. The stream of conversion results is clocked through each device in the chain and is eventually clocked onto the SDO pin of the AD7767 device labeled A. The conversion results of all the devices in the chain must be clocked onto the SDO pin of the final device in the chain while its DRDY signal is active low. This is illustrated in the examples shown (Figure 37 and Figure 38), where the conversion results from the devices labeled A, B, C, and D are clocked onto SDO (A) during the time between the falling edge of DRDY (A) and the rising edge of DRDY (A). CHOOSING THE SCLK FREQUENCY As shown in Figure 37, the number of SCLK falling edges that occur during the period when DRDY (A) is active low must match the number of devices in the chain multiplied by 24 (the number of bits that must be clocked through onto SDO (A) for each device). The period of SCLK (tSCLK) required for a known daisy-chain length using a known common MCLK frequency must, therefore, be established in advance. Note that the maximum SCLK frequency is governed by t8 and is specified in the Timing Specifications table for different VDRIVE voltages. In the case where CS is tied logic low, ⎡ t ⎤ t SCLK ≤ ⎢ READ ⎥ ⎣ 24 × K ⎦ (1) where: K is the number of AD7767 devices in the chain. tSCLK is the period of the SCLK. tREAD equals tDRDY − t5. In the case where CS is used in the daisy-chain interface, ) − (t 6 + t 7 + t 13 ) ⎤ ⎡ (t t SCLK ≤ ⎢ READ ⎥ 24 × K ⎣ ⎦ (2) where: K is the number of AD7767 devices in the chain. tSCLK is the period of the SCLK. tREAD equals tDRDY − t5. Note that the maximum value of SCLK is governed by t8 and is specified in the Timing Specifications table for different VDRIVE voltages. Rev. C | Page 18 of 24 AD7767 DAISY-CHAIN MODE CONFIGURATION AND TIMING DIAGRAMS SYNC/PD CS SYNC/PD SYNC/PD CS CS AD7767 SDI AD7767 (D) SDO SDI SYNC/PD SCLK (C) SDO SDI SCLK MCLK SYNC/PD CS CS AD7767 (B) SDO SDI (A) SDO SDI SCLK MCLK DRDY AD7767 SCLK MCLK MCLK 06859-013 SCLK MCLK Figure 36. Daisy-Chain Configuration with Four AD7767 Devices MCLK 1 8×n DRDY (A) CS 24 × tSCLK 24 × tSCLK 24 × tSCLK 24 × tSCLK SCLK SDO (A) AD7767 (A) AD7767 (B) AD7767 (C) SDI (A) = SDO (B) AD7767 (B) AD7767 (C) AD7767 (D) SDI (B) = SDO (C) AD7767 (C) AD7767 (D) SDI (C) = SDO (D) AD7767 (D) AD7767 (D) AD7767 (A) AD7767 (B) AD7767 (D) 06859-014 AD7767 (C) Figure 37. Daisy-Chain Timing Diagram (n = 1 for AD7767, n = 2 for AD7767-1, n = 4 for AD7767-2) When Driving the AD7767 1 MCLK DRDY (A) CS SDO (A) MSB (A) LSB (A) MSB (B) LSB (B) MSB (C) LSB (C) LSB (B) MSB (C) LSB (C) MSB (D) LSB (D) t16 SDI (A) = SDO (B) MSB (B) t17 Figure 38. Daisy-Chain SDI Setup and Hold Timing Rev. C | Page 19 of 24 06859-015 SCLK AD7767 DRIVING THE AD7767 The AD7767 must be driven with fully differential inputs. The common-mode voltage of the differential inputs to the AD7767 device and therefore the limits on the differential inputs are set by the reference voltage (VREF) applied to the device. The commonmode voltage of the AD7767 is VREF/2. When the AD7767 VREF+ pin has a 5 V supply (using ADR445, ADR435, or ADR425), the common mode is at 2.5 V, meaning that the maximum inputs that can be applied on the AD7767 differential inputs are a 5 V p-p input around 2.5 V. VREF VREF 2 R1 and R2 set the attenuation ratio between the input range and the ADC range (VREF). R1, R2, and CF are chosen depending on the desired input resistance, signal bandwidth, antialiasing, and noise contribution. The ratio of R2 to R1 should be equal to the ratio of REF to the peak-to-peak input voltage. For example, for the ±10 V range with a 4 kΩ impedance, R2 = 1 kΩ and R1 = 4 kΩ. R3 and R4 set the common mode on the IN− input, and R5 and R6 set the common mode on the IN+ input of the ADC. The common mode, which is equal to the voltage present at VOFFSET1, should be close to VREF/2. The voltage present should roughly be set to the ratio of VOFFSET1 to 1 + R2/R1. 1kΩ VIN+ 3.3nF ADP3330-2.5 1kΩ 0V AIN+ ADA4841-1 VREF 2.5V 15Ω 2.2nF VIN– 1kΩ VREF 2 1 4 V IN+ AD7767 3.3nF 5 VIN– 06859-016 VREF+ 1kΩ 2 AIN– 2.2nF Figure 39. Maximum Differential Inputs to the AD7767 ADA4841-1 * 2.5V TO 5V 1kΩ ADR4xx 1kΩ 06859-020 An analog voltage of 2.5 V supplies the AD7767 AVDD pin. However, the AD7767 allows the user to apply a reference voltage of up to 5 V. This provides the user with an increased full-scale range, offering the user the option of using the AD7767 with a greater LSB voltage. Figure 39 shows the maximum inputs to the AD7767. REFERENCE VOLTAGE 15Ω *SEE VREF+ INPUT SIGNAL SECTION FOR DETAILS. Figure 40. Driving the AD7767 from a Fully Differential Source VIN DIFFERENTIAL SIGNAL SOURCE VOUT = 5V REF ADR445 ADR425 0.1µF Voffset1 R6 R5 An example of recommended driving circuitry that can be used in conjunction with the AD7767/AD7767-1/AD7767-2 is shown in Figure 40. Figure 40 shows how the ADA4841-1 device can be used to drive an input to the AD7767/AD7767-1/AD7767-2 from a differential source. Each of the differential paths is driven by an ADA4841-1 device. R3 Voffset2 2.5V LDO 100µF REF 15Ω OUTN 2.2nF 15Ω IN SINGLE-ENDED SIGNAL SOURCE VREF+ VIN+ AVDD AD7767 2.2nF OUTP 100nF ADP3330-2.5 R4 5.2V 100nF 0.1µF VIN– AGND DGND FB For applications using a single-ended analog signal, either bipolar or unipolar, the ADA4941-1 single-ended-to-differential driver creates a fully differential input to the AD7767/AD7767-1/ AD7767-2. The schematic is shown in Figure 41. ADA4941 Vin R1 –0.2V R2 06859-018 0V AVDD CF Figure 41. Driving the AD7767 from a Single-Ended Source Table 8. Resistor Values Required When Using the Differential to Single-Ended Circuit with ADA4941 (See Figure 41) VIN (V) +20, −20 +10, −10 +5, −5 VOFFSET1 (V) 2.5 2.5 2.5 VOFFSET2 (V) 2.203 2.000 1.667 OUT+ (V) −0.01, +4.96 0.01, 4.99 0.00, 5.00 OUT− (V) 5.01, 0.04 4.99, 0.01 5.00, 0.00 Rev. C | Page 20 of 24 R1 (kΩ) 8.06 4.02 2 R2 (kΩ) 1 1 1 R4 (kΩ) 12.7 15 20 R3 = R5 = R6 (kΩ) 10 10 10 AD7767 ANTIALIASING fMCLK DIGITAL FILTER IMAGE AT fMCLK FIRST IMAGE POINT DIDD AD7767-2 1.5 IREF 0.5 0 0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1000k FREQUENCY (Hz) Figure 43. AD7767 Current vs. MCLK Frequency 2.5 2.0 Table 9. Antialias Filter Order, Attenuation at at First Image Point AD7767-1 2.0 AIDD Table 9 shows the attenuation achieved by various orders of front-end antialias filters prior to the signal entering the AD7767/ AD7767-1/AD7767-2 at the image of the digital filter stop band, which is 1.024 MHz − 0.547 × ODR. DIDD CURRENT (mA) Filter Order First Second Third First Second Third First Second Third 2.5 1.0 Figure 42. AD7767/AD7767-1/AD7767 Spectrum Model AD7767 3.0 06859-226 fMCLK – (0.547 × ODR) 3.5 Attenuation at 1.024 MHz – 0.547 × ODR 27 dB 50 dB 70 dB 33 dB 62 dB 89 dB 38 dB 74 dB 110 dB 1.5 1.0 AIDD 0.5 IREF 0 0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1000k FREQUENCY (Hz) 06859-227 BAND OF INTEREST 4.0 06859-231 DIGITAL FILTER 100dB ANTIALIAS PROTECTION 4.5 CURRENT (mA) The AD7767/AD7767-1/AD7767-2 sample the analog input at a maximum rate of 1.024 MHz. The on-board digital filter provides up to 100 dB attenuation for any possible aliasing frequency in the range from the beginning of the filter stop band (0.547 × ODR) to where the image of the digital filter pass band occurs. This occurs at MCLK minus the filter stop band (MCLK − 0.547 × ODR), as shown in Figure 42. applied divided by the decimation rate employed by the device in use. For instance, operating the AD7767 device with an MCLK of 800 kHz results in an output data rate of 100 kHz due to the decimate-by-8 filtering. Figure 44. AD7767-1 Current vs. MCLK Frequency 1.4 DIDD 1.2 CURRENT (mA) The AD7764 and AD7765 ∑-Δ devices are available to customers that require extra antialias protection. These devices sample the signal internally at a rate of 20 MHz to achieve up to a maximum of 156 kHz or 312 kHz output data rate. This means that the first alias point of these devices when run at the maximum speeds is 19.921 MHz and 19.843 MHz, respectively. POWER DISSIPATION 1.0 AIDD 0.8 0.6 0.4 Rev. C | Page 21 of 24 IREF 0.2 0 0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1000k FREQUENCY (Hz) Figure 45. AD7767-2 Current vs. MCLK Frequency 06859-228 The AD7767/AD7767-1/AD7767-2 offer exceptional performance at ultralow power. Figure 43, Figure 44, and Figure 45 show how the current consumption of the AD7767/AD7767-1/ AD7767-2 scales with the MCLK frequency applied to the device. Both the digital and analog currents scale as the MCLK frequency is reduced. The actual throughput equals the MCLK frequency AD7767 VREF+ INPUT SIGNAL MULTIPLEXING ANALOG INPUT CHANNELS The AD7767/AD7767-1/AD7767-2 VREF + pin is supplied with a voltage in the range of 2.4 V to 2 × AVDD (nominally 5 V). It is recommended that the VREF+ input be generated by a low noise voltage reference. Examples of such references are the ADR445, ADR435, ADR425 (5 V output), and ADR421 (2.5 V output). Typical reference supply circuits are shown in Figure 46. The AD7767/AD7767-1/AD7767-2 can be used with a multiplexer configuration. As per any converter that uses a digital filtering block, the maximum switching rate or the output data rate per channel is a function of the digital filter settling time. The reference voltage input pin (VREF+) also acts as a power supply to the AD7767/AD7767-1/AD7767-2 device. For a 5 V VREF+ input, a full-scale input of 5 V on both VIN+ and VIN− can be applied while voltage supplies to pins AVDD remain at 2.5 V. This configuration reduces the number of different supplies required. The output of the low noise voltage reference does not require a buffer; however, decoupling the output of the low noise reference is important. Place a 0.1 μF capacitor at the output of the voltage reference devices (ADR445, ADR435, ADR425, and ADR421) and follow the decoupling advice provided for the reference device chosen. A user multiplexing the analog inputs to a converter that employs a digital filter must wait the full digital filter settling time before a valid conversion result can be achieved; after this settling time, the channel can be switched. Then, the full settling time must again be observed before a valid conversion result is available and the input is switched once more. The AD7767 filter settling time equals 74 divided by the output data rate in use. The maximum switching frequency in a multiplexed application is, therefore, 1/(74/ODR), where the output data rate (ODR) is a function of the applied MCLK frequency and the decimation rate employed by the device in question. For example, applying a 1.024 MHz MCLK frequency to the AD7767 results in a maximum output data rate of 128 kHz, which in turn allows a 1.729 kHz multiplexer switching rate. The AD7767-1 and the AD7767-2 employ digital filters with longer settling time to achieve greater precision; thus, the maximum switching frequency for these devices is 864 Hz and 432 Hz, respectively. As mentioned, the nominal supply to the VREF+ pin is 5 V to achieve the full dynamic range available. When a 2.5 V VREF+ input is used (that is, in low power applications), the signal-tonoise ratio and dynamic range figures (generated using a 5 V VREF+ input) quoted in the Specifications section decrease by 6 dB, a direct result of halving the available input range. The AD7767/AD7767-1/AD7767-2 device requires a 100 μF capacitor to ground, which acts as a decoupling capacitor and as a reservoir of charge for the VREF+ pin. Place this capacitor as close to the AD7767/AD7767-1/AD7767-2 device as possible. Reducing the value of this capacitor (C40 in Figure 46) to 10 μF typically degrades noise performance by 1 dB. C40 can be an electrolytic or tantalum capacitor. VIN C34 10µF C35 0.1µF VOUT ADR4xx C39 0.1µF VREF+ C40 100µF AD7767/ AD7767-1/ AD7767-2 06859-021 REFERENCE SUPPLY V+ Figure 46. AD7767/AD7767-1/AD7767-2 Reference Input Configuration Rev. C | Page 22 of 24 AD7767 OUTLINE DIMENSIONS 5.10 5.00 4.90 16 9 4.50 4.40 4.30 6.40 BSC 1 8 PIN 1 1.20 MAX 0.15 0.05 0.20 0.09 0.65 BSC 0.30 0.19 COPLANARITY 0.10 SEATING PLANE 8° 0° 0.75 0.60 0.45 COMPLIANT TO JEDEC STANDARDS MO-153-AB Figure 47. 16-Lead Thin Shrink Small Outline Package [TSSOP] (RU-16) Dimensions shown in millimeters ORDERING GUIDE Model 1 AD7767BRUZ AD7767BRUZ-RL7 AD7767BRUZ-1 AD7767BRUZ-1-RL7 AD7767BRUZ-2 AD7767BRUZ-2-RL7 EVAL-AD7767EDZ EVAL-AD7767-1EDZ EVAL-AD7767-2EDZ EVAL-CED1Z 1 Temperature Range −40°C to +105°C −40°C to +105°C −40°C to +105°C −40°C to +105°C −40°C to +105°C −40°C to +105°C Package Description 16-Lead Thin Shrink Small Outline Package [TSSOP] 16-Lead Thin Shrink Small Outline Package [TSSOP] 16-Lead Thin Shrink Small Outline Package [TSSOP] 16-Lead Thin Shrink Small Outline Package [TSSOP] 16-Lead Thin Shrink Small Outline Package [TSSOP] 16-Lead Thin Shrink Small Outline Package [TSSOP] Evaluation Board Evaluation Board Evaluation Board Converter Evaluation and Development Board Z = RoHS Compliant Part. Rev. C | Page 23 of 24 Package Option RU-16 RU-16 RU-16 RU-16 RU-16 RU-16 AD7767 NOTES ©2007–2010 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D06859-0-5/10(C) Rev. C | Page 24 of 24