FEATURES FUNCTIONAL BLOCK DIAGRAM High performance 24-bit ∑-∆ ADC 115 dB dynamic range at 78.125 kHz output data rate 109 dB dynamic range at 312.5 kHz output data rate 312.5 kHz maximum fully filtered output word rate Pin-selectable oversampling rates of 64×, 128×, and 256× Low power mode Flexible serial peripheral interface (SPI) Fully differential modulator input On-chip differential amplifier for signal buffering On-chip reference buffer Full band, low-pass, finite impulse response (FIR) filter Overrange alert pin Digital gain correction registers Power-down mode Synchronization of multiple devices via the SYNC pin Daisy chaining APPLICATIONS Data acquisition systems Vibration analysis Instrumentation VOUTA– VOUTA+ VIN+ VIN– MCLK GND AVDD1 VINA+ DIFF MULTIBIT Σ-Δ MODULATOR VINA– AVDD2 AVDD3 AVDD4 DVDD VREF + BUF RECONSTRUCTION REFGND SYNC RESET/PWRDWN DECIMATION INTERFACE LOGIC AND OFFSET AND GAIN CORRECTION REGISTERS FIR FILTER ENGINE OVERRANGE DEC_RATE RBIAS AD7764 FSO SCO SDI SDO FSI 06518-001 Data Sheet 24-Bit, 312 kSPS, 109 dB Sigma-Delta ADC with On-Chip Buffers and Serial Interface AD7764 Figure 1. Table 1. Related Devices Device No. AD7760 AD7762 AD7763 AD7765 AD7766 AD7767 Description 2.5 MSPS, 100 dB, parallel output, on-chip buffer 625 kSPS, 109 dB, parallel output, on-chip buffer 625 kSPS, 109 dB, serial output, on-chip buffers 156 kSPS, 112 dB, serial output, on-chip buffers 128/64/32 kSPS, 8.5 mW, 109 dB SNR 128/64/32 kSPS, 8.5 mW, 109 dB SNR GENERAL DESCRIPTION The AD7764 is a high performance, 24-bit, sigma-delta (Σ-Δ) analog-to-digital converter (ADC). It combines wide input bandwidth, high speed, and performance of 109 dB dynamic range at a 312.5 kHz output data rate. With excellent dc specifications, the converter is ideal for high speed data acquisition of ac signals where dc data is also required. Using the AD7764 eases front-end antialias filtering requirements, simplifying the design process significantly. The AD7764 offers pin-selectable decimation rates of 64×, 128×, and 256×. Other features include an integrated buffer to drive the reference, as well as a fully differential amplifier to buffer and level shift the input to the modulator. An overrange alert pin indicates when an input signal exceeds the acceptable range. The addition of internal gain and internal overrange registers makes the AD7764 a compact, highly integrated data acquisition device requiring minimal peripheral components. The AD7764 also offers a low power mode, significantly reducing power dissipation without reducing the output data rate or available input bandwidth. Rev. B The differential input is sampled at up to 40 MSPS by an analog modulator. The modulator output is processed by a series of low-pass filters. The external clock frequency applied to the AD7764 determines the sample rate, filter corner frequencies, and output word rate. The AD7764 device boasts a full band, on-board FIR filter. The full stop-band attenuation of the filter is achieved at the Nyquist frequency. This feature offers increased protection from signals that lie above the Nyquist frequency being aliased back into the input signal bandwidth. The reference voltage supplied to the AD7764 determines the input range. With a 4 V reference, the analog input range is ±3.2768 V differential, biased around a common mode of 2.048 V. This common-mode biasing is achieved using the on-chip differential amplifier, further reducing the external signal conditioning requirements. The AD7764 is available in a 28-lead TSSOP package and is specified over the industrial temperature range of −40°C to +85°C. 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 ©2007–2017 Analog Devices, Inc. All rights reserved. Technical Support www.analog.com AD7764* PRODUCT PAGE QUICK LINKS Last Content Update: 03/29/2017 COMPARABLE PARTS DESIGN RESOURCES View a parametric search of comparable parts. • AD7764 Material Declaration • PCN-PDN Information EVALUATION KITS • Quality And Reliability • AD7764/AD7765 Evaluation Kit • Symbols and Footprints DOCUMENTATION DISCUSSIONS Data Sheet View all AD7764 EngineerZone Discussions. • AD7764: 24-Bit, 312 kSPS, 109 dB Sigma-Delta ADC with On-Chip Buffers and Serial Interface Data Sheet SAMPLE AND BUY SOFTWARE AND SYSTEMS REQUIREMENTS Visit the product page to see pricing options. • AD7764/AD7765 Evaluation Board Software TECHNICAL SUPPORT TOOLS AND SIMULATIONS Submit a technical question or find your regional support number. • Sigma-Delta ADC Tutorial DOCUMENT FEEDBACK REFERENCE MATERIALS Submit feedback for this data sheet. Technical Articles • MS-2210: Designing Power Supplies for High Speed ADC This page is dynamically generated by Analog Devices, Inc., and inserted into this data sheet. A dynamic change to the content on this page will not trigger a change to either the revision number or the content of the product data sheet. This dynamic page may be frequently modified. AD7764 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Synchronization .......................................................................... 24 Applications ....................................................................................... 1 Overrange Alerts ........................................................................ 24 Functional Block Diagram .............................................................. 1 Power Modes ............................................................................... 25 General Description ......................................................................... 1 Decimation Rate Pin .................................................................. 25 Revision History ............................................................................... 2 Daisy-Chaining ............................................................................... 26 Specifications..................................................................................... 3 Reading Data in Daisy-Chain Mode........................................ 26 Timing Specifications .................................................................. 6 Writing Data in Daisy-Chain Mode ........................................ 27 Absolute Maximum Ratings ............................................................ 8 Clocking the AD7764 .................................................................... 28 ESD Caution .................................................................................. 8 MCLK Jitter Requirements ....................................................... 28 Pin Configuration and Function Descriptions ............................. 9 Decoupling and Layout Information ........................................... 29 Typical Performance Characteristics ........................................... 11 Supply Decoupling ..................................................................... 29 Terminology .................................................................................... 15 Reference Voltage Filtering ....................................................... 29 Theory of Operation ...................................................................... 16 Differential Amplifier Components ........................................ 29 Σ-Δ Modulation and Digital Filtering ..................................... 16 Layout Considerations ............................................................... 29 AD7764 Antialias Protection .................................................... 19 Using the AD7764 ...................................................................... 30 Input Structure ................................................................................ 20 Bias Resistor Selection ............................................................... 30 On-Chip Differential Amplifier ............................................... 21 AD7764 Registers ........................................................................... 31 Modulator Input Structure ........................................................ 22 Control Register ......................................................................... 31 Driving the Modulator Inputs Directly ................................... 22 Status Register ............................................................................. 31 AD7764 Serial Interface ................................................................. 23 Gain Register—Address 0x0004 ............................................... 32 Reading Data ............................................................................... 23 Overrange Register—Address 0x0005 ..................................... 32 Reading Status and Other Registers ......................................... 23 Outline Dimensions ....................................................................... 33 Writing to the AD7764 .............................................................. 23 Ordering Guide .......................................................................... 33 Functionality ................................................................................... 24 REVISION HISTORY 3/2017—Rev. A to Rev. B Changes to Features Section and General Description Section....... 1 Changes to Table 2 ............................................................................ 3 Changes to Table 3 ............................................................................ 6 Change to Figure 3 Caption and Figure 4 Caption ...................... 7 Changes to Table 4 ............................................................................ 8 Changes to Table 5 ............................................................................ 9 Added Figure 29; Renumbered Sequentially .............................. 14 Changes to Terminology Section.................................................. 15 Changes to Σ-Δ Modulation and Digital Filtering Section and Table 6 ....................................................................................... 16 Added Figure 34.............................................................................. 17 Change to Table 7 ........................................................................... 17 Added Table 8 and Table 9 ............................................................ 18 Changes to On-Chip Differential Amplifier Section and Table 10..................................................................................... 21 Changes to AD7764 Serial Interface Section Title and Reading Data Section .................................................................................... 23 Changes to Functionality Section Title........................................ 24 Changes to Table 14 ........................................................................ 25 Changes to Table 15 ....................................................................... 26 Changes to Table 17, Table 18, and Table 19............................... 31 11/2009—Rev. 0 to Rev. A Changes to Table 2.............................................................................4 Changes to Table 3.............................................................................6 Changes to Table 4.............................................................................8 Changes to Typical Performance Characteristics Section, Introductory Text ........................................................................... 11 Changes to Σ-Δ Modulation and Digital Filtering Section....... 16 Added AD7764 Antialias Protection Section ............................. 17 Changes to Figure 35...................................................................... 19 Added Driving the Modulator Inputs Directly Section, Including Figure 39 and Figure 40, Renumbered Subsequent Figures ..... 20 Changes to Synchronization Section, Added Figure 41 ............ 22 Changes to Power Modes Section, Added Figure 44 ................. 23 Changes to Example 2 Section ...................................................... 26 Changes to Using the AD7764 Section........................................ 28 6/2007—Revision 0: Initial Version Rev. B | Page 2 of 33 Data Sheet AD7764 SPECIFICATIONS AVDD1 = DVDD = 2.5 V, AVDD2 = AVDD3 = AVDD4 = 5 V, VREF+ = 4.096 V, MCLK amplitude = 5 V, TA = 25°C, normal power mode, using the on-chip amplifier with components, as shown in the Optimal row in Table 10, unless otherwise noted. 1 Table 2. Parameter DYNAMIC PERFORMANCE Decimate 256× Normal Power Mode Dynamic Range Signal-to-Noise Ratio (SNR) 2 Spurious-Free Dynamic Range (SFDR) Total Harmonic Distortion (THD) Low Power Mode Dynamic Range SNR2 THD Decimate 128× Normal Power Mode Dynamic Range SNR2 SFDR THD Intermodulation Distortion (IMD) Low Power Mode Dynamic Range SNR2 THD IMD Test Conditions/Comments MCLK = 40 MHz, output data rate (ODR) = 78.125 kHz, fIN = 1 kHz sine wave Modulator inputs shorted Differential amplifier inputs shorted Input amplitude = −0.5 dB Nonharmonic Input amplitude = −0.5 dB Input amplitude = −6 dB Input amplitude = −60 dB MCLK = 40 MHz, ODR = 78.125 kHz, fIN = 1 kHz sine wave Modulator inputs shorted Differential amplifier inputs shorted Input amplitude = −0.5 dB Input amplitude = −0.5 dB Input amplitude = −6 dB Input amplitude = −60 dB MCLK = 40 MHz, ODR = 156.25 kHz, fIN = 1 kHz sine wave Modulator inputs shorted Differential amplifier inputs shorted Min Typ 110 115 113.4 109 130 dB dB dB dBFS −105 −103 −71 dB dB dB 113 112 109 −105 −111 −76 dB dB dB dB dB dB 106 110 106 108 105 Nonharmonic Input amplitude = −0.5 dB Input amplitude = −6 dB Input amplitude = −6 dB, fIN A = 50.3 kHz, fIN B = 47.3 kHz Second-order terms Third-order terms MCLK = 40 MHz, ODR = 156.25 kHz, fIN = 1 kHz sine wave Modulator inputs shorted Differential amplifier inputs shorted Input amplitude = −0.5 dB Input amplitude = −0.5 dB Input amplitude = −6 dB Input amplitude = −6 dB Input amplitude = −6 dB, fIN A = 50.3 kHz, fIN B = 47.3 kHz Second-order terms Third-order terms Rev. B | Page 3 of 33 109 105 Max −100 Unit 112 110.4 107 130 −105 −103 dB dB dB dBFS dB dB −117 −108 dB dB 110 109 107 −105 −111 dB dB dB dB dB dB −100 −134 −110 dB dB AD7764 Parameter Decimate 64× Normal Power Mode Dynamic Range SNR2 SFDR Total Harmonic Distortion (THD) IMD Low Power Mode Dynamic Range SNR2 Data Sheet Test Conditions/Comments MCLK = 40 MHz, ODR = 312.5 kHz, fIN = 1 kHz sine wave Modulator inputs shorted Differential amplifier inputs shorted Min Typ 105 109 107.3 104 130 −105 −103 102.7 Nonharmonic Input amplitude = −0.5 dB Input amplitude = −6 dB Input amplitude = −6 dB, fIN A = 100.3 kHz, fIN B = 97.3 kHz Second-order terms Third-order terms Modulator inputs shorted Differential amplifier inputs shorted Input amplitude = −0.5 dB Max dB dB dB dBFS dB dB −118 −108 105 106 105.3 103 102 SFDR THD DC ACCURACY Resolution Integral Nonlinearity Zero Error Nonharmonic Input amplitude = −0.5 dB Input amplitude = −6 dB Guaranteed monotonic to 24 bits Normal power mode Low power mode Normal power mode Including on-chip amplifier Low power mode Gain Error Zero Error Drift Gain Error Drift DIGITAL FILTER CHARACTERISTICS Pass-Band Ripple Pass Band 3 Including on-chip amplifier Does not include on-chip amplifier Does not include on-chip amplifier Normal and low power modes 110 −105 −111 Group Delay ANALOG INPUT Differential Input Voltage Input Capacitance REFERENCE INPUT/OUTPUT VREF+ Input Voltage VREF+ Input DC Leakage Current VREF+ Input Capacitance 0.03 0.024 0.1 −1 dB frequency ODR × 0.4016 ODR × 0.4096 ODR × 0.5 −3 dB Bandwidth3 Stop Band3 Stop-Band Attenuation −100 24 0.0036 0.0014 0.006 0.04 0.002 0.018 0.04 0.00006 0.00005 Beginning of stop band Decimate 64× and decimate 128× modes Decimate 256× See Table 8 and Table 9 Modulator input pins: VIN+ − VIN−, VREF+ = 4.096 V At on-chip differential amplifier inputs At modulator inputs −120 −115 5 Rev. B | Page 4 of 33 dB dB dB dB dB dB dBFS dB dB Bits % % % % % % % %FS/°C %FS/°C dB kHz kHz kHz dB dB ±3.2768 V p-p pF pF 4.096 ±1 V µA pF 5 29 AVDD3 = 5 V ± 5% Unit Data Sheet Parameter DIGITAL INPUT/OUTPUT MCLK Input Amplitude Input Capacitance Input Leakage Current VINH VINL VOH 4 VOL ON-CHIP DIFFERENTIAL AMPLIFIER Input Impedance Bandwidth for 0.1 dB Flatness Common-Mode Input Voltage 5 Common-Mode Output Voltage AD7764 Test Conditions/Comments DVDD Normal Power Mode AIDD1 (Modulator) AIDD2 (General) 6 AIDD3 (Differential Amplifier) AIDD4 (Reference Buffer) DIDD6 Low Power Mode AIDD1 (Modulator) AIDD2 (General) 6 AIDD3 (Differential Amplifier) AIDD4 (Reference Buffer) DIDD6 POWER DISSIPATION Normal Power Mode Low Power Mode Power-Down Mode 7 Typ 2.25 Max Unit 5.25 V pF μA/pin V 7.3 ±1 0.8 × DVDD 0.2 × DVDD 0.1 V V V 125 2.2 MΩ kHz V 2.2 >1 Common-mode voltage at amplifier input pins; VINA+ and VINA− On-chip differential amplifier pins, VOUTA+ and VOUTA− POWER REQUIREMENTS AVDD1 (Modulator Supply) AVDD2 (General Supply) AVDD3 (Differential Amplifier Supply) AVDD4 (Reference Buffer Supply) Min 0.8 2.048 V 2.375 4.75 2.625 5.25 V V 5 V supply required for 4.096 V reference 5 V supply required for 4.096 V reference 3.15 3.15 5.25 5.25 ±5% 2.375 2.625 V V min/ max V MCLK = 40 MHz AVDD3 = 5 V AVDD4 = 5 V MCLK = 40 MHz 19 13 10 9 37 mA mA mA mA mA MCLK = 40 MHz AVDD3 = 5 V AVDD4 = 5 V MCLK = 40 MHz 10 7 5.5 5 20 mA mA mA mA mA MCLK = 40 MHz, decimate 64× MCLK = 40 MHz, decimate 64× PWRDWN pin held logic low 300 160 1 371 215 mW mW mW See the Terminology section. SNR specifications in decibels are referred to a full-scale input, FS, and are tested with an input signal at 0.5 dB below full scale, unless otherwise specified. 3 The output data rate (ODR) = [(MCLK/2)]/decimation rate. That is, the maximum ODR for the AD7764 = [(40 MHz)/2)/64] = 312.5 kHz. 4 Tested with a 400 µA load current. 5 Specified min and max values relate to the common mode voltage at which the protection circuitry on the pins (VINA+ and VINA−) starts to turn on. Prior to this turn on, the THD of the AD7764 degrades at common modes approaching 2 V, or on the lower side approaching 1 V. 6 Tested at MCLK = 40 MHz. This current scales linearly with the applied MCLK frequency. 7 Tested at 125°C. 1 2 Rev. B | Page 5 of 33 AD7764 Data Sheet TIMING SPECIFICATIONS AVDD1 = DVDD = 2.5 V, AVDD2 = AVDD3 = AVDD4 = 5 V, VREF+ = 4.096 V, TA = 25°C, CLOAD = 25 pF. Table 3. Parameter fMCLK Min 500 fICLK 250 Limit at TMIN, TMAX Typ Max 40 20 t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t121 t13 t14 t15 tR MIN tR HOLD tR SETUP tS MIN tS HOLD tS SETUP 1 1 × tICLK 1 × tICLK 1 2 2.5 3.5 8 40 2 9.5 2.5 32 × tSCO 12 1 × tSCO 32 × tSCO 12 12 0 1 × tMCLK 5 5 4 × tMCLK 5 5 Unit kHz MHz kHz MHz sec sec ns ns ns ns ns ns sec ns sec sec ns ns ns sec ns ns sec ns ns Description Applied master clock frequency Internal modulator clock derived from MCLK SCO high period SCO low period SCO rising edge to FSO falling edge Data access time, FSO falling edge to data active MSB data access time, SDO active to SDO valid Data hold time (SDO valid to SCO rising edge) Data access time (SCO rising edge to SDO valid) SCO rising edge to FSO rising edge FSO low period Setup time from FSI falling edge to SCO falling edge FSI low period FSI low period SDI setup time for the first data bit SDI setup time SDI hold time Minimum time for a valid RESET pulse Minimum time between the MCLK rising edge and RESET rising edge Minimum time between the RESET rising edge and MCLK rising edge Minimum time for a valid SYNC pulse Minimum time between the MCLK falling edge and SYNC rising edge Minimum time between the SYNC rising edge and MCLK falling edge This is the maximum time FSI can be held low when writing to an individual device (a device that is not daisy-chained). Rev. B | Page 6 of 33 Data Sheet AD7764 Timing Diagrams 32 × tSCO t1 SCO (O) t8 t2 t9 t3 FSO (O) t4 SDO (O) D23 D22 D21 D20 t7 D19 D1 D0 ST4 ST3 ST2 ST1 ST0 0 0 06518-002 t6 t5 0 Figure 2. Serial Read Timing Diagram t1 SCO (O) t2 t12 t10 t11 t14 t13 SDI (I) RA15 t15 RA14 RA13 RA12 RA11 RA10 RA9 RA8 RA1 RA0 D15 D14 D1 D0 06518-003 FSI (I) Figure 3. Register Write Timing Diagram SCO (O) ≥8 × tSCO FSO (O) STATUS REGISTER CONTENTS [31:16] SDO (O) DON’T CARE BITS [15:0] NEXT DATA READ FOLLOWING THE WRITE TO CONTROL REGISTER SDI (I) CONTROL REGISTER ADDR (0x0001) 06518-004 FSI (I) CONTROL REGISTER INSTRUCTION Figure 4. Status Register Read Cycle Timing Diagram Rev. B | Page 7 of 33 AD7764 Data Sheet ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 4. Parameter AVDD1 to Ground AVDD2, AVDD3, AVDD4 to Ground DVDD to Ground VINA+, VINA− to Ground1 VIN+, VIN− to Ground1 Digital Input Voltage to Ground2 VREF+ to Ground3 AGND to DGND Input Current to Any Pin Except Supplies4 Operating Temperature Range, Commercial Storage Temperature Range Junction Temperature θJA Thermal Impedance (1s0p)5 θJA Thermal Impedance (2s2p)6, 7 θJC Thermal Impedance8 Solder Reflow Temperature9 ESD Rating −0.3 V to +2.8 V −0.3 V to +6 V −0.3 V to +2.8 V −0.3 V to +6 V −0.3 V to +6 V −0.3 V to +2.8 V −0.3 V to +6 V −0.3 V to +0.3 V ±10 mA −40°C to +85°C −65°C to +150°C 150°C 143°C/W 71.1°C/W 20°C/W 260°C 1 kV Stresses at or above those listed under Absolute Maximum Ratings may cause permanent damage to the product. This is a stress rating only; functional operation of the product at these or any other conditions above those indicated in the operational section of this specification is not implied. Operation beyond the maximum operating conditions for extended periods may affect product reliability. ESD CAUTION The absolute maximum voltage for VIN−, VIN+, VINA−, and VINA+ is 6.0 V or AVDD3 + 0.3 V, whichever is lower. The absolute maximum voltage on the digital input is 3.0 V or DVDD + 0.3 V, whichever is lower. 3 The absolute maximum voltage on the VREF+ input is 6.0 V or AVDD4 + 0.3 V, whichever is lower. 4 Transient currents of up to 100 mA do not cause SCR latch-up. 5 1s0p means a single-layer printed circuit board (PCB), which includes one signal layer and zero power layers. 6 2s2p means a 4-layer PCB, which includes 2 signal layers and 2 power layers. 7 θJA for a 2s2p PCB is derived from simulation. 8 The revised θJC (thermal impedance) is derived from simulation. 9 Maximum reflow temperature as per JEDEC J-SDT-020. 1 2 Rev. B | Page 8 of 33 Data Sheet AD7764 VINA– 1 28 AVDD3 VOUTA+ 2 27 VREF + VINA+ 3 26 REFGND VOUTA– 4 25 AVDD4 VIN– 5 24 AVDD1 AD7764 23 AGND1 TOP VIEW (Not to Scale) 22 RBIAS 21 AVDD2 OVERRANGE 9 20 AGND2 SCO 10 19 MCLK FSO 11 18 DEC_RATE SDO 12 17 DVDD SDI 13 16 RESET/PWRDWN FSI 14 15 SYNC VIN+ 6 AVDD2 7 AGND3 8 06518-005 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS Figure 5. Pin Configuration Table 5. Pin Function Descriptions Pin No. 1 2 3 4 5 6 7, 21 Mnemonic VINA− VOUTA+ VINA+ VOUTA− VIN− VIN+ AVDD2 8 9 AGND3 OVERRANGE 10 SCO 11 12 FSO SDO 13 SDI 14 FSI 15 SYNC 16 RESET/PWRDWN 17 DVDD 18 DEC_RATE 19 MCLK 20 22 AGND2 RBIAS Description Negative Input to the Differential Amplifier. Positive Output from the Differential Amplifier. Positive Input to the Differential Amplifier. Negative Output from the Differential Amplifier. Negative Input to the Modulator. Positive Input to the Modulator. 5 V Power Supply. Decouple Pin 7 to AGND3 (Pin 8) with a 100 nF capacitor. Decouple Pin 21 to AGND1 (Pin 23) with a 100 nF capacitor. Power Supply Ground for the Analog Circuitry. Overrange Pin. This pin outputs a logic high to indicate that the user applied an analog input that is approaching the limit of the analog input to the modulator. Serial Clock Out. This clock signal is derived from the internal ICLK signal. The frequency of this clock is equal to ICLK. See the Clocking the AD7764 section for more information. Frame Sync Out. This signal frames the serial data output and is 32 SCO periods wide. Serial Data Out. Data and status are output on this pin during each serial transfer. Each bit is clocked out on an SCO rising edge and is valid on the falling edge. See the AD7764 Serial Interface section for further details. Serial Data In. The first data bit (MSB) must be valid on the next SCO falling edge after the FSI event is latched. Thirty-two bits are required for each write; the first 16-bit word contains the device and register address, and the second word contains the data. See the AD7764 Serial Interface section for more information. Frame Sync Input. The status of this pin is checked on the falling edge of SCO. If this pin is low, then the first data bit is latched in on the next SCO falling edge. See the AD7764 Serial Interface section for more information. Synchronization Input. A falling edge on this pin resets the internal filter. Use this pin to synchronize multiple devices in a system. See the Synchronization section for more information. Reset/Power-Down Pin. When a logic low is sensed on this pin, the device is powered down and all internal circuitry is reset. 2.5 V Power Supply for the Digital Circuitry and FIR Filter. Decouple this pin to the ground plane with a 100 nF capacitor. Decimation Rate Pin. This pin selects one of the three decimation rate modes. When 2.5 V is applied to this pin, a decimation rate of 64× is selected. Select a decimation rate of 128× by leaving this pin floating. Select a decimation rate of 256× by setting this pin to ground. Master Clock Input. A low jitter digital clock must be applied to this pin. The output data rate depends on the frequency of this clock. See the Clocking the section for more information. Power Supply Ground for the Analog Circuitry. Bias Current Setting Pin. This pin must be decoupled to the ground plane. For more information, see the Bias Resistor Selection section. Rev. B | Page 9 of 33 AD7764 Pin No. 23 24 25 26 27 28 Mnemonic AGND1 AVDD1 AVDD4 REFGND VREF+ AVDD3 Data Sheet Description Power Supply Ground for the Analog Circuitry. 2.5 V Power Supply for the Modulator. Decouple this pin to AGND1 (Pin 23) with a 100 nF capacitor. 3.3 V to 5 V Power Supply for the Reference Buffer. Decouple this pin to AGND1 (Pin 23) with a 100 nF capacitor. Reference Ground. This pin is the ground connection for the reference voltage. Reference Input. 3.3 V to 5 V Power Supply for the Differential Amplifier. Decouple this pin to the ground plane with a 100 nF capacitor. Rev. B | Page 10 of 33 Data Sheet AD7764 TYPICAL PERFORMANCE CHARACTERISTICS 0 –25 –25 –50 –50 –75 –100 –125 –150 –150 100k 156.249k FREQUENCY (Hz) –175 0 –25 –25 –50 –50 AMPLITUDE (dB) 0 –75 –100 –150 –150 78.124k FREQUENCY (Hz) 0 –25 –25 –50 –50 AMPLITUDE (dB) 0 –75 –100 –150 39.062k FREQUENCY (Hz) 06518-008 –150 30k 40k 50k 60k 70k –100 –125 20k 30k –75 –125 10k 20k Figure 10. Low Power Mode, FFT,1 kHz, −0.5 dB Input Tone, 128× Decimation Rate 0 0 10k FREQUENCY (Hz) Figure 7. Normal Power Mode, FFT,1 kHz, −0.5 dB Input Tone, 128× Decimation Rate –175 150k –175 06518-007 60k 125k –100 –125 40k 100k –75 –125 20k 75k Figure 9. Low Power Mode, FFT,1 kHz, −0.5 dB Input Tone, 64× Decimation Rate 0 0 50k FREQUENCY (Hz) Figure 6. Normal Power Mode, FFT,1 kHz, −0.5 dB Input Tone, 64× Decimation Rate –175 25k 06518-211 50k Figure 8. Normal Power Mode, FFT,1 kHz, −0.5 dB Input Tone, 256× Decimation Rate –175 0 5k 10k 15k 20k 25k 30k 35k FREQUENCY (Hz) Figure 11. Low Power Mode, FFT,1 kHz, −0.5 dB Input Tone, 256× Decimation Rate Rev. B | Page 11 of 33 06518-210 0 AMPLITUDE (dB) –100 –125 –175 AMPLITUDE (dB) –75 06518-212 AMPLITUDE (dB) 0 06518-006 AMPLITUDE (dB) AVDD1 = DVDD = 2.5 V, AVDD2 = AVDD3 = AVDD4 = 5 V, VREF+ = 4.096 V, MCLK amplitude = 5 V, TA = 25°C. Linearity plots measured to 16-bit accuracy. Input signal reduced to avoid modulator overload and digital clipping; fast Fourier transforms (FFTs) of −0.5 dB tones are generated from 262,144 samples in normal power mode. All other FFTs are generated from 8192 samples. Data Sheet 0 –25 –25 –50 –50 –75 –100 –125 –150 –150 100k 150k FREQUENCY (Hz) –175 0 –25 –25 –50 –50 AMPLITUDE (dB) 0 –75 –100 –75 –100 –125 –125 –150 –150 50k 75k FREQUENCY (Hz) –175 06518-201 25k –25 –50 –50 AMPLITUDE (dB) –25 –75 –100 –75 –100 –125 –150 –150 15k 20k 25k 30k 35k FREQUENCY (Hz) 06518-202 –125 10k 75k Figure 16. Low Power Mode, FFT,1 kHz, −6 dB Input Tone, 128× Decimation Rate 0 5k 50k FREQUENCY (Hz) 0 0 25k 0 Figure 13. Normal Power Mode, FFT,1 kHz, −6 dB Input Tone, 128× Decimation Rate –175 150k Figure 15. Low Power Mode, FFT,1 kHz, −6 dB Input Tone, 64× Decimation Rate 0 0 100k FREQUENCY (Hz) Figure 12. Normal Power Mode, FFT,1 kHz, −6 dB Input Tone, 64× Decimation Rate –175 50k 06518-204 50k Figure 14. Normal Power Mode, FFT,1 kHz, −6 dB Input Tone, 256× Decimation Rate –175 0 5k 10k 15k 20k 25k 30k 35k FREQUENCY (Hz) Figure 17. Low Power Mode, FFT,1 kHz, −6 dB Input Tone, 256× Decimation Rate Rev. B | Page 12 of 33 06518-205 0 AMPLITUDE (dB) –100 –125 –175 AMPLITUDE (dB) –75 06518-203 AMPLITUDE (dB) 0 06518-200 AMPLITUDE (dB) AD7764 Data Sheet AD7764 25 40 35 DVDD 20 DVDD CURRENT (mA) CURRENT (mA) 30 25 AVDD1 20 AVDD2 15 AVDD3 15 AVDD1 10 AVDD2 10 5 5 AVDD3 AVDD4 0 5 10 15 20 25 30 35 40 MCLK FREQUENCY (MHz) 0 06518-010 0 0 5 10 15 20 25 30 35 40 45 MCLK FREQUENCY (MHz) Figure 18. Normal Power Mode, Current Consumption vs. MCLK Frequency, 64× Decimation Rate 06518-011 AVDD4 Figure 21. Low Power Mode, Current Consumption vs. MCLK Frequency, 64× Decimation Rate 40 25 DVDD 35 DVDD 20 CURRENT (mA) CURRENT (mA) 30 25 AVDD1 20 AVDD2 15 15 AVDD1 10 AVDD2 10 5 AVDD3 AVDD4 5 10 20 15 30 25 40 35 45 MCLK FREQUENCY (MHz) 0 06518-114 0 AVDD3 AVDD4 0 Figure 19. Normal Power Mode, Current Consumption vs. MCLK Frequency, 128× Decimation Rate 0 5 10 15 25 20 35 30 40 45 MCLK FREQUENCY (MHz) 06518-115 5 Figure 22. Low Power Mode, Current Consumption vs. MCLK Frequency, 128× Decimation Rate 40 20 18 35 DVDD DVDD 16 30 CURRENT (mA) AVDD1 20 AVDD2 15 AVDD1 12 10 AVDD2 8 6 10 4 AVDD3 5 5 10 15 20 25 MCLK FREQUENCY (MHz) 30 35 40 Figure 20. Normal Power Mode, Current Consumption vs. MCLK Frequency, 256× Decimation Rate AVDD4 0 06518-112 0 0 AVDD3 2 AVDD4 0 5 10 15 20 25 MCLK FREQUENCY (MHz) 30 35 40 06518-113 CURRENT (mA) 14 25 Figure 23. Low Power Mode, Current Consumption vs. MCLK Frequency, 256× Decimation Rate Rev. B | Page 13 of 33 AD7764 Data Sheet 2.0 0 1.5 –20 –40 AMPLITUDE (dB) 1.0 0 –0.5 –1.0 –60 –80 –100 –120 –140 –1.5 –160 15k 20k 25k 30k 35k 40k 45k 50k 55k 59,535 CODE –180 06518-208 –2.0 6k 10k 0 20k 40k 60k 78,124 FREQUENCY (Hz) Figure 24. DNL Plot 06518-209 DNL (LSB) 0.5 Figure 27. Normal Power Mode, IMD, fIN A = 49.7 kHz, fIN B = 50.3 kHz, 50 kHz Center Frequency, 128× Decimation Rate 0.003225 0.00300 –40°C 0.003000 +85°C 0.00225 0.00150 0.00225 +25°C +25°C INL (%) INL (%) 0.00075 0 0.00150 –0.00075 +85°C –40°C 0.00075 –0.00150 6k 10k 15k 20k 25k 30k 35k 40k 45k 50k 55k 59,535 16-BIT CODE SCALING 0 –0.00012 06518-206 –0.00300 6k 10k 15k 20k 25k 30k 35k 40k 45k 50k 55k 59,535 16-BIT CODE SCALING Figure 25. Normal Power Mode INL 06518-207 –0.00225 Figure 28. Low Power Mode INL 110 109 140 –140 120 –120 100 –100 80 –80 SNR (dB) 107 106 NORMAL SNR 105 104 THD SNR SINAD 60 –60 40 –40 20 –0 THD (dB) SNR, SINAD (dB) 108 103 0 64 128 DECIMATION RATE 192 256 Figure 26. Normal and Low Power Mode, SNR vs. Decimation Rate, 1 kHz, −0.5 dB Input Tone 0 0 06518-009 102 0 0.5 1.0 1.5 2.0 2.5 3.0 COMMON-MODE VOLTAGE (V) Figure 29. SINAD vs. DC Common-Mode Input Voltage at Amplifier Input Pins (VINA−, VINA+); Optimal Component Values Used (See Table 10) Rev. B | Page 14 of 33 06518-329 LOW SNR Data Sheet AD7764 TERMINOLOGY Signal-to-Noise Ratio (SNR) SNR is the ratio of the rms value of the actual input signal 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 (dB). Total Harmonic Distortion (THD) THD is the ratio of the rms sum of the harmonics to the fundamental. For the AD7764, THD is defined as THD(dB) = 20log In this case, the second-order terms are typically distanced in frequency from the original sine waves, and the third-order terms are typically 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 per the THD specification, where the calculation 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. V22 + V32 + V42 + V52 + V62 V1 Differential Nonlinearity (DNL) DNL is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. where: V2, V3, V4, V5, and V6 are the rms amplitudes of the second to the sixth harmonics. V1 is the rms amplitude of the fundamental. Nonharmonic Spurious-Free Dynamic Range (SFDR) Nonharmonic 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 dynamic range is expressed in decibels. Intermodulation Distortion 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 is equal to 0. For example, the secondorder terms include (fa + fb) and (fa − fb), while the third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). The AD7764 is tested using the CCIF standard, where two input frequencies near the top end of the input bandwidth are used. 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 code transition (from 100 … 000 to 100 … 001) occurs for an analog voltage 1/2 LSB above the nominal negative full scale. The last code transition (from 011 … 110 to 011 … 111) occurs for an analog voltage 1 ½ LSB below the nominal full scale. 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. Rev. B | Page 15 of 33 AD7764 Data Sheet THEORY OF OPERATION The digital filtering that follows the modulator removes the large out-of-band quantization noise (see Figure 32) while also reducing the data rate from fICLK at the input of the filter to fICLK /64 or less at the output of the filter, depending on the decimation rate used. 0 Σ-Δ MODULATION AND DIGITAL FILTERING –20 –40 AMPLITUDE (dB) The input waveform applied to the modulator is sampled, and an equivalent digital word is output to the digital filter at a rate equal to fICLK. By employing oversampling, the quantization noise is spread across a wide bandwidth from 0 to fICLK. This means that the noise energy contained in the signal band of interest is reduced (see Figure 30). To further reduce the quantization noise, a high-order modulator is employed to shape the noise spectrum so that most of the noise energy is shifted out of the signal band (see Figure 31). PASS-BAND RIPPLE = 0.05dB –0.1dB FREQUENCY = 125.1kHz –3dB FREQUENCY = 128kHz STOP BAND = 156.25kHz –60 –80 –100 –120 –140 –160 0 50 100 150 200 250 FREQUENCY (kHz) QUANTIZATION NOISE Figure 33. Filter Frequency Response (312.5 kHz ODR) 06518-012 BAND OF INTEREST fICLK/2 The AD7764 employs a sequence of three FIR filters in series to provide a digital filter with a low ripple pass band, a steep transition band, and excellent stop-band rejection, which starts at the Nyquist frequency. Achieving the stop-band rejection at the Nyquist frequency is beneficial because it prevents signals slightly greater than the Nyquist frequency, which are not protected by and external antialias filter from aliasing back in-band. Figure 30. Σ-Δ ADC, Quantization Noise fICLK/2 06518-013 NOISE SHAPING BAND OF INTEREST The AD7764 digital filter allows data to be output at three different output data rates (for any given MCLK input frequency) through setting the decimation ratio through the series of filters. Figure 31. Σ-Δ ADC, Noise Shaping DIGITAL FILTER CUTOFF FREQUENCY fICLK/2 The first filter receives data from the modulator at fICLK MHz, where it is decimated 4× to the output data at (fICLK/4) MHz. The second filter allows the decimation rate to be chosen from 8× to 32×. The third filter has a fixed decimation rate of 2×. 06518-014 BAND OF INTEREST Figure 32. Σ-Δ ADC, Digital Filter Cutoff Frequency The modulator sampling rate, fICLK, is dependent on the power mode chosen. Table 6 details the relationship between MCLK and fICLK across power mode. The AD7764 low power mode divides the MCLK by a factor of 4, reducing current consumption in both the analog and digital domains. Table 6. Modulator Sampling Rate vs. Power Mode Power Mode Normal Low 300 06518-015 The AD7764 features an on-chip fully differential amplifier to feed the Σ-Δ modulator pins, an on-chip reference buffer, and a FIR filter block to perform the required digital filtering of the Σ-Δ modulator output. Using this Σ-Δ conversion technique with the added digital filtering, the analog input is converted to an equivalent digital word. Digital filters exhibit a group delay and settling time. The group delay of the filter is the delay from the change in analog input to when it is output by the digital filter. It is comprised of the computation plus the filter delays. The delay until valid data is available (when the FILTER_SETTLE status bit is set) is approximately twice the filter delay plus the computation delay. Modulator Sampling Rate (fICLK) MCLK/2 MCLK/4 Rev. B | Page 16 of 33 Data Sheet AD7764 The group delay and settling time of a digital filter is apparent in the behavior of the ADC response to a SYNC input pulse. Table 8 and Table 9 describe the response of the AD7764 in both normal and low power modes to a SYNC pulse, in addition to providing the group delay and settling times. Figure 34 shows the effect of SYNC and the subsequent signals from the AD7764. The SYNC rising edge sets a known point in time from which the digital filter begins to process inputs from the modulator. This is useful in building a simultaneous sampling solution with multiple AD7764 devices, all clocked by the same MCLK. The logic level of the SYNC pin is sampled by the rising edge of MCLK. Transitioning SYNC from low to high on an MCLK falling edge is recommended. Following the rising edge of SYNC, a number of MCLK periods pass before the first falling edge of FSO indicates a conversion output; this number of MCLK periods is defined as tSYNC_OFFSET. Beyond tSYNC_OFFSET, a number of conversion periods, each indicated by the falling edge of FSO, occur before the data from the filter is fully settled. During this time, outputs from the ADC have data that is a filtered mix of inputs to the modulator both before and after the time at which the SYNC signal transitioned from logic low to logic high. Table 7. Group Delay (tGD_ODR) Expressed in Periods of the Output Data Rate Decimation Rate 64× 128× 256× Group Delay (tGD_ODR) in Output Data Rate Periods Normal Power Mode Low Power Mode 28 29 28 28 28 28 Because the group delay is not an exact number of conversion periods, a more precise way to describe the term is in MCLK periods. The exact region within a given conversion period where the analog input change occurs determines which output data period the ADC output responds to with a change in the digital output. Figure 34 shows that, if the step change on the input occurs at later point in time within the output period, the digital output remains updated within the same ODR period. Described as the fine group delay, tGDMCLK, this delay consists of computation delay added to the actual delay through the filter provided in MCLK periods. Expressing the group delay in this manner allows it to be shown independent of the discrete steps of the ADC output data rate. All outputs from the ADC exhibit a group delay. The constant delay, due to the digital filter, is described by the number of conversion periods (tODR) that pass between a change occurring on the analog input being seen on the digital conversion output. Figure 34 illustrates the group delay, showing a scenario where there is a step change on the analog inputs after the filter initially settles. As shown, a given number of periods of the output data rate occur before the digital output shows the step change. MCLK FSO SYNC SDO (CONVERSION RESULT) tSYNC_FS tSYNC OFFSET FILTER SETTLE BIT ADC ANALOG INPUTS ADC ANALOG OUTPUTS tGD MCLK1 1FINE GROUP DELAY IN t MLCK 2COARSE GROUP DELAY IN t ODR Figure 34. AD7764 Digital Filtering; Response to SYNC, Settling Time, and Group Delay Rev. B | Page 17 of 33 06518-233 tGDODR2 AD7764 Data Sheet Table 8. Filter Group Delay and Settling Time in Normal Power Mode tSYNC_OFFSET (tMCLK) 87 tSYNC_FS SYNC to FILTER_ SETTLE Bit (tMCLK) 7128 Modulator Sampling Rate (fICLK) MCLK/2 Oversampling Ratio (OSR) 64 Decimation Rate 64× Filter Computation Delay (tMCLK) 80 Filter Delay (tMCLK) 3504 tGDMCLK ADC Group Delay (tMCLK) 3584 128× 113 6960 7073 141 13,966 MCLK/2 128 256× 443 13,608 14,051 252 27,901 MCLK/2 256 Modulator Sampling Rate (fICLK) MCLK/4 Oversampling Ratio (OSR) 32 Filter Pass Band ODR × 0.4 ODR × 0.4 ODR × 0.4 Output Data Rate (ODR) MCLK/64 MCLK/128 MCLK/256 Table 9. Filter Group Delay and Settling Time for Low Power Mode Decimation Rate 64× Filter Computation Delay (tMCLK) 120 Filter Delay (tMCLK) 3552 tGDMCLK ADC Group Delay (tMCLK) 3672 128× 162 7008 7170 167 14,248 MCLK/4 64 256× 224 13,920 14,144 277 27,926 MCLK/4 128 tSYNC_OFFSET (tMCLK) 113 SYNC to FILTER_ SETTLE Bit (tMCLK) 7282 Rev. B | Page 18 of 33 Filter Pass Band ODR × 0.4 ODR × 0.4 ODR × 0.4 Output Data Rate (ODR) MCLK/64 MCLK/128 MCLK/256 Data Sheet AD7764 AD7764 ANTIALIAS PROTECTION The decimation of the AD7764, along with its counterparts in the AD776x family, namely the AD7760, AD7762, AD7763, and AD7765, provides top of the range antialias protection. The decimation filter of the AD7764 features more than 115 dB of attenuation across the full stop band, which ranges from the Nyquist frequency, namely ODR/2, up to ICLK – ODR/2 (where ODR is the output data rate). Starting the stop band at the Nyquist frequency prevents any signal component above Nyquist (and up to ICLK – ODR/2) from aliasing into the desired signal bandwidth. Taking as an example the AD7764 in normal power and in decimate ×128 mode, the first possible alias frequency is at the ICLK frequency minus the pass band of the digital filter (see Figure 35). SIMPLFIES ANTIALIAS FILTER ROLL-OFF REQUIRED DIGITAL FILTER RESPONSE IMAGE DIGITAL FILTER RESPONSE NOISE SHAPING NYQUIST = 78kHz ODR = 156kHz FIRST ALIAS POINT 20MHz TO 78kHz FREQUENCY (Hz) MODULATOR SAMPLING RATE = MCLK/2 = 20MHz Figure 35. Antialias Example Using the AD7764 in Normal Mode, Decimate ×128 Using MCLK/2 = ICLK = 20 MHz Rev. B | Page 19 of 33 06518-213 AMPLITUDE (dB) NO ALIASING OF SIGNALS INTO PASSBAND AROUND NYQUIST FREQUENCY Figure 33 shows the frequency response of the decimation filter when the AD7764 is operated with a 40 MHz MCLK in decimate ×128 mode. Note that the first stop-band frequency occurs at Nyquist. The frequency response of the filter scales with both the decimation rate chosen and the MCLK frequency applied. When using low power mode, the modulator sample rate is MCLK/4. AD7764 Data Sheet INPUT STRUCTURE The AD7764 requires a 4.096 V input to the reference pin, VREF+, supplied by a high precision reference, such as the ADR444. Because the input to the Σ-Δ modulator of the device is fully differential, the effective differential reference range is 8.192 V. VREF + (Diff) = 2 × 4.096 = 8.192 V As is inherent in Σ-Δ modulators, only a certain portion of this full reference can be used. With the AD7764, 80% of the full differential reference can be applied to the differential inputs of the modulator. This means that a maximum of ±3.2768 V p-p full scale can be applied to each of the AD7764 modulator inputs (Pin 5 and Pin 6), with the AD7764 being specified with an input −0.5 dB down from full scale (−0.5 dBFS). The AD7764 modulator inputs must have a common-mode input of 2.048 V. Figure 36 shows the relative scaling between the differential voltages applied to the modulator pins and the respective 24-bit, twos complement digital outputs. Modulator_InputFULL SCALE = 8.192 V × 0.8 = 6.5536 V INPUT VOLTAGE (V) OVERRANGE REGION TWOS COMPLEMENT DIGITAL OUTPUT +4.096V VIN+ = 3.6855V VIN– = 0.4105V +3.2768V = MODULATOR FULL-SCALE = 80% OF 4.096V 0111 1111 1111 1111 1111 1111 0111 1000 1101 0110 1111 1101 –0.5dBFS INPUT 0000 0000 0000 0000 0000 0001 0000 0000 0000 0000 0000 0000 1111 1111 1111 1111 1111 1111 VIN+ = 2.048V VIN– = 2.048V DIGITAL OUTPUT ON SDO PIN –0.5dBFS INPUT VIN+ = 0.4105V VIN– = 3.6855V 1000 0111 0010 1001 0000 0010 1000 0000 0000 0000 0000 0000 80% OF 4.096V = MODULATOR FULL-SCALE = –3.2768V –4.096V OVERRANGE REGION Figure 36. AD7764 Scaling: Modulator Input Voltage vs. Digital Output Code Rev. B | Page 20 of 33 06518-120 INPUT TO MODULATOR PIN 5 AND PIN 6 VIN– AND VIN+ Data Sheet AD7764 ON-CHIP DIFFERENTIAL AMPLIFIER The AD7764 contains an on-board differential amplifier recommended to drive the modulator input pins. Pin 1, Pin 2, Pin 3, and Pin 4 on the AD7764 are the differential input and output pins of the amplifier. The external components, RIN, RFB, CFB, CS, and RM, are placed around Pin 1 through Pin 6 to create the recommended configuration. To achieve the specified performance, configure the differential amplifier as a first-order antialias filter, as shown in Figure 37, using the component values listed in Table 10. The inputs to the differential amplifier are then routed through the external component network before being applied to the modulator inputs, VIN− and VIN+ (Pin 5 and Pin 6). Using the optimal values in the table as an example yields a 25 dB attenuation at the first alias point of 19.6 MHz. CFB The common-mode input at each of the differential amplifier input pins (VINA+ and VINA−) can range from −0.5 V dc to 2.2 V dc. The amplifier has a constant output common-mode voltage of 2.048 V, that is, VREF/2, the requisite common mode voltage for the modulator input pins (VIN+ and VIN−). Figure 38 shows the signal conditioning that occurs using the differential amplifier configuration shown in Table 10 with a ±2.5 V input signal to the differential amplifier. The amplifier in this example is biased around ground and is scaled to provide ±3.0935 V p-p (−0.5 dBFS) on each modulator input with a 2.048 V common mode. +2.5V +3.632V 0V +2.048V VIN+ A +0.464V –2.5V RFB B 2 5 DIFF AMP 3 4 6 RM RFB RM (Ω) 43 36 to 47 –2.5V +0.464V Figure 38. Differential Amplifier Signal Conditioning Table 10. On-Chip Differential Filter Component Values RFB (kΩ) 3.01 2.4 to 4.87 +2.048V VOUTA– Figure 37. Differential Amplifier Configuration RIN (kΩ) 4.75 2.37 to 5.76 0V VIN+ CFB Value Optimal Tolerance Range1 CS (pF) 8.2 0 to 10 CFB (pF) 47 20 to 100 CM (pF) 33 33 to 56 To obtain maximum performance from the AD7764, it is recommended to drive the ADC with differential signals. Figure 39 shows how a bipolar, single-ended signal biased around ground drives the AD7764 with the use of an external operational amplifier, such as the AD8021. CFB RFB 2R VIN 1 VIN– VIN– CM RIN VINA+ B 06518-122 1 CS +3.632V +2.5V RM RIN 06518-024 A VOUTA+ The values shown are the acceptable tolerances for each component when altered relative to the optimal values that achieve the stated specifications of the device. The range of values for each of the components in the differential amplifier configuration is listed in Table 10. When using the differential amplifier to gain the input voltages to the required modulator input range, it is recommended to implement the gain function by changing RIN and leaving RFB as the listed optimal value. Rev. B | Page 21 of 33 2R RM RIN AD8021 VIN– CS R DIFF AMP RM CM VIN+ RIN RFB CFB Figure 39. Single-Ended to Differential Conversion 06518-026 VINA– AD7764 Data Sheet MODULATOR INPUT STRUCTURE SCO (O) CONTROL REGISTER ADDRESS 0x0001 CPB1 SS3 The AD7764 modulator inputs must have a common-mode voltage of 2.048 V and adhere to the amplitudes as described in the Input Structure section. ANALOG MODULATOR CS2 SS2 An example of a typical circuit to drive the AD7764 for applications requiring excellent ac and dc performance is shown in Figure 42. Either the AD8606 or AD8656 can drive the AD7764 modulator inputs directly. SH4 SS4 06518-027 CPB2 Figure 40. Equivalent Input Circuit The SS1 and SS3 sampling switches are driven by ICLK, whereas the SS2 and SS4 sampling switches are driven by ICLK. When ICLK is high, the analog input voltage is connected to CS1. On the falling edge of ICLK, the SS1 and SS3 switches open, and the analog input is sampled on CS1. Similarly, when ICLK is low, the analog input voltage is connected to CS2. On the rising edge of ICLK, the SS2 and SS4 switches open, and the analog input is sampled on CS2. Best practice is to short the differential amplifier inputs to ground through the typical input resistors and leave the typical feedback resistors in place. C22 10kΩ 10kΩ C12 ANALOG INPUT1 10kΩ 10kΩ 4.99kΩ The CPA, CPB1, and CPB2 capacitors represent parasitic capacitances that include the junction capacitances associated with the MOS switches. 1.024V U2 AD8606 AD8655 CS2 (pF) 13 CPA (pF) 13 4.99kΩ AD8606 AD8655 2.048V 51Ω 0Ω 51Ω 0Ω U1 6 5 VIN– VIN+ AD7764 VINA– VOUTA+ VINA+ VOUTA– Table 11. Equivalent Component Values CS1 (pF) 13 AMP OFF MODE DATA 0x0001 Figure 41. Writing to the AD7764 Control Register, Turning Off the On-Board Differential Amplifier SH3 CPA SH2 SDI (I) CS1 SS1 SH1 FSI (I) 2 1 RFB CPB1/CPB2 (pF) 5 RIN 4 3 RFB RIN 1 –0.5dBFS INPUT SIGNAL AS DESCRIBED IN INPUT STRUCTURE SECTION. 2 SET C1 AND C2 AS REQUIRED FOR APPLICATION INPUT BW AND DRIVING THE MODULATOR INPUTS DIRECTLY ANTI-ALIAS REQUIREMENT. The AD7765 can be configured so that the on-board differential amplifier can be disabled and the modulator can be driven directly using discrete amplifiers. This allows the user to lower the power dissipation. 06518-302 VIN+ 32 × tSCO 06518-301 The AD7764 employs a double sampling front end, as shown in Figure 40. For simplicity, only the equivalent input circuitry for VIN+ is shown. The equivalent circuitry for VIN− is the same. Figure 42. Driving the AD7764 Modulator Inputs Directly from a SingleEnded Source (On-Board Differential Amplifier Powered Down) To power down the on board differential amplifier, the user issues a write to set the AMP OFF bit in the control register to logic high (see Figure 41). Rev. B | Page 22 of 33 Data Sheet AD7764 AD7764 SERIAL INTERFACE READING DATA READING STATUS AND OTHER REGISTERS The AD7764 uses fully synchronous serial data interface where the ADC is the master providing frame, serial clock, and serial data outputs. The timing diagram in Figure 2 shows how the AD7764 transmits conversion results. The AD7764 features a gain correction register, an overrange register, and a read only status register. To read back the contents of these registers, the user must first write to the control register of the device and set the bit that corresponds to the register to be read. The next read operation outputs the contents of the selected register (on the SDO pin) instead of a conversion result. The data read from the AD7764 is clocked out using the serial clock output (SCO). The SCO frequency is half that of the MCLK input to the AD7764. The conversion result output on the serial data output (SDO) line is framed by the frame synchronization output, FSO, which is sent logic low for 32 SCO cycles. Each bit of the new conversion result is clocked onto the SDO line on the rising SCO edge and is valid on the falling SCO edge. The 32-bit result consists of the 24 data bits followed by five status bits followed further by three zeros. The five status bits are listed in Table 12 and described in this section. Table 12. Status Bits During a Data Read D7 FILTER_SETTLE D6 OVR D5 LPWR D4 DEC_RATE 1 D3 DEC_RATE 0 The FILTER_SETTLE bit indicates whether the data output from the AD7764 is valid. After resetting the device (using the RESET pin) or clearing the digital filter (using the SYNC pin), the FILTER_SETTLE bit goes logic low to indicate that the full settling time of the filter has not yet passed and that the data is not yet valid. The FILTER_SETTLE bit also goes to zero when the input to the device asserts the overrange alerts. The OVR (overrange) bit is described in the Overrange Alerts section. The LPWR bit is set to logic high when the AD7764 operates in low power mode. See the Power Modes section for further details. The DEC_RATE 1 bit and the DEC_RATE 0 bit indicate the decimation ratio used. Table 13 is a truth table for the decimation rate bits. Table 13. Decimation Rate Status Bits Decimate 64× 128× 256× 1 DEC_RATE 1 0 1 0 DEC_RATE 0 1 X1 0 To ensure that the next read cycle contains the contents of the register written to, the write operation to that register must be completed a minimum of 8 × tSCO before the falling edge of FSO, which indicates the start of the next read cycle. See Figure 4 for further details. The AD7764 Registers section provides more information on the relevant bits in the control register. WRITING TO THE AD7764 A write operation to the AD7764 is shown in Figure 3. The serial writing operation is synchronous to the SCO signal. The status of the frame synchronization input, FSI, is checked on the falling edge of the SCO signal. If the FSI line is low, then the first data bit on the serial data in (SDI) line is latched in on the next SCO falling edge. Set the active edge of the FSI signal to occur at a position when the SCO signal is high or low to allow setup and hold times from the SCO falling edge to be met. The width of the FSI signal can be set to between 1 and 32 SCO periods wide. A second, or subsequent, falling edge that occurs before 32 SCO periods elapses is ignored. Figure 3 details the format for the serial data being written to the AD7764 through the SDI pin. Thirty-two bits are required for a write operation. The first 16 bits select the register address that the data being read is intended for. The second 16 bits contain the data for the selected register. Writing to the AD7764 is allowed at any time, even while reading a conversion result. Note that, after writing to the device, valid data is not output until after the settling time for the filter elapses. The FILTER_SETTLE status bit is asserted at this point to indicate that the filter has settled and that valid data is available at the output. X means don’t care. If the DEC_RATE 1 bit is set to 1, the AD7764 is in decimate 128× mode. Rev. B | Page 23 of 33 AD7764 Data Sheet FUNCTIONALITY SYNCHRONIZATION OVERRANGE ALERTS The SYNC input to the AD7764 provides a synchronization function that allows the user to begin gathering samples of the analog front-end input from a known point in time. The AD7764 offers an overrange function in both a pin and status bit output. The overrange alerts indicate when the voltage applied to the AD7764 modulator input pins exceeds the limit set in the overrange register, indicating that the voltage applied is approaching a level that places the modulator in an overrange condition. To set this limit, the user must program the register. The default overrange limit is set to 80% of the VREF voltage (see the AD7764 Registers section). The AD7764 SYNC pin is polled by the falling edge of MCLK. The AD7764 device goes into SYNC when an MCLK falling edge senses that the SYNC input signal is logic low. At this point, the digital filter sequencer is reset to 0. The filter is held in a reset state (in SYNC mode) until the first MCLK falling edge senses SYNC to be logic high Where possible, ensure that all transitions of SYNC occur synchronously with the rising edge of MCLK (that is, as far away as possible from the MCLK falling edge, or decision edge). Otherwise, abide by the timing specified in Figure 43, which excludes the SYNC rising edge from occurring in a 10 ns window centered around the MCLK fallings edge. Keep SYNC logic low for a minimum of four MCLK periods. When the MCLK falling edge senses that SYNC has returned to logic high, the AD7764 filters begin to gather input samples simultaneously. The FSO falling edges are also synchronized, allowing for simultaneous output of conversion data. The OVR status bit is output as Bit D6 on SDO during a data conversion and can be checked in the AD7764 status register. This bit is less dynamic than the OVERRANGE pin output. It is updated on each conversion result output; that is, the bit changes at the output data rate. If the modulator samples a voltage input that exceeds the overrange limit during the process of gathering samples for a particular conversion result output, then the OVR bit is set to logic high. LOGIC LEVEL HIGH LOW t OUTPUT FREQUENCY OF FIR FILTER 1 = ICLK/4 MCLK OUTPUT DATA RATE (ODR) (ICLK/DECIMATION RATE OVERRANGE LIMIT tS SETUP SYNC OVR BIT tS HOLD LOGIC LEVEL 06518-303 tS MIN 4 × tMCLK Figure 43. SYNC Timing Relative to MCLK Following a SYNC pulse, the digital filter needs time to settle before valid data can be read from the AD7764. To ensure there is valid data on the SDO line, by check the FILTER_SETTLE status bit (see D7 in Table 12) that is output with each conversion result. The time from the rising edge of SYNC until the FILTERSETTLE bit asserts depends on the filter configuration used. See the Theory of Operation section and the values listed in Table 8 and Table 9 for details on calculating the time until FILTER_ SETTLE asserts. Note that the FILTER_SETTLE bit is designed as a reactionary flag to indicate when the conversion data output is valid. OVERRANGE LIMIT OBSOLUTE INPUT TO AD7764 [(VIN+) – (VIN–)] HIGH LOW t 06518-016 In the case of a system with multiple AD7764 devices, connect common MCLK, SYNC and RESET signals to each AD7764. The OVERRANGE pin outputs logic high to alert the user that the modulator has sampled an input voltage greater in magnitude than the overrange limit as set in the overrange register. The OVERRANGE pin is set to logic high when the modulator samples an input above the overrange limit. After the input returns below the limit, the OVERRANGE pin returns to zero. The OVERRANGE pin is updated after the first FIR filter stage. The output of OVERRANGE changes at the ICLK/4 frequency. OVERRANGE PIN OUTPUT The SYNC function allows multiple AD7764 devices, operated from the same master clock that use common SYNC and RESET signals, to be synchronized so that each ADC simultaneously updates its output register. Note that all devices being synchronized must operate in the same power mode and at the same decimation rate. Figure 44. OVERRANGE Pin and OVR Bit vs. Absolute Voltage Applied to the Modulator The output points from FIR Filter 1 in Figure 44 are not drawn to scale relative to the output data rate points. The FIR Filter 1 output is updated either 16×, 32×, or 64× faster than the output data rate, depending on the decimation rate in operation. Rev. B | Page 24 of 33 Data Sheet AD7764 POWER MODES The best practice is to ensure that all transitions of RESET occur synchronously with the falling edge of MCLK; otherwise, adhere to the timing requirements shown in Figure 46. Low Power Mode During power-up, the AD7764 defaults to operate in normal power mode. There is no register write required. Keep RESET at logic low for a minimum of one MCLK period for a valid reset to occur. The AD7764 also offers low power mode. To operate the device in low power mode, set the LPWR bit in the control register to logic high (see Figure 45). Operating the AD7764 in low power mode has no impact on the output data rate or available bandwidth. In cases where multiple AD7764 devices are being synchronized using the SYNC pulse and in the case of daisy-chaining multiple AD7764 devices, a common RESET pulse must be provided in addition to the common SYNC and MCLK signals. SCO (O) 32 × tSCO MCLK FSI (I) LOW POWER MODE DATA 0x0010 Figure 45. Write Scheme for Low Power Mode tR SETUP RESET 06518-304 CONTROL REGISTER ADDRESS 0x0001 06518-017 SDI (I) tR MIN RESET/PWRDWN Mode 1 × tMCLK The AD7764 features a RESET/PWRDWN pin. Holding the input to this pin logic low places the AD7764 in power-down mode. All internal circuitry is reset. Apply a RESET pulse to the AD7764 after initial power-up of the device. The AD7764 RESET pin is polled by the rising edge of MCLK. The AD7764 device goes into reset when an MCLK rising senses the RESET input signal to be logic low. AD7764 comes out of RESET on the first MCLK rising edge that senses RESET to be logic high. tR HOLD Figure 46. RESET Timing Synchronous to MCLK DECIMATION RATE PIN The decimation rate of the AD7764 is selected using the DEC_RATE pin. Table 14 shows the voltage input settings required for each of the three decimation rates. Table 14. DEC_RATE Pin Settings Decimation Rate 64× 128× 256× Rev. B | Page 25 of 33 DEC_RATE Pin State DVDD Floating Ground Maximum ODR (kHz) 312.5 156.25 78.125 AD7764 Data Sheet DAISY-CHAINING This 32-bit conversion result is then followed by the conversion results from the AD7764 (B), AD7764 (C), and AD7764 (D) devices with all conversion results output in an MSB-first sequence. The signals output from the daisy chain are the stream of conversion results from the SDO pin of AD7764 (A) and the FSO signal output by the first device in the chain, AD7764 (A). Daisy-chaining allows numerous devices to use the same digital interface lines. This feature is especially useful for reducing component count and wiring connections, such as in isolated multiconverter applications or for systems with a limited interfacing capacity. Data readback is analogous to clocking a shift register. When daisy-chaining is used, all devices in the chain must operate in a common power mode and at a common decimation rate. The falling edge of FSO signals the MSB of the first conversion output in the chain. FSO stays logic low throughout the 32 SCO clock periods needed to output the AD7764 (A) result and then goes logic high during the output of the conversion results from the AD7764 (B), AD7764 (C), and AD7764 (D devices. The block diagram in Figure 47 shows how to connect devices to achieve daisy-chain functionality. Figure 47 shows four AD7764 devices daisy-chained together with a common MCLK signal applied. This configuration works in decimate 128× or decimate 256× mode only. The maximum number of devices that can be daisy-chained is dependent on the decimation rate selected. Calculate the maximum number of devices that can be daisy-chained by simply dividing the chosen decimation rate by 32 (the number of bits that must be clocked out for each conversion). Table 15 provides the maximum number of chained devices for each decimation rate. READING DATA IN DAISY-CHAIN MODE Referring to Figure 47, note that the SDO line of AD7764 (A) provides the output data from the chain of AD7764 converters. Also, note that for the last device in the chain, AD7764 (D), the SDI pin is connected to ground. All of the devices in the chain must use common MCLK and SYNC signals. Table 15. Maximum Daisy Chain Length for all Decimation Rates To enable the daisy-chain conversion process, apply a common SYNC pulse to all devices (see the Synchronization section). Decimation Rate 256× 128× 64× After a SYNC pulse is applied to all devices, the filter settling time must pass before the FILTER_SETTLE bit is asserted, indicating valid conversion data at the output of the chain of devices. As shown in Figure 48, the first conversion result is output from the device labeled AD7764 (A). Maximum Chain Length 8 devices 4 devices 2 devices FSI AD7764 (D) AD7764 (C) FSI AD7764 (B) FSI SDI SDO FSI SDI SYNC SDO SYNC MCLK FSI SDI SDO SYNC MCLK AD7764 (A) FSO SDI SDO SYNC MCLK MCLK 06518-018 SYNC MCLK Figure 47. Daisy-Chaining Four Devices in Decimate 128× Mode Using a 40 MHz MCLK Signal 32 × tSCO 32 × tSCO 32 × tSCO 32 × tSCO AD7764 (A) 32-BIT OUTPUT AD7764 (B) 32-BIT OUTPUT AD7764 (C) 32-BIT OUTPUT AD7764 (D) 32-BIT OUTPUT SDI (A) = SDO (B) AD7764 (B) AD7764 (C) AD7764 (D) SDI (B) = SDO (C) AD7764 (C) AD7764 (D) SDI (C) = SDO (D) AD7764 (D) SCO SDO (A) AD7764 (A) 32-BIT OUTPUT AD7764 (B) 32-BIT OUTPUT AD7764 (B) AD7764 (C) AD7764 (C) AD7764 (D) AD7764 (D) Figure 48. Daisy-Chain Mode, Data Read Timing Diagram (for the Daisy-Chain Configuration Shown in Figure 47) Rev. B | Page 26 of 33 06518-019 FSO (A) Data Sheet AD7764 WRITING DATA IN DAISY-CHAIN MODE Writing to AD7764 devices in daisy-chain mode is similar to writing to a single device. The serial writing operation is synchronous to the SCO signal. The status of the frame synchronization input, FSI, is checked on the falling edge of the SCO signal. If the FSI line is low, then the first data bit on the serial data in the SDI line is latched in on the next SCO falling edge. Writing data to the AD7764 in daisy-chain mode operates with the same timing structure as writing to a single device (see Figure 3). The difference between writing to a single device and writing to a number of daisy-chained devices is in the implementation of the FSI signal. The number of devices that are in the daisy chain determines the period for which the FSI signal must remain logic low. To write to n number of devices in the daisy chain, the period between the falling edge of FSI and the rising edge of FSI must be between 32 × (n − 1) to 32 × n SCO periods. For example, if three AD7764 devices are being written to in daisy-chain mode, FSI is logic low for between 32 × (3 − 1) to 32 × 3 SCO pulses. This means that the rising edge of FSI must occur between the 64th and 96th SCO periods. The AD7764 devices can be written to at any time. The falling edge of FSI overrides all attempts to read data from the SDO pin. In the case of a daisy chain, the FSI signal remaining logic low for more than 32 SCO periods indicates to the AD7764 device that there are more devices further on in the chain. This means that the AD7764 directs data that is input on the SDI pin to its SDO pin. This ensures that data is passed to the next device in the chain. FSI AD7764 (C) AD7764 (D) SDI SDI FSI FSI FSI SDO SDO SDI SDI FSI FSO SDI SDO SYNC MCLK MCLK MCLK SDO SYNC SYNC SYNC AD7764 (A) AD7764 (B) SCO MCLK 06518-020 SYNC MCLK Figure 49. Writing to an AD7764 Daisy-Chain Configuration FSI t10 32 × tSCO 32 × tSCO 32 × tSCO 31 × tSCO SCO SDI (C) = SDO (D) SDI (D) SDI (C) SDI (B) = SDO (C) SDI (B) SDI (A) = SDO (B) SDI (A) Figure 50. Daisy-Chain Write Timing Diagram; Writing to Four AD7764 Devices Rev. B | Page 27 of 33 06518-021 SDI (D) AD7764 Data Sheet CLOCKING THE AD7764 The AD7764 requires an external low jitter clock source. This signal is applied to the MCLK pin. An internal clock signal (ICLK) is derived from the MCLK input signal. The ICLK controls the internal operation of the AD7764. The maximum ICLK frequency is 20 MHz. To generate the ICLK, ICLK = MCLK/2 For output data rates equal to those used in audio systems, a 12.288 MHz ICLK frequency can be used. As shown in Table 6, output data rates of 96 kHz and 48 kHz are achievable with this ICLK frequency. The input amplitude also has an effect on these jitter figures. For example, if the input level is 3 dB below full scale, the allowable jitter is increased by a factor of √2, increasing the first example to 144.65 ps rms. This happens when the maximum slew rate is decreased by a reduction in amplitude. Figure 51 and Figure 52 illustrate this point, showing the maximum slew rate of a sine wave of the same frequency but with different amplitudes. MCLK JITTER REQUIREMENTS 1.0 0.5 The MCLK jitter requirements depend on a number of factors and are given by –0.5 where: OSR (the oversampling ratio) = fICLK/ODR. fIN is the maximum input frequency. SNR(dB) is the target SNR. 06518-022 t j(rms ) 0 OSR = SNR (dB) 2 × π × f IN × 10 20 –1.0 Figure 51. Maximum Slew Rate of a Sine Wave with an Amplitude of 2 V p-p Example 1 1.0 Take Example 1 from Table 6, where ODR = 312.5 kHz. fICLK = 20 MHz. fIN (max) = 156.25 kHz. SNR = 104 dB. t j (rms ) = 0.5 0 64 = 51.41 ps 2 × π × 156.25 × 10 3 ×10 5.2 Example 2 –1.0 Figure 52. Maximum Slew Rate of the Same Frequency Sine Wave as in Figure 51 with an Amplitude of 1 V p-p Take Example 2 from Table 6, where ODR = 48 kHz. fICLK = 12.288 MHz. fIN (max) = 19.2 kHz. SNR = 109 dB. t j(rms ) = 06518-023 –0.5 This is the maximum allowable clock jitter for a full-scale, 156.25 kHz input tone with the given ICLK and output data rate. 256 = 470 ps 2 × π × 19.2 × 103 × 105.45 Rev. B | Page 28 of 33 Data Sheet AD7764 DECOUPLING AND LAYOUT INFORMATION ADR444 The decoupling of the supplies applied to the AD7764 is important in achieving maximum performance. Each supply pin must be decoupled to the correct ground pin with a 100 nF, 0603 case size capacitor. Pay particular attention to decoupling Pin 7 (AVDD2) directly to the nearest ground pin (Pin 8). The digital ground pin, AGND2 (Pin 20), is routed directly to ground. Also, connect REFGND (Pin 26) directly to ground. Decouple the DVDD (Pin 17) and AVDD3 (Pin 28) supplies to the ground plane at a point away from the device. It is recommended to decouple the supplies that are connected to the following supply pins through 0603 size, 100 nF capacitors to a star ground point linked to Pin 23 (AGND1): VREF+ (Pin 27) AVDD4 (Pin 25) AVDD1 (Pin 24) AVDD2 (Pin 21) 10µF 100µF 100nF The components recommended for use around the on-chip differential amplifier are detailed in Table 10. Matching the components on both sides of the differential amplifier is important to minimize distortion of the signal applied to the amplifier. A tolerance of 0.1% or better is required for these components. Symmetrical routing of the tracks on both sides of the differential amplifier also assists in achieving the stated performance. Figure 55 shows a typical layout for the components around the differential amplifier. Note that the traces for both differential paths are as symmetrical as possible and that the feedback resistors and capacitors are placed on the underside of the PCB to enable the simplest routing. RIN RFB CFB VINA– VINA+ GND RIN VREF + (PIN 27) Figure 55. Typical Layout Structure for Components Surrounding the Differential Amplifier AVDD1 (PIN 24) LAYOUT CONSIDERATIONS VIA TO GND FROM PIN 20 06518-133 AVDD2 (PIN 21) GND PIN 15 VREF + PIN 27 + GND 4 100nF 200Ω DIFFERENTIAL AMPLIFIER COMPONENTS AVDD3 (PIN 28) PIN 23 STAR-POINT GND 6 Figure 54. Reference Connection A layout decoupling scheme for these supplies, which connect to the right side of the AD7764, is shown in Figure 53. Note the star point ground created at Pin 23. AVDD4 (PIN 25) + VOUT VIN 06518-135 • • • • 2 7.5V 06518-134 SUPPLY DECOUPLING Figure 53. Supply Decoupling REFERENCE VOLTAGE FILTERING A low noise reference source, such as the ADR444 or ADR434 (4.096 V), is suitable for use with the AD7764. Decouple and filter the reference voltage supplied to the AD7764 as shown in Figure 54. The recommended scheme for the reference voltage supply is a 200 Ω series resistor connected to a 100 μF tantalum capacitor, followed by a 10 nF decoupling capacitor very close to the VREF+ pin. The use of correct components is essential to achieve optimum performance, and the correct layout is equally important. The AD7764 product page contains the Gerber files for the AD7764 evaluation board. Use the Gerber files as a reference when designing any system using the AD7764. Carefully consider the use of ground planes. To ensure that the return currents through the decoupling capacitors are flowing to the correct ground pin, place the ground side of the capacitors as close as possible to the ground pin associated with that supply, as recommended in the Supply Decoupling section. Rev. B | Page 29 of 33 AD7764 Data Sheet USING THE AD7764 Use the following sequence to power up and use the AD7764: 1. 2. 3. 4. 5. Apply power to the device. Apply the MCLK signal. Take RESET low for a minimum of one MCLK cycle, preferably synchronous to the falling MCLK edge. If multiple devices are to be synchronized, apply a common RESET to all devices. Wait a minimum of two MCLK cycles after RESET is released. If multiple devices are being synchronized, a SYNC pulse must be applied to the devices, preferably synchronous with the MCLK rising edge. In the case where devices are not being synchronized, no SYNC pulse is required; apply a logic high signal to the SYNC pin. Data can then be read from the device using the default gain and overrange threshold values. The conversion data read is not valid, however, until the settling time of the filter elapses. After this time elapses, the FILTER_SETTLE status bit is set, indicating that the data is valid. Values for the gain and overrange thresholds can be written to or read from the respective registers at this stage. BIAS RESISTOR SELECTION The AD7764 requires a resistor to be connected between the RBIAS and AGNDx pins. Select the resistor value to give a current of 25 µA through the resistor to ground. For a 4.096 V reference voltage, the correct resistor value is 160 kΩ. When applying the SYNC pulse, • • The issue of a SYNC pulse to the device must not coincide with a write to the device. Ensure that the SYNC pulse is taken low for a minimum of four MCLK periods. Rev. B | Page 30 of 33 Data Sheet AD7764 AD7764 REGISTERS registers. Writing to these registers involves writing the register address followed by a 16-bit data-word. The register addresses, details of the individual bits, and default values are provided in the following sections. The AD7764 has a number of user-programmable registers. The control register sets the functionality of the on-chip buffer and differential amplifier and provides the option to power down the AD7764. There are also digital gain and overrange threshold CONTROL REGISTER Table 16. Control Register (Address 0x0001, Default Value 0x0000) MSB D15 0 D14 RD OVR D13 RD GAIN D12 0 D11 RD STAT D10 0 D9 SYNC D8 0 D7 BYPASS REF D6 0 D5 0 D4 0 D3 PWR DOWN D2 LPWR D1 REF BUF OFF LSB D0 AMP OFF Table 17. Bit Descriptions of the Control Register Bit 14 Mnemonic RD OVR 1, 2 13 11 9 RD GAIN2 RD STAT2 SYNC1 7 3 2 1 0 BYPASS REF PWR DOWN LPWR REF BUF OFF AMP OFF 1 2 Description Read overrange. If this bit is set, the next read operation outputs the contents of the overrange threshold register instead of a conversion result. Read gain. If this bit is set, the next read operation outputs the contents of the digital gain register. Read status. If this bit is set, the next read operation outputs the contents of the status register. Synchronize. Setting this bit initiates an internal synchronization routine. Setting this bit simultaneously on multiple devices synchronizes all filters. Bypass reference. Setting this bit bypasses the reference buffer if the buffer is off. Power-down. A logic high powers the device down without resetting. Writing a 0 to this bit powers the device back up. Low power mode. Set this bit to Logic 1 when the AD7764 is in low power mode. Reference buffer off. Asserting this bit powers down the reference buffer. Amplifier off. Asserting this bit switches the differential amplifier off. Bit 14 to Bit 11 and Bit 9 are self-clearing bits. Only one of the bits from D14 to D11 can be set in any write operation. The user must select only one function from these bits. That bit, one of Bit D14 to D11, read determines the contents of the data output within the next FSO frame on the SDO pin. STATUS REGISTER Table 18. Status Register (Read Only) MSB D15 PARTNO D14 1 D13 0 D12 0 D11 1 D10 FILTER_SETTLE D9 LPWR D8 OVR D7 0 D6 1 D5 0 D4 REF BUF OFF D3 AMP OFF D2 0 D1 DEC 1 LSB D0 DEC 0 Table 19. Bit Descriptions of the Status Register Bits 15 10 Mnemonic PARTNO FILTER_SETTLE 9 8 4 3 2 1 0 LPWR OVR REF BUF OFF AMP OFF 0 DEC_RATE 1 DEC_RATE 0 Description Part number. This bit is set to 1 for the AD7764. Filter settling bit. This bit corresponds to the FILTER_SETTLE bit in the status word output in the second 16-bit read operation. It indicates when data is valid. Low power mode. This bit is set when operating in low power mode. Overrange. If the current analog input exceeds the current overrange threshold, this bit is set. Reference buffer off. This bit is set when the reference buffer is disabled. Amplifier off. This bit is set when the input amplifier is disabled. Zero. This bit is set to Logic 0. Decimation rate. This bit corresponds to the decimation rate in use. Decimation rate. This bit corresponds to the decimation rate in use. Rev. B | Page 31 of 33 AD7764 Data Sheet GAIN REGISTER—ADDRESS 0x0004 OVERRANGE REGISTER—ADDRESS 0x0005 Nonbit Mapped, Default Value: 0xA000 Nonbit Mapped, Default Value: 0xCCCC The gain register is scaled such that 0x8000 corresponds to a gain of 1.0. The default value of this register is 1.25 (0xA000). This value results in a full-scale digital output when the input is at 80% of VREF+, tying in with the maximum analog input range of ±80% of VREF+ p-p. The overrange register value is compared to the output of the first decimation filter to obtain an overload indication with minimum propagation delay. This comparison is prior to any gain scaling. The default value is 0xCCCC, which corresponds to 80% of VREF+ (the maximum permitted analog input voltage). Assuming VREF+ = 4.096 V, the bit is then set when the input voltage exceeds approximately 6.55 V p-p differential. The overrange bit is set immediately if the analog input voltage exceeds 100% of VREF+ for more than four consecutive samples at the modulator rate. Rev. B | Page 32 of 33 Data Sheet AD7764 OUTLINE DIMENSIONS 9.80 9.70 9.60 28 15 4.50 4.40 4.30 6.40 BSC 1 14 PIN 1 0.65 BSC 0.15 0.05 COPLANARITY 0.10 0.30 0.19 1.20 MAX SEATING PLANE 0.20 0.09 8° 0° 0.75 0.60 0.45 COMPLIANT TO JEDEC STANDARDS MO-153-AE Figure 56. 28-Lead Thin Shrink Small Outline [TSSOP] (RU-28) Dimensions shown in millimeters ORDERING GUIDE Model 1 AD7764BRUZ AD7764BRUZ-REEL7 EVAL-AD7764EDZ 1 Temperature Range –40°C to +85°C –40°C to +85°C Package Description 28-Lead Thin Shrink Small Outline [TSSOP] 28-Lead Thin Shrink Small Outline [TSSOP] Evaluation Board Z = RoHS Compliant Part. ©2007–2017 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D06518-0-3/17(B) Rev. B | Page 33 of 33 Package Option RU-28 RU-28