3-Channel, Low Noise, Low Power, 20-Bit ∑-Δ ADC with On-Chip In-Amp and Reference AD7785 FEATURES Industrial process control Instrumentation Portable instrumentation Blood analysis Smart transmitters Liquid/gas chromatography 6-digit DVM Up to 20 bits effective resolution RMS noise 40 nV @ 4.17 Hz 85 nV @ 16.7 Hz Current: 400 μA typical Power-down: 1 μA maximum Low noise programmable gain instrumentation amp Band gap reference with 4 ppm/°C drift typical Update rate: 4.17 Hz to 470 Hz 3 differential inputs Internal clock oscillator Simultaneous 50 Hz/60 Hz rejection Programmable current sources On-chip bias voltage generator Burnout currents Power supply: 2.7 V to 5.25 V –40°C to +105°C temperature range Independent interface power supply 16-lead TSSOP package Interface 3-wire serial SPI®, QSPI™, MICROWIRE™, and DSP compatible Schmitt trigger on SCLK GENERAL DESCRIPTION The AD7785 is a low power, low noise, complete analog front end for high precision measurement applications. The AD7785 contains a low noise 20-bit ∑-Δ ADC with three differential analog inputs. The on-chip, low noise instrumentation amplifier means that signals of small amplitude can be interfaced directly to the ADC. With a gain setting of 64, the rms noise is 40 nV when the update rate equals 4.17 Hz. The device contains a precision low noise, low drift internal band gap reference and can accept an external differential reference. Other on-chip features include programmable excitation current sources, burnout currents, and a bias voltage generator. The bias voltage generator sets the common-mode voltage of a channel to AVDD/2. The AD7785 can be operated with either the internal clock or an external clock. The output data rate from the device is software-programmable and can be varied from 4.17 Hz to 470 Hz. The device operates with a power supply from 2.7 V to 5.25 V. It consumes a current of 400 μA typical and is housed in a 16-lead TSSOP package. APPLICATIONS Thermocouple measurements RTD measurements Thermistor measurements Gas analysis FUNCTIONAL BLOCK DIAGRAM AVDD REFIN(+)/AIN3(+) REFIN(–)/AIN3(–) VBIAS BAND GAP REFERENCE GND AVDD AIN1(+) AIN1(–) AIN2(+) AIN2(–) MUX BUF Σ-Δ ADC IN-AMP SERIAL INTERFACE AND CONTROL LOGIC AVDD GND IOUT1 DOUT/RDY DIN SCLK CS DVDD INTERNAL CLOCK AD7785 IOUT2 CLK 06721-001 GND Figure 1. Rev. 0 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 Analog Devices, Inc. All rights reserved. AD7785 TABLE OF CONTENTS Features .............................................................................................. 1 Offset Register ............................................................................ 19 Applications....................................................................................... 1 Full-Scale Register...................................................................... 19 General Description ......................................................................... 1 ADC Circuit Information.............................................................. 20 Functional Block Diagram .............................................................. 1 Overview ..................................................................................... 20 Revision History ............................................................................... 2 Digital Interface.......................................................................... 21 Specifications..................................................................................... 3 Circuit Description......................................................................... 25 Timing Characteristics..................................................................... 6 Analog Input Channel ............................................................... 25 Timing Diagrams.......................................................................... 7 Instrumentation Amplifier........................................................ 25 Absolute Maximum Ratings............................................................ 8 Bipolar/Unipolar Configuration .............................................. 25 ESD Caution.................................................................................. 8 Data Output Coding .................................................................. 25 Pin Configuration and Function Descriptions............................. 9 Burnout Currents ....................................................................... 26 Output Noise and Resolution Specifications .............................. 11 Excitation Currents .................................................................... 26 External Reference...................................................................... 11 Bias Voltage Generator .............................................................. 26 Internal Reference ...................................................................... 12 Reference ..................................................................................... 26 Typical Performance Characteristics ........................................... 13 Reset ............................................................................................. 26 On-Chip Registers .......................................................................... 14 AVDD Monitor ............................................................................. 27 Communications Register......................................................... 14 Calibration................................................................................... 27 Status Register ............................................................................. 15 Grounding and Layout .............................................................. 27 Mode Register ............................................................................. 15 Applications Information .............................................................. 29 Configuration Register .............................................................. 17 Temperature Measurement using a Thermocouple............... 29 Data Register ............................................................................... 18 Temperature Measurement using an RTD.............................. 30 ID Register................................................................................... 18 Outline Dimensions ....................................................................... 31 IO Register................................................................................... 18 Ordering Guide .......................................................................... 31 REVISION HISTORY 4/07—Revision 0: Initial Version Rev. 0 | Page 2 of 32 AD7785 SPECIFICATIONS AVDD = 2.7 V to 5.25 V; DVDD = 2.7 V to 5.25 V; GND = 0 V; all specifications TMIN to TMAX, unless otherwise noted. Table 1. Parameter ADC CHANNEL Output Update Rate No Missing Codes 2 Resolution Output Noise and Update Rates Integral Nonlinearity Offset Error 3 Offset Error Drift vs. Temperature 4 Full-Scale Error3, 5 Gain Drift vs. Temperature4 Power Supply Rejection ANALOG INPUTS Differential Input Voltage Ranges Absolute AIN Voltage Limits2 Unbuffered Mode Buffered Mode In-Amp Active Common-Mode Voltage, VCM Analog Input Current Buffered Mode or In-Amp Active Average Input Current2 Average Input Current Drift Unbuffered Mode Average Input Current Average Input Current Drift Normal Mode Rejection2 Internal Clock @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz External Clock @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz Common-Mode Rejection @ DC @ 50 Hz, 60 Hz2 @ 50 Hz, 60 Hz2 AD7785B 1 Unit 4.17 to 470 20 Hz nom Bits min Test Conditions/Comments See Output Noise and Resolution Specifications See Output Noise and Resolution Specifications ±15 ±1 ±10 ±10 ±1 ±3 100 ppm of FSR max μV typ nV/°C typ μV typ ppm/°C typ ppm/°C typ dB min ±VREF/Gain V nom VREF = REFIN(+) − REFIN(−) or internal reference, gain = 1 to 128 GND – 30 mV AVDD + 30 mV GND + 100 mV AVDD – 100 mV GND + 300 mV AVDD – 1.1 0.5 V min V max V min V max V min V max V min Gain = 1 or 2 ±1 ±250 ±2 nA max pA max pA/°C typ Gain = 1 to 16, external reference Gain = 32 to 128, external reference AIN = 1 V/gain, gain ≥ 4, external reference Gain = 1 or 2 Gain = 4 to 128 VCM = (AIN(+) + AIN(−))/2, gain = 4 to 128 Gain = 1 or 2, update rate < 100 Hz Gain = 4 to 128, update rate < 100 Hz Gain = 1 or 2 Input current varies with input voltage ±400 ±50 nA/V typ pA/V/°C typ 65 80 90 dB min dB min dB min 80 dB typ, 50 ± 1 Hz, 60 ± 1 Hz, FS[3:0] = 1010 6 90 dB typ, 50 ± 1 Hz, FS[3:0] = 10016 100 dB typ, 60 ± 1 Hz, FS[3:0] = 10006 80 94 90 dB min dB min dB min 90 dB typ, 50 ± 1 Hz, 60 ± 1 Hz, FS[3:0] = 10106 100 dB typ, 50 ± 1 Hz, FS[3:0] = 10016 100 dB typ, 60 ± 1 Hz, FS[3:0] = 10006 100 100 100 dB min dB min dB min AIN = 1 V/gain, gain ≥ 4 50 ± 1 Hz, 60 ± 1 Hz, FS[3:0] = 10106 50 ± 1 Hz (FS[3:0] = 1001)6, 60 ± 1 Hz (FS[3:0] = 1000)6 Rev. 0 | Page 3 of 32 AD7785 Parameter REFERENCE Internal Reference Internal Reference Initial Accuracy Internal Reference Drift2 Power Supply Rejection External Reference External REFIN Voltage Reference Voltage Range2 Absolute REFIN Voltage Limits2 Average Reference Input Current Average Reference Input Current Drift Normal Mode Rejection Common-Mode Rejection EXCITATION CURRENT SOURCES (IEXC1 and IEXC2) Output Current Initial Tolerance at 25°C Drift Current Matching Drift Matching Line Regulation (VDD) Load Regulation Output Compliance TEMPERATURE SENSOR Accuracy Sensitivity BIAS VOLTAGE GENERATOR VBIAS VBIAS Generator Start-Up Time INTERNAL/EXTERNAL CLOCK Internal Clock Frequency2 Duty Cycle External Clock Frequency Duty Cycle AD7785B 1 Unit Test Conditions/Comments 1.17 ± 0.01% 4 15 85 V min/max ppm/°C typ ppm/°C max dB typ AVDD = 4 V, TA = 25°C 2.5 0.1 AVDD V nom V min V max REFIN = REFIN(+) − REFIN(−) GND − 30 mV AVDD + 30 mV 400 ±0.03 V min V max nA/V typ nA/V/°C typ Same as for analog inputs 100 dB typ 10/210/1000 ±5 200 ±0.5 50 2 0.2 AVDD − 0.65 AVDD − 1.1 GND − 30 mV μA nom % typ ppm/°C typ % typ ppm/°C typ %/V typ %/V typ V max V max V min ±2 0.81 °C typ mV/°C typ Applies if user calibrates the temperature sensor Applies if user calibrates the temperature sensor AVDD/2 See Figure 10 V nom ms/nF typ Dependent on the capacitance on the AIN pin 64 ± 3% 50:50 kHz min/max % typ 64 kHz nom 45:55 to 55:45 % typ 0.8 0.4 2.0 V max V max V min When VREF = AVDD, the differential input must be limited to 0.9 × VREF /gain if the in-amp is active Matching between IEXC1 and IEXC2; VOUT = 0 V AVDD = 5 V ± 5% 10 μA or 210 μA currents selected 1 mA currents selected A 128 kHz external clock can be used if the divide-by-2 function is used (Bit CLK1 = CLK0 = 1) Applies for external 64 kHz clock; a 128 kHz clock can have a less stringent duty cycle LOGIC INPUTS CS2 VINL, Input Low Voltage VINH, Input High Voltage Rev. 0 | Page 4 of 32 DVDD = 5 V DVDD = 3 V DVDD = 3 V or 5 V AD7785 Parameter SCLK, CLK, and DIN (SchmittTriggered Input)2 VT(+) VT(–) VT(+) − VT(−) VT(+) VT(–) VT(+) − VT(−) Input Currents Input Capacitance LOGIC OUTPUTS (INCLUDING CLK) VOH, Output High Voltage2 VOL, Output Low Voltage2 VOH, Output High Voltage2 VOL, Output Low Voltage2 Floating-State Leakage Current Floating-State Output Capacitance Data Output Coding SYSTEM CALIBRATION2 Full-Scale Calibration Limit Zero-Scale Calibration Limit Input Span POWER REQUIREMENTS 7 Power Supply Voltage AVDD to GND DVDD to GND Power Supply Currents IDD Current IDD (Power-Down Mode) AD7785B 1 Unit Test Conditions/Comments 1.4/2 0.8/1.7 0.1/0.17 0.9/2 0.4/1.35 0.06/0.13 V min/V max V min/V max V min/V max V min/V max V min/V max V min/V max DVDD = 5 V DVDD = 5 V DVDD = 5 V DVDD = 3 V DVDD = 3 V DVDD = 3 V ±10 10 μA max pF typ VIN = DVDD or GND All digital inputs DVDD − 0.6 0.4 4 0.4 V min V max V min V max DVDD = 3 V, ISOURCE = 100 μA DVDD = 3 V, ISINK = 100 μA DVDD = 5 V, ISOURCE = 200 μA DVDD = 5 V, ISINK = 1.6 mA (DOUT/RDY)/ 800 μA (CLK) ±10 10 Offset binary μA max pF typ +1.05 × FS −1.05 × FS 0.8 × FS 2.1 × FS V max V min V min V max 2.7/5.25 2.7/5.25 V min/max V min/max 140 μA max 185 μA max 400 μA max 500 μA max 1 μA max 1 110 μA typ @ AVDD = 3 V, 125 μA typ @ AVDD = 5 V, unbuffered mode, external reference 130 μA typ @ AVDD = 3 V, 165 μA typ @ AVDD = 5 V, buffered mode, gain = 1 or 2, external reference 300 μA typ @ AVDD = 3 V, 350 μA typ @ AVDD = 5 V, gain = 4 to 128, external reference 400 μA typ @ AVDD = 3 V, 450 μA typ @ AVDD = 5 V, gain = 4 to 128, internal reference Temperature range is –40°C to +105°C. Specification is not production tested, but is supported by characterization data at initial product release. Following a calibration, this error is in the order of the noise for the programmed gain and update rate selected. 4 Recalibration at any temperature removes these errors. 5 Full-scale error applies to both positive and negative full-scale and applies at the factory calibration conditions (AVDD = 4 V, gain = 1, TA = 25°C). 6 FS[3:0] are the four bits used in the mode register to select the output word rate. 7 Digital inputs equal to DVDD or GND with excitation currents and bias voltage generator disabled. 2 3 Rev. 0 | Page 5 of 32 AD7785 TIMING CHARACTERISTICS AVDD = 2.7 V to 5.25 V, DVDD = 2.7 V to 5.25 V, GND = 0 V, Input Logic 0 = 0 V, Input Logic 1 = DVDD, unless otherwise noted. Table 2. Parameter 1, 2 t3 t4 Read Operation t1 t2 3 t5 5, 6 t6 t7 Write Operation t8 t9 t10 t11 Limit at TMIN, TMAX (B Version) 100 100 Unit ns min ns min Conditions/Comments SCLK high pulse width SCLK low pulse width 0 60 80 0 60 80 10 80 0 10 ns min ns max ns max ns min ns max ns max ns min ns max ns min ns min CS falling edge to DOUT/RDY active time DVDD = 4.75 V to 5.25 V DVDD = 2.7 V to 3.6 V SCLK active edge to data valid delay 4 DVDD = 4.75 V to 5.25 V DVDD = 2.7 V to 3.6 V Bus relinquish time after CS inactive edge SCLK inactive edge to CS inactive edge SCLK inactive edge to DOUT/RDY high 0 30 25 0 ns min ns min ns min ns min CS falling edge to SCLK active edge setup time4 Data valid to SCLK edge setup time Data valid to SCLK edge hold time CS rising edge to SCLK edge hold time 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 1.6 V. See Figure 3 and Figure 4. 3 These numbers are measured with the load circuit shown in Figure 2 and defined as the time required for the output to cross the VOL or VOH limits. 4 SCLK active edge is the falling edge of SCLK. 5 These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit shown in Figure 2. The measured number is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus relinquish times of the part and, as such, are independent of external bus loading capacitances. 6 RDY returns high after a read of the ADC. In single conversion mode and continuous conversion mode, the same data can be read again, if required, while RDY is high, although care should be taken to ensure that subsequent reads do not occur close to the next output update. In continuous read mode, the digital word can be read only once. 2 ISINK (1.6mA WITH DVDD = 5V, 100µA WITH DVDD = 3V) TO OUTPUT PIN 1.6V ISOURCE (200µA WITH DVDD = 5V, 100µA WITH DVDD = 3V) Figure 2. Load Circuit for Timing Characterization Rev. 0 | Page 6 of 32 06721-002 50pF AD7785 TIMING DIAGRAMS CS (I) t6 t1 t5 MSB DOUT/RDY (O) LSB t7 t2 t3 06721-003 SCLK (I) t4 NOTES 1. I = INPUT, O = OUTPUT Figure 3. Read Cycle Timing Diagram CS (I) t11 t8 SCLK (I) t9 t10 MSB LSB NOTES 1. I = INPUT, O = OUTPUT Figure 4. Write Cycle Timing Diagram Rev. 0 | Page 7 of 32 06721-004 DIN (I) AD7785 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 3. Parameter AVDD to GND DVDD to GND Analog Input Voltage to GND Reference Input Voltage to GND Digital Input Voltage to GND Digital Output Voltage to GND AIN/Digital Input Current Operating Temperature Range Storage Temperature Range Maximum Junction Temperature TSSOP θJA Thermal Impedance θJC Thermal Impedance Lead Temperature, Soldering Reflow Rating −0.3 V to +7 V −0.3 V to +7 V −0.3 V to AVDD + 0.3 V −0.3 V to AVDD + 0.3 V −0.3 V to DVDD + 0.3 V −0.3 V to DVDD + 0.3 V 10 mA −40°C to +105°C −65°C to +150°C 150°C 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 listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ESD CAUTION 150.4°C/W 27.6°C/W 260°C Rev. 0 | Page 8 of 32 AD7785 SCLK 1 16 DIN CLK 2 15 DOUT/RDY CS 3 14 DVDD IOUT1 4 AIN1(+) 5 AD7785 13 AVDD TOP VIEW (Not to Scale) 12 GND IOUT2 AIN1(–) 6 11 AIN2(+) 7 10 REFIN(–)/AIN3(–) AIN2(–) 8 9 REFIN(+)/AIN3(+) 06721-005 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS Figure 5. Pin Configuration Table 4. Pin Function Descriptions Pin No. 1 Mnemonic SCLK 2 CLK 3 CS 4 IOUT1 5 6 7 8 9 AIN1(+) AIN1(−) AIN2(+) AIN2(−) REFIN(+)/AIN3(+) 10 REFIN(−)/AIN3(−) 11 IOUT2 12 13 14 GND AVDD DVDD Description Serial Clock Input. This serial clock input is for data transfers to and from the ADC. The SCLK has a Schmitttriggered input, making the interface suitable for opto-isolated applications. The serial clock can be continuous with all data transmitted in a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information being transmitted to or from the ADC in smaller batches of data. Clock In/Clock Out. The internal clock can be made available at this pin. Alternatively, the internal clock can be disabled, and the ADC can be driven by an external clock. This allows several ADCs to be driven from a common clock, allowing simultaneous conversions to be performed. Chip Select Input. This is an active low logic input used to select the ADC. CS can be used to select the ADC in systems with more than one device on the serial bus or as a frame synchronization signal in communicating with the device. CS can be hardwired low, allowing the ADC to operate in 3-wire mode with SCLK, DIN, and DOUT used to interface with the device. Output of Internal Excitation Current Source. The internal excitation current source can be made available at this pin. The excitation current source is programmable so that the current can be 10 μA, 210 μA, or 1 mA. Either IEXC1 or IEXC2 can be switched to this output. Analog Input. AIN1(+) is the positive terminal of the differential analog input pair AIN1(+)/AIN1(−). Analog Input. AIN1(−) is the negative terminal of the differential analog input pair AIN1(+)/AIN1(−). Analog Input. AIN2(+) is the positive terminal of the differential analog input pair AIN2(+)/AIN2(−). Analog Input. AIN2(−) is the negative terminal of the differential analog input pair AIN2(+)/AIN2(−). Positive Reference Input/Analog Input. An external reference can be applied between REFIN(+) and REFIN(−). REFIN(+) can lie anywhere between AVDD and GND + 0.1 V. The nominal reference voltage REFIN(+) − REFIN(−) is 2.5 V, but the part functions with a reference from 0.1 V to AVDD. Alternatively, this pin can function as AIN3(+) where AIN3(+) is the positive terminal of the differential analog input pair AIN3(+)/AIN3(−). Negative Reference Input/Analog Input. REFIN(−) is the negative reference input for REFIN. This reference input can lie anywhere between GND and AVDD − 0.1 V. This pin also functions as AIN3(−), which is the negative terminal of the differential analog input pair AIN3(+)/AIN3(−). Output of Internal Excitation Current Source. The internal excitation current source can be made available at this pin. The excitation current source is programmable so that the current can be 10 μA, 210 μA, or 1 mA. Either IEXC1 or IEXC2 can be switched to this output. Ground Reference Point. Supply Voltage, 2.7 V to 5.25 V. Digital Interface Supply Voltage. The logic levels for the serial interface pins are related to this supply, which is between 2.7 V and 5.25 V. The DVDD voltage is independent of the voltage on AVDD; therefore, AVDD can equal 5 V with DVDD at 3 V or vice versa. Rev. 0 | Page 9 of 32 AD7785 Pin No. 15 Mnemonic DOUT/RDY 16 DIN Description Serial Data Output/Data Ready Output. DOUT/RDY serves a dual purpose. It functions as a serial data output pin to access the output shift register of the ADC. The output shift register can contain data from any of the on-chip data or control registers. In addition, DOUT/RDY operates as a data ready pin, going low to indicate the completion of a conversion. If the data is not read after the conversion, the pin goes high before the next update occurs. The DOUT/RDY falling edge can be used as an interrupt to a processor, indicating that valid data is available. With an external serial clock, the data can be read using the DOUT/RDY pin. With CS low, the data/control word information is placed on the DOUT/RDY pin on the SCLK falling edge and is valid on the SCLK rising edge. Serial Data Input. This serial data input is to the input shift register on the ADC. Data in this shift register is transferred to the control registers within the ADC; the register selection bits of the communications register identify the appropriate register. Rev. 0 | Page 10 of 32 AD7785 OUTPUT NOISE AND RESOLUTION SPECIFICATIONS EXTERNAL REFERENCE Table 5 shows the output rms noise of the AD7785 for some of the update rates and gain settings. The numbers given are for the bipolar input range with an external 2.5 V reference. These numbers are typical and are generated with a differential input voltage of 0 V. Table 6 shows the effective resolution, with the output peak-to-peak (p-p) resolution shown in parentheses. It is important to note that the effective resolution is calculated using the rms noise, while the p-p resolution is based on the p-p noise. The p-p resolution represents the resolution for which there is no code flicker. These numbers are typical and are rounded to the nearest LSB. Table 5. Output RMS Noise (μV) vs. Gain and Output Update Rate Using an External 2.5 V Reference Update Rate (Hz) 4.17 8.33 16.7 33.2 62 123 242 470 Gain of 1 0.64 1.04 1.55 2.3 2.95 4.89 11.76 11.33 Gain of 2 0.6 0.96 1.45 2.13 2.85 4.74 9.5 9.44 Gain of 4 0.29 0.38 0.54 0.74 0.92 1.49 4.02 3.07 Gain of 8 0.22 0.26 0.36 0.5 0.58 1 1.96 1.79 Gain of 16 0.1 0.13 0.18 0.23 0.29 0.48 0.88 0.99 Gain of 32 0.065 0.078 0.11 0.17 0.2 0.32 0.45 0.63 Gain of 64 0.039 0.057 0.087 0.124 0.153 0.265 0.379 0.568 Gain of 128 0.041 0.055 0.086 0.118 0.144 0.283 0.397 0.593 Gain of 64 20 (18.5) 20 (18) 20 (17.5) 19 (16.5) 19 (16.5) 18 (15.5) 17.5 (15) 17 (14.5) Gain of 128 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17 (14.5) 16.5 (14) 16 (13.5) Table 6. Typical Resolution (Bits) vs. Gain and Output Update Rate Using an External 2.5 V Reference Update Rate (Hz) 4.17 8.33 16.7 33.2 62 123 242 470 Gain of 1 20 (20) 20 (19.5) 20 (19) 20 (18.5) 20 (18) 20 (17.5) 18.5 (16) 18.5 (16) Gain of 2 20 (19.5) 20 (19) 20 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18 (15.5) 18 (15.5) Gain of 4 20 (19.5) 20 (19) 20 (18.5) 20 (18) 20 (18) 19.5 (17) 18 (15.5) 18.5 (16) Gain of 8 20 (19) 20 (18.5) 20 (18) 20 (17.5) 20 (17.5) 19 (16.5) 18 (15.5) 18.5 (16) Rev. 0 | Page 11 of 32 Gain of 16 20 (19) 20 (18.5) 20 (18) 20 (18) 20 (17.5) 19.5 (17) 18.5 (16) 18 (15.5) Gain of 32 20 (18.5) 20 (18.5) 20 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) AD7785 INTERNAL REFERENCE Table 7 shows the output rms noise of the AD7785 for some of the update rates and gain settings. The numbers given are for the bipolar input range with the internal 1.17 V reference. These numbers are typical and are generated with a differential input voltage of 0 V. Table 8 shows the effective resolution, with the output peak-to-peak (p-p) resolution given in parentheses. It is important to note that the effective resolution is calculated using the rms noise, while the p-p resolution is calculated based on p-p noise. The p-p resolution represents the resolution for which there is no code flicker. These numbers are typical and are rounded to the nearest LSB. Table 7. Output RMS Noise (μV) vs. Gain and Output Update Rate Using the Internal Reference Update Rate (Hz) 4.17 8.33 16.7 33.2 62 123 242 470 Gain of 1 0.81 1.18 1.96 2.99 3.6 5.83 11.22 12.46 Gain of 2 0.67 1.11 1.72 2.48 3.25 5.01 8.64 10.58 Gain of 4 0.32 0.41 0.55 0.83 1.03 1.69 2.69 4.58 Gain of 8 0.2 0.25 0.36 0.48 0.65 0.96 1.9 2 Gain of 16 0.13 0.16 0.25 0.33 0.46 0.67 1.04 1.27 Gain of 32 0.065 0.078 0.11 0.17 0.2 0.32 0.45 0.63 Gain of 64 0.04 0.058 0.088 0.13 0.15 0.25 0.35 0.50 Gain of 128 0.039 0.059 0.088 0.12 0.15 0.26 0.34 0.49 Gain of 64 20 (17.5) 19 (16.5) 18.5 (16) 18 (15.5) 18 (15.5) 17 (14.5) 16.5 (14) 16 (13.5) Gain of 128 19 (16.5) 18 (15.5) 17.5 (15) 17 (14.5) 17 (14.5) 16 (13.5) 15.5 (13) 15 (12.5) Table 8. Typical Resolution (Bits) vs. Gain and Output Update Rate Using the Internal Reference Update Rate (Hz) 4.17 8.33 16.7 33.2 62 123 242 470 Gain of 1 20 (19) 20 (18.5) 20 (17.5) 19.5 (17) 19.5 (17) 18.5 (16) 17.5 (15) 17.5 (15) Gain of 2 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 18 (15.5) 17 (14.5) 17 (14.5) Gain of 4 20 (18.5) 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 18.5 (16) 17.5 (15) 17 (14.5) Gain of 8 20.5 (18) 20 (17.5) 19.5 (17) 19 (16.5) 19 (16.5) 18 (15.5) 17 (14.5) 17 (14.5) Rev. 0 | Page 12 of 32 Gain of 16 20 (17.5) 20 (17.5) 19 (16.5) 19 (16.5) 18.5 (16) 17.5 (15) 17 (14.5) 17 (14.5) Gain of 32 20 (17.5) 20 (17.5) 19.5 (17) 18.5 (16) 18.5 (16) 18 (15.5) 17.5 (15) 17 (14.5) AD7785 TYPICAL PERFORMANCE CHARACTERISTICS 8388800 8388750 20 OCCURRENCE (%) 8388650 8388600 10 8388450 06721-006 8388500 0 200 400 600 800 0 1000 06721-009 8388550 –1.75 –1.05 –0.70 –0.35 READING NUMBER 90 14 80 0.70 1.05 1.40 1.75 70 POWER-UP TIME (ms) 12 10 8 6 4 60 50 40 30 20 10 06721-007 2 0 8388482 8388520 8388560 8388600 8388640 8388680 0 8388720 8388750 CODE 06721-010 OCCURRENCE 0.35 Figure 9. Excitation Current Matching (1 mA) at Ambient Temperature 16 0 –0.4 0 0.4 800 1000 0.8 1.2 1.6 2.0 MATCHING (%) 2.0 1.5 1.0 0.5 0 06721-011 RMS NOISE (µV) 06721-008 10 –0.8 600 VDD = 5V UPDATE RATE = 16.6Hz TA = 25°C 2.5 20 –1.2 400 Figure 10. Bias Voltage Generator Power-Up Time vs. Load Capacitance 3.0 –2.0 200 LOAD CAPACITANCE (nF) Figure 7. Noise Distribution Histogram (Internal Reference, Gain = 64, Update Rate = 16.7 Hz) 0 0 MATCHING (%) Figure 6. Typical Noise Plot (Internal Reference, Gain = 64, Update Rate = 16.7 Hz) OCCURRENCE (%) CODE READ 8388700 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 REFERENCE VOLTAGE (V) Figure 11. RMS Noise vs. Reference Voltage (Gain = 1) Figure 8. Excitation Current Matching (210 μA) at Ambient Temperature Rev. 0 | Page 13 of 32 5.0 AD7785 ON-CHIP REGISTERS The ADC is controlled and configured via a number of on-chip registers, which are described on the following pages. In the following descriptions, set implies a Logic 1 state and cleared implies a Logic 0 state, unless otherwise stated. COMMUNICATIONS REGISTER RS2, RS1, RS0 = 0, 0, 0 The communications register is an 8-bit write-only register. All communications to the part must start with a write operation to the communications register. The data written to the communications register determines whether the next operation is a read or write operation, and to which register this operation takes place. For read or write operations, once the subsequent read or CR7 WEN(0) CR6 R/W(0) CR5 RS2(0) CR4 RS1(0) write operation to the selected register is complete, the interface returns to where it expects a write operation to the communications register. This is the default state of the interface and, on power-up or after a reset, the ADC is in this default state waiting for a write operation to the communications register. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high returns the ADC to this default state by resetting the entire part. Table 9 outlines the bit designations for the communications register. CR0 through CR7 indicate the bit location, CR denoting the bits are in the communications register. CR7 denotes the first bit of the data stream. The number in parentheses indicates the poweron/reset default status of that bit. CR3 RS0(0) CR2 CREAD(0) CR1 0(0) CR0 0(0) Table 9. Communications Register Bit Designations Bit Location CR7 CR6 CR5 to CR3 CR2 CR1 to CR0 Bit Name WEN Description Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually occurs. If a 1 is the first bit written, the part does not clock on to subsequent bits in the register. It stays at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits are loaded to the communications register. A 0 in this bit location indicates that the next operation is a write to a specified register. A 1 in this position R/W indicates that the next operation is a read from the designated register. RS2 to RS0 Register Address Bits. These address bits are used to select which of the ADC’s registers are being selected during this serial interface communication. See Table 10. CREAD Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the serial interface is configured so that the data register can be continuously read. For example, the contents of the data register are placed on the DOUT pin automatically when the SCLK pulses are applied after the RDY pin goes low to indicate that a conversion is complete. The communications register does not have to be written to for data reads. To enable continuous read mode, the instruction 01011100 must be written to the communications register. To exit the continuous read mode, the instruction 01011000 must be written to the communications register while the RDY pin is low. While in continuous read mode, the ADC monitors activity on the DIN line so that it can receive the instruction to exit continuous read mode. Additionally, a reset occurs if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read mode until an instruction is to be written to the device. 0 These bits must be programmed to Logic 0 for correct operation. Table 10. Register Selection RS2 0 0 0 0 0 1 1 1 1 RS1 0 0 0 1 1 0 0 1 1 RS0 0 0 1 0 1 0 1 0 1 Register Communications Register During a Write Operation Status Register During a Read Operation Mode Register Configuration Register Data Register ID Register IO Register Offset Register Full-Scale Register Rev. 0 | Page 14 of 32 Register Size 8-bit 8-bit 16-bit 16-bit 24-bit (20-bit conversion followed by four 1s) 8-bit 8-bit 24-bit 24-bit AD7785 STATUS REGISTER RS2, RS1, RS0 = 0, 0, 0; Power-On/Reset = 0x88 The status register is an 8-bit read-only register. To access the ADC status register, the user must write to the communications register, select the next operation to be a read, and load Bit RS2, Bit RS1, and Bit RS0 with 0. Table 11 outlines the bit designations for the status register. SR0 through SR7 indicate the bit locations, and SR denotes that the bits are in the status register. SR7 denotes the first bit of the data stream. The number in parentheses indicates the power-on/reset default status of that bit. SR7 RDY(1) SR6 ERR(0) SR5 0(0) SR4 0(0) SR3 1 (1) SR2 CH2(0) SR1 CH1(0) SR0 CH0(0) Table 11. Status Register Bit Designations Bit Location SR7 Bit Name RDY SR6 ERR SR5 to SR4 SR3 SR2 to SR0 0 1 CH2 to CH0 Description Ready Bit for ADC. Cleared when data is written to the ADC data register. The RDY bit is set automatically after the ADC data register has been read or a period before the data register is updated with a new conversion result to indicate to the user not to read the conversion data. It is also set when the part is placed in power-down mode. The end of a conversion is indicated by the DOUT/RDY pin also. This pin can be used as an alternative to the status register for monitoring the ADC for conversion data. ADC Error Bit. This bit is written to at the same time as the RDY bit. Set to indicate that the result written to the ADC data register has been clamped to all 0s or all 1s. Error sources include overrange and underrange. Cleared by a write operation to start a conversion. These bits are automatically cleared. This bit is automatically set on the AD7785. These bits indicate which channel is being converted by the ADC. MODE REGISTER RS2, RS1, RS0 = 0, 0, 1; Power-On/Reset = 0x000A The mode register is a 16-bit register from which data can be read or to which data can be written. This register is used to select the operating mode, update rate, and clock source. Table 12 outlines the bit designations for the mode register. MR0 through MR15 indicate the bit locations, MR denoting the bits are in the mode register. MR15 denotes the first bit of the data stream. The number in parentheses indicates the power-on/reset default status of that bit. Any write to the setup register resets the modulator and filter and sets the RDY bit. MR15 MD2(0) MR7 CLK1(0) MR14 MD1(0) MR6 CLK0(0) MR13 MD0(0) MR5 0(0) MR12 0(0) MR4 0(0) MR11 0(0) MR3 FS3(1) MR10 0(0) MR2 FS2(0) MR9 0(0) MR1 FS1(1) MR8 0(0) MR0 FS0(0) Table 12. Mode Register Bit Designations Bit Location MR15 to MR13 MR12 to MR8 MR7 to MR6 Bit Name MD2 to MD0 0 CLK1 to CLK0 MR5 to MR4 MR3 to MR0 0 FS3 to FS0 Description Mode Select Bits. These bits select the operational mode of the AD7785 (see Table 13). These bits must be programmed with a Logic 0 for correct operation. These bits are used to select the clock source for the AD7785. Either an on-chip 64 kHz clock or an external clock can be used. The ability to override using an external clock allows several AD7785 devices to be synchronized. In addition, 50 Hz/60 Hz is improved when an accurate external clock drives the AD7785. CLK1 CLK0 ADC Clock Source 0 0 Internal 64 kHz Clock. Internal clock is not available at the CLK pin. 0 1 Internal 64 kHz Clock. This clock is made available at the CLK pin. 1 0 External 64 kHz Clock Used. An external clock gives better 50 Hz/60 Hz rejection. See specifications for external clock. 1 1 External Clock Used. The external clock is divided by 2 within the AD7785. These bits must be programmed with a Logic 0 for correct operation. Filter Update Rate Select Bits (see Table 14). Rev. 0 | Page 15 of 32 AD7785 Table 13. Operating Modes MD2 0 MD1 0 MD0 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 Mode Continuous Conversion Mode (Default). In continuous conversion mode, the ADC continuously performs conversions and places the result in the data register. RDY goes low when a conversion is complete. The user can read these conversions by placing the device in continuous read mode, whereby the conversions are automatically placed on the DOUT line when SCLK pulses are applied. Alternatively, the user can instruct the ADC to output the conversion by writing to the communications register. After power-on, a channel change, or a write to the mode, configuration, or IO registers, the first conversion is available after a period of 2/fADC. Subsequent conversions are available at a frequency of fADC. Single Conversion Mode. When single conversion mode is selected, the ADC powers up and performs a single conversion. The oscillator requires 1 ms to power up and settle. The ADC then performs the conversion, which takes a time of 2/fADC. The conversion result is placed in the data register, RDY goes low, and the ADC returns to power-down mode. The conversion remains in the data register, and RDY remains active low until the data is read or another conversion is performed. Idle Mode. In idle mode, the ADC filter and modulator are held in a reset state, although the modulator clocks are still provided. Power-Down Mode. In power-down mode, all the AD7785 circuitry is powered down, including the current sources, burnout currents, bias voltage generator, and CLKOUT circuitry. Internal Zero-Scale Calibration. An internal short is automatically connected to the enabled channel. A calibration takes 2 conversion cycles to complete. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured offset coefficient is placed in the offset register of the selected channel. Internal Full-Scale Calibration. A full-scale input voltage is automatically connected to the selected analog input for this calibration. When the gain equals 1, a calibration takes 2 conversion cycles to complete. For higher gains, 4 conversion cycles are required to perform the full-scale calibration. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured full-scale coefficient is placed in the full-scale register of the selected channel. Internal full-scale calibrations cannot be performed when the gain equals 128. With this gain setting, a system fullscale calibration can be performed. A full-scale calibration is required each time the gain of a channel is changed to minimize the full-scale error. System Zero-Scale Calibration. The user should connect the system zero-scale input to the channel input pins as selected by the CH2 to CH0 bits. A system offset calibration takes 2 conversion cycles to complete. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured offset coefficient is placed in the offset register of the selected channel. System Full-Scale Calibration. The user should connect the system full-scale input to the channel input pins as selected by the CH2 to CH0 bits. A calibration takes 2 conversion cycles to complete. RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured full-scale coefficient is placed in the full-scale register of the selected channel. A full-scale calibration is required each time the gain of a channel is changed. Table 14. Update Rates Available FS3 0 0 0 0 0 0 0 0 1 FS2 0 0 0 0 1 1 1 1 0 FS1 0 0 1 1 0 0 1 1 0 FS0 0 1 0 1 0 1 0 1 0 fADC (Hz) x 470 242 123 62 50 39 33.2 19.6 tSETTLE (ms) x 4 8 16 32 40 48 60 101 Rejection @ 50 Hz/60 Hz (Internal Clock) 90 dB (60 Hz only) Rev. 0 | Page 16 of 32 AD7785 FS3 1 1 1 1 1 1 1 FS2 0 0 0 1 1 1 1 FS1 0 1 1 0 0 1 1 FS0 1 0 1 0 1 0 1 fADC (Hz) 16.7 16.7 12.5 10 8.33 6.25 4.17 tSETTLE (ms) 120 120 160 200 240 320 480 Rejection @ 50 Hz/60 Hz (Internal Clock) 80 dB (50 Hz only) 65 dB (50 Hz and 60 Hz) 66 dB (50 Hz and 60 Hz) 69 dB (50 Hz and 60 Hz) 70 dB (50 Hz and 60 Hz) 72 dB (50 Hz and 60 Hz) 74 dB (50 Hz and 60 Hz) CONFIGURATION REGISTER RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710 The configuration register is a 16-bit register from which data can be read or to which data can be written. This register is used to configure the ADC for unipolar or bipolar mode, enable or disable the buffer, enable or disable the burnout currents, select the gain, and select the analog input channel. Table 15 outlines the bit designations for the filter register. CON0 through CON15 indicate the bit locations; CON denotes that the bits are in the configuration register. CON15 denotes the first bit of the data stream. The number in parentheses indicates the power-on/reset default status of that bit. CON15 VBIAS1(0) CON14 VBIAS0(0) CON13 BO(0) CON12 U/B(0) CON11 BOOST(0) CON10 G2(1) CON9 G1(1) CON8 G0(1) CON7 REFSEL(0) CON6 0(0) CON5 0(0) CON4 BUF(1) CON3 0(0) CON2 CH2(0) CON1 CH1(0) CON0 CH0(0) Table 15. Configuration Register Bit Designations Bit Location CON15 to CON14 Bit Name VBIAS1 to VBIAS0 CON13 BO CON12 U/B CON11 BOOST CON10 to CON8 G2 to G0 Description Bias Voltage Generator Enable. The negative terminal of the analog inputs can be biased up to AVDD/2. These bits are used in conjunction with the boost bit. VBIAS1 VBIAS0 Bias Voltage 0 0 Bias voltage generator disabled 0 1 Bias voltage connected to AIN1(−) 1 0 Bias voltage connected to AIN2(−) 1 1 Reserved Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal path are enabled. When BO = 0, the burnout currents are disabled. The burnout currents can be enabled only when the buffer or in-amp is active. The burnout currents are available on Channels AIN1 and AIN2. Unipolar/Bipolar Bit. Set by user to enable unipolar coding. Therefore, a zero differential input results in 0x00000 output, and a full-scale differential input results in 0xFFFFF output. Cleared by the user to enable bipolar coding. Negative full-scale differential input results in an output code of 0x00000, zero differential input results in an output code of 0x80000, and a positive full-scale differential input results in an output code of 0xFFFFF. This bit is used in conjunction with the VBIAS1 and VBIAS0 bits. When set, the current consumed by the bias voltage generator is increased. This reduces its power-up time. Gain Select Bits. Written by the user to select the ADC input range as follows: G2 G1 G0 Gain ADC Input Range (2.5 V Reference) 0 0 0 1 (In-amp not used) 2.5 V 0 0 1 2 (In-amp not used) 1.25 V 0 1 0 4 625 mV 0 1 1 8 312.5 mV 1 0 0 16 156.2 mV 1 0 1 32 78.125 mV 1 1 0 64 39.06 mV 1 1 1 128 19.53 mV Rev. 0 | Page 17 of 32 AD7785 Bit Location CON7 Bit Name REFSEL CON6 to CON5 CON4 0 BUF CON3 CON2 to CON0 0 CH2 to CH0 Description Reference Select Bit. The reference source for the ADC is selected using this bit. REFSEL Reference Source 0 External Reference Applied between REFIN(+) and REFIN(–). 1 Internal Reference Selected. These bits must be programmed with a Logic 0 for correct operation. Configures the ADC for buffered or unbuffered mode of operation. If cleared, the ADC operates in unbuffered mode, lowering the power consumption of the device. If set, the ADC operates in buffered mode, allowing the user to place source impedances on the front end without contributing gain errors to the system. The buffer can be disabled when the gain equals 1 or 2. For higher gains, the buffer is automatically enabled. With the buffer disabled, the voltage on the analog input pins can be from 30 mV below GND to 30 mV above AVDD. When the buffer is enabled, it requires some headroom, so the voltage on any input pin must be limited to 100 mV within the power supply rails. This bit must be programmed with a Logic 0 for correct operation. Channel Select Bits. Written by the user to select the active analog input channel to the ADC. CH2 0 0 0 0 1 1 1 1 CH1 0 0 1 1 0 0 1 1 CH0 0 1 0 1 0 1 0 1 Channel AIN1(+) – AIN1(–) AIN2(+) – AIN2(–) AIN3(+) – AIN3(–) AIN1(−) – AIN1(−) Reserved Reserved Temp Sensor AVDD Monitor Calibration Pair 0 1 2 0 Automatically selects gain = 1 and internal reference Automatically selects gain = 1/6 and 1.17 V reference DATA REGISTER RS2, RS1, RS0 = 0, 1, 1; Power-On/Reset = 0x00000F The conversion result from the ADC is stored in this data register. This is a read-only register. On completion of a read operation from this register, the RDY bit/pin is set. This is a 24-bit register. The 20-bit conversion is contained in the 20 MSBs. The 4 LSBs are set to 1. ID REGISTER RS2, RS1, RS0 = 1, 0, 0; Power-On/Reset = 0xXB The identification number for the AD7785 is stored in the ID register. This is a read-only register. IO REGISTER RS2, RS1, RS0 = 1, 0, 1; Power-On/Reset = 0x00 The IO register is an 8-bit register from which data can be read or to which data can be written. This register is used to enable and select the value of the excitation currents. Table 16 outlines the bit designations for the IO register. IO0 through IO7 indicate the bit locations; IO denotes that the bits are in the IO register. IO7 denotes the first bit of the data stream. The number in parentheses indicates the poweron/reset default status of that bit. IO7 0(0) IO6 0(0) IO5 0(0) IO4 0(0) IO3 IEXCDIR1(0) IO2 IEXCDIR0(0) Rev. 0 | Page 18 of 32 IO1 IEXCEN1(0) IO0 IEXCEN0(0) AD7785 Table 16. IO Register Bit Designations Bit Location IO7 to IO4 IO3 to IO2 Bit Name 0 IEXCDIR1 to IEXCDIR0 Description These bits must be programmed with a Logic 0 for correct operation. Direction of current sources select bits. IEXCDIR1 0 IO1 to IO0 IEXCEN1 to IEXCEN0 IEXCDIR0 0 Current Source Direction Current Source IEXC1 connected to Pin IOUT1, Current Source IEXC2 connected to Pin IOUT2. 0 1 Current Source IEXC1 connected to Pin IOUT2, Current Source IEXC2 connected to Pin IOUT1. 1 0 Both current sources connected to Pin IOUT1. Permitted when the current sources are set to 10 μA or 210 μA only. 1 1 Both current sources connected to Pin IOUT2. Permitted when the current sources are set to 10 μA or 210 μA only. These bits are used to enable and disable the current sources along with selecting the value of the excitation currents. IEXCEN1 IEXCEN0 Current Source Value 0 0 Excitation Current Disabled 0 1 10 μA 1 0 210 μA 1 1 1 mA OFFSET REGISTER FULL-SCALE REGISTER RS2, RS1, RS0 = 1, 1, 0; Power-On/Reset = 0x800000 RS2, RS1, RS0 = 1, 1, 1; Power-On/Reset = 0x5XXX00 Each analog input channel has a dedicated offset register that holds the offset calibration coefficient for the channel. This register is 24 bits wide and its power-on/reset value is 0x8000(00). The offset register is used in conjunction with its associated full-scale register to form a register pair. The poweron reset value is automatically overwritten if an internal or system zero-scale calibration is initiated by the user. The offset register is a read/write register. However, the AD7785 must be in idle mode or power-down mode when writing to the offset register. The full-scale register is a 24-bit register that holds the full-scale calibration coefficient for the ADC. The AD7785 has 3 full-scale registers, each channel having a dedicated full-scale register. The full-scale registers are read/write registers; however, when writing to the full-scale registers, the ADC must be placed in power-down mode or idle mode. These registers are configured on power-on with factory-calibrated full-scale calibration coefficients, the calibration being performed at gain = 1. Therefore, every device has different default coefficients. The coefficients are different depending on whether the internal reference or an external reference is selected. The default value is automatically overwritten if an internal or system full-scale calibration is initiated by the user, or the full-scale register is written to. Rev. 0 | Page 19 of 32 AD7785 ADC CIRCUIT INFORMATION The AD7785 uses slightly different filter types, depending on the output update rate so that the rejection of quantization noise and device noise is optimized. When the update rate is from 4.17 Hz to 12.5 Hz, a Sinc3 filter, along with an averaging filter, is used. When the update rate is from 16.7 Hz to 39 Hz, a modified Sinc3 filter is used. This filter provides simultaneous 50 Hz/60 Hz rejection when the update rate equals 16.7 Hz. A Sinc4 filter is used when the update rate is from 50 Hz to 242 Hz. Finally, an integrate-only filter is used when the update rate equals 470 Hz. OVERVIEW The AD7785 is a low power ADC that incorporates a ∑-Δ modulator, a buffer, reference, in-amp, and an on-chip digital filter intended for the measurement of wide dynamic range, low frequency signals such as those in pressure transducers, weigh scales, and temperature measurement applications. The part has three differential inputs that can be buffered or unbuffered. The device can be operated with the internal 1.17 V reference, or an external reference can be used. Figure 12 shows the basic connections required to operate the part. Figure 13 to Figure 16 show the frequency response of the different filter types for several update rates. The output rate of the AD7785 (fADC) is user-programmable. The allowable update rates, along with their corresponding settling times, are listed in Table 14. Normal mode rejection is the major function of the digital filter. Simultaneous 50 Hz and 60 Hz rejection is optimized when the update rate equals 16.7 Hz or less as notches are placed at both 50 Hz and 60 Hz with these update rates (see Figure 14.) GND AVDD VBIAS THERMOCOUPLE JUNCTION R REFIN(+) REFIN(–) BAND GAP REFERENCE AIN1(+) AIN1(–) GND AVDD R C AIN2(+) MUX BUF AIN2(–) SERIAL INTERFACE AND CONTROL LOGIC Σ-Δ ADC IN-AMP REFIN(+) RREF GND 0 –20 (dB) –40 –60 06721-013 –80 20 40 60 80 100 FREQUENCY (Hz) Figure 13. Filter Profile with Update Rate = 4.17 Hz Rev. 0 | Page 20 of 32 CS 06721-012 CLK Figure 12. Basic Connection Diagram 0 SCLK AD7785 IOUT2 –100 DIN DVDD INTERNAL CLOCK AVDD REFIN(–) DOUT/RDY 120 AD7785 0 DIGITAL INTERFACE –20 (dB) –40 –60 –100 06721-014 –80 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (Hz) Figure 14. Filter Profile with Update Rate = 16.7 Hz 0 –20 (dB) –40 –60 –100 06721-015 –80 0 500 1000 1500 2000 2500 3000 FREQUENCY (Hz) Figure 15. Filter Profile with Update Rate = 242 Hz 0 –10 The programmable functions of the AD7785 are controlled using a set of on-chip registers. Data is written to these registers via the serial interface of the device; read access to the on-chip registers is also provided by this interface. All communications with the device must start with a write to the communications register. After power-on or reset, the device expects a write to its communications register. The data written to this register determines whether the next operation is a read operation or a write operation and determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the part begins with a write operation to the communications register followed by a write to the selected register. A read operation from any other register (except when continuous read mode is selected) starts with a write to the communications register followed by a read operation from the selected register. The serial interface of the AD7785 consists of four signals: CS, DIN, SCLK, and DOUT/RDY. The DIN line is used to transfer data into the on-chip registers, and DOUT/RDY is used for accessing from the on-chip registers. SCLK is the serial clock input for the device, and all data transfers (either on DIN or DOUT/RDY) occur with respect to the SCLK signal. The DOUT/RDY pin operates as a data-ready signal also; the line going low when a new data-word is available in the output register. It is reset high when a read operation from the data register is complete. It also goes high prior to the updating of the data register to indicate when not to read from the device to ensure that a data read is not attempted while the register is being updated. CS is used to select a device. It can be used to decode the AD7785 in systems where several components are connected to the serial bus. –30 –40 –50 –60 06721-016 (dB) –20 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 FREQUENCY (Hz) Figure 16. Filter Response at 470 Hz Update Rate Rev. 0 | Page 21 of 32 AD7785 Figure 3 and Figure 4 show timing diagrams for interfacing to the AD7785 with CS being used to decode the part. Figure 3 shows the timing for a read operation from the AD7785 output shift register, and Figure 4 shows the timing for a write operation to the input shift register. It is possible to read the same word from the data register several times, even though the DOUT/RDY line returns high after the first read operation. However, care must be taken to ensure that the read operations have been completed before the next output update occurs. In continuous read mode, the data register can be read only once. The serial interface can operate in 3-wire mode by tying CS low. In this case, the SCLK, DIN, and DOUT/RDY lines are used to communicate with the AD7785. The end of the conversion can be monitored using the RDY bit in the status register. This scheme is suitable for interfacing to microcontrollers. If CS is required as a decoding signal, it can be generated from a port pin. For microcontroller interfaces, it is recommended that SCLK idle high between data transfers. The AD7785 can be operated with CS being used as a frame synchronization signal. This scheme is useful for DSP interfaces. In this case, the first bit (MSB) is effectively clocked out by CS, because CS would normally occur after the falling edge of SCLK in DSPs. The SCLK can continue to run between data transfers, provided the timing numbers are obeyed. The serial interface can be reset by writing a series of 1s on the DIN input. If a Logic 1 is written to the AD7785 line for at least 32 serial clock cycles, the serial interface is reset. This ensures that the interface can be reset to a known state if the interface gets lost due to a software error or some glitch in the system. Reset returns the interface to the state in which it is expecting a write to the communications register. This operation resets the contents of all registers to their power-on values. Following a reset, the user should allow a period of 500 μs before addressing the serial interface. Single Conversion Mode In single conversion mode, the AD7785 is placed in shutdown mode between conversions. When a single conversion is initiated by setting MD2, MD1, MD0 to 0, 0, 1 in the mode register, the AD7785 powers up, performs a single conversion, and then returns to power-down mode. The on-chip oscillator requires 1 ms to power up. A conversion requires a time period of 2 × tADC. DOUT/RDY goes low to indicate the completion of a conversion. When the data-word has been read from the data register, DOUT/RDY goes high. If CS is low, DOUT/RDY remains high until another conversion is initiated and completed. The data register can be read several times, if required, even when DOUT/RDY has gone high. Continuous Conversion Mode This is the default power-up mode. The AD7785 continuously converts, the RDY pin in the status register going low each time a conversion is completed. If CS is low, the DOUT/ RDY line also goes low when a conversion is complete. To read a conversion, the user writes to the communications register indicating that the next operation is a read of the data register. The digital conversion is placed on the DOUT/ RDY pin as soon as SCLK pulses are applied to the ADC. DOUT/RDY returns high when the conversion is read. The user can read this register additional times, if required. However, the user must ensure that the data register is not being accessed at the completion of the next conversion, otherwise the new conversion word is lost. The AD7785 can be configured to continuously convert or to perform a single conversion. See Figure 17 through Figure 19. Rev. 0 | Page 22 of 32 AD7785 CS DIN 0x08 0x200A 0x58 DATA 06721-017 DOUT/RDY SCLK Figure 17. Single Conversion CS 0x58 0x58 DIN DATA DATA 06721-018 DOUT/RDY SCLK Figure 18. Continuous Conversion Rev. 0 | Page 23 of 32 AD7785 Continuous Read read before the next conversion is complete. If the user has not read the conversion before the completion of the next conversion, or if insufficient serial clocks are applied to the AD7785 to read the word, the serial output register is reset when the next conversion is completed, and the new conversion is placed in the output serial register. Rather than write to the communications register to access the data each time a conversion is complete, the AD7785 can be configured so that the conversions are placed on the DOUT/RDY line automatically. By writing 01011100 to the communications register, the user needs only to apply the 24 SCLK cycles to the ADC, and the 20-bit result followed by four 1s is automatically placed on the DOUT/RDY line when a conversion is complete. The ADC should be configured for continuous conversion mode. To exit the continuous read mode, the instruction 01011000 must be written to the communications register while the DOUT/RDY pin is low. While in the continuous read mode, the ADC monitors activity on the DIN line so that it can receive the instruction to exit the continuous read mode. Additionally, a reset occurs if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read mode until an instruction is written to the device. When DOUT/RDY goes low to indicate the end of a conversion, sufficient SCLK cycles must be applied to the ADC, and the data conversion is placed on the DOUT/RDY line. When the conversion is read, DOUT/RDY returns high until the next conversion is available. In this mode, the data can be read only once. In addition, the user must ensure that the data-word is CS 0x5C DIN DATA DATA DATA 06721-019 DOUT/RDY SCLK Figure 19. Continuous Read Rev. 0 | Page 24 of 32 AD7785 CIRCUIT DESCRIPTION ANALOG INPUT CHANNEL The AD7785 has three differential analog input channels. These are connected to the on-chip buffer amplifier when the device is operated in buffered mode and directly to the modulator when the device is operated in unbuffered mode. In buffered mode (the BUF bit in the mode register is set to 1), the input channel feeds into a high impedance input stage of the buffer amplifier. Therefore, the input can tolerate significant source impedances and is tailored for direct connection to external resistive-type sensors, such as strain gauges or resistance temperature detectors (RTDs). When BUF = 0, the part is operated in unbuffered mode. This results in a higher analog input current. Note that this unbuffered input path provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on the input pins can cause gain errors, depending on the output impedance of the source that is driving the ADC input. Table 17 shows the allowable external resistance/capacitance values for unbuffered mode such that no gain error at the 20-bit level is introduced. Table 17. External R-C Combination for 20-Bit No Gain Error C (pF) R (Ω) 50 9k 100 6k 500 1.5 k 1000 900 5000 200 The AD7785 can be operated in unbuffered mode only when the gain equals 1 or 2. At higher gains, the buffer is automatically enabled. The absolute input voltage range in buffered mode is restricted to a range between GND + 100 mV and AVDD – 100 mV. When the gain is set to 4 or higher, the in-amp is enabled. The absolute input voltage range when the in-amp is active is restricted to a range between GND + 300 mV and AVDD − 1.1 V. Take care in setting up the common-mode voltage so that these limits are not exceeded to avoid degradation in linearity and noise performance. The absolute input voltage in unbuffered mode includes the range between GND – 30 mV and AVDD + 30 mV as a result of being unbuffered. The negative absolute input voltage limit does allow the possibility of monitoring small true bipolar signals with respect to GND. INSTRUMENTATION AMPLIFIER Amplifying the analog input signal by a gain of 1 or 2 is performed digitally within the AD7785. However, when the gain equals 4 or higher, the output from the buffer is applied to the input of the on-chip instrumentation amplifier. This low noise in-amp means that signals of small amplitude can be gained within the AD7785 while still maintaining excellent noise performance. For example, when the gain is set to 64, the rms noise is 40 nV typically, which is equivalent to 20 bits effective resolution or 18.5 bits peak-to-peak resolution. The AD7785 can be programmed to have a gain of 1, 2, 4, 8, 16, 32, 64, and 128 using Bit G2 to Bit G0 in the configuration register. Therefore, with an external 2.5 V reference, the unipolar ranges are from 0 mV to 20 mV to 0 V to 2.5 V while the bipolar ranges are from ±20 mV to ±2.5 V. When the in-amp is active (gain ≥ 4), the common-mode voltage (AIN(+) + AIN(–))/2 must be greater than or equal to 0.5 V. If the AD7785 is operated with an external reference that has a value equal to AVDD, the analog input signal must be limited to 90% of VREF/gain when the in-amp is active, for correct operation. BIPOLAR/UNIPOLAR CONFIGURATION The analog input to the AD7785 can accept either unipolar or bipolar input voltage ranges. A bipolar input range does not imply that the part can tolerate negative voltages with respect to system GND. Unipolar and bipolar signals on the AIN(+) input are referenced to the voltage on the AIN(–) input. For example, if AIN(−) is 2.5 V, and the ADC is configured for unipolar mode and a gain of 1, the input voltage range on the AIN(+) pin is 2.5 V to 5 V. If the ADC is configured for bipolar mode, the analog input range on the AIN(+) input is 0 V to 5 V. The bipolar/unipolar option is chosen by programming the U/B bit in the configuration register. DATA OUTPUT CODING When the ADC is configured for unipolar operation, the output code is natural (straight) binary with a zero differential input voltage resulting in a code of 00000 hex, a midscale voltage resulting in a code of 80000, and a full-scale input voltage resulting in a code of FFFFF. The output code for any analog input voltage can be represented as Code = (2N × AIN × GAIN)/VREF When the ADC is configured for bipolar operation, the output code is offset binary with a negative full-scale voltage resulting in a code of 00000 hex, a zero differential input voltage resulting in a code of 80000 hex, and a positive full-scale input voltage resulting in a code of FFFFF hex. The output code for any analog input voltage can be represented as Code = 2N – 1 × [(AIN × GAIN /VREF) + 1] where: AIN is the analog input voltage. GAIN is the in-amp setting (1 to 128). N = 20. Rev. 0 | Page 25 of 32 AD7785 BURNOUT CURRENTS Burnout currents are available on Channels AIN1 and AIN2. The burnout currents are 100 nA constant current generators, one sourcing current from AVDD to AIN(+) and one sinking current from AIN(–) to GND. The currents are switched to the selected analog input pair. Both currents are either on or off, depending on the burnout current enable (BO) bit in the configuration register. These currents can be used to verify that an external transducer is still operational before attempting to take measurements on that channel. Once the burnout currents are turned on, they flow in the external transducer circuit, and a measurement of the input voltage on the analog input channel can be taken. If the resultant voltage measured is full scale, the user needs to verify why this is the case. A full-scale reading could mean that the front-end sensor is open circuit. It could also mean that the front-end sensor is overloaded and is justified in outputting full scale, or the reference may be absent, thus clamping the data to all 1s. When reading all 1s from the output, the user needs to check these three cases before making a judgment. If the voltage measured is 0 V, it may indicate that the transducer has short circuited. For normal operation, these burnout currents are turned off by writing a 0 to the BO bit in the configuration register. The current sources work over the normal absolute input voltage range specifications with buffers on. EXCITATION CURRENTS The AD7785 also contains two matched, software-configurable, constant current sources that can be programmed to equal 10 μA, 210 μA, or 1 mA. Both source currents from the AVDD are directed to either the IOUT1 or IOUT2 pin of the device. These current sources are controlled via bits in the IO register. The configuration bits enable the current sources, direct the current sources to IOUT1 or IOUT2, and select the value of the current. These current sources can be used to excite external resistive bridge or RTD sensors. BIAS VOLTAGE GENERATOR A bias voltage generator is included on the AD7785. This biases the negative terminal of the selected input channel to AVDD/2. It is useful in thermocouple applications, because the voltage generated by the thermocouple must be biased about some dc voltage if the gain is greater than 2. This is necessary because the instrumentation amplifier requires headroom to ensure that signals close to GND or AVDD are converted accurately. The bias voltage generator is controlled using the VBIAS1 and VBIAS0 bits in conjunction with the boost bit in the configuration register. The power-up time of the bias voltage generator is dependent on the load capacitance. To accommodate higher load capacitances, the AD7785 has a boost bit. When this bit is set to 1, the current consumed by the bias voltage generator increases, so that the power-up time is considerably reduced. Figure 10 shows the power-up time when boost equals 0 and 1 for different load capacitances. The current consumption of the AD7785 increases by 40 μA when the bias voltage generator is enabled, and boost equals 0. With the boost function enabled, the current consumption increases by 250 μA. REFERENCE The AD7785 has an embedded 1.17 V reference that can be used to supply the ADC, or an external reference can be applied. The embedded reference is a low noise, low drift reference, the drift being 4 ppm/°C typically. For external references, the ADC has a fully differential input capability for the channel. The reference source for the AD7785 is selected using the REFSEL bit in the configuration register. When the internal reference is selected, it is internally connected to the modulator. It is not available on the REFIN pins. The common-mode range for these differential inputs is from GND to AVDD. The reference input is unbuffered; therefore, excessive R-C source impedances introduce gain errors. The reference voltage REFIN (REFIN(+) − REFIN(−)) is 2.5 V nominal, but the AD7785 is functional with reference voltages from 0.1 V to AVDD. In applications where the excitation (voltage or current) for the transducer on the analog input also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source is removed because the application is ratiometric. If the AD7785 is used in a nonratiometric application, a low noise reference should be used. Recommended 2.5 V reference voltage sources for the AD7785 include the ADR381 and ADR391, which are low noise, low power references. Also, note that the reference inputs provide a high impedance, dynamic load. Because the input impedance of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the output impedance of the source that is driving the reference inputs. Reference voltage sources like those recommended previously (such as the ADR391) typically have low output impedances and are, therefore, tolerant to having decoupling capacitors on REFIN(+) without introducing gain errors in the system. Deriving the reference input voltage across an external resistor means that the reference input sees a significant external source impedance. External decoupling on the REFIN pins is not recommended in this type of circuit configuration. RESET The circuitry and serial interface of the AD7785 can be reset by writing 32 consecutive 1s to the device. This resets the logic, the digital filter, and the analog modulator while all on-chip registers are reset to their default values. A reset is automatically performed on power-up. When a reset is initiated, the user must allow a period of 500 μs before accessing any of the onchip registers. A reset is useful if the serial interface becomes asynchronous due to noise on the SCLK line. Rev. 0 | Page 26 of 32 AD7785 AVDD MONITOR Along with converting external voltages, the ADC can be used to monitor the voltage on the AVDD pin. When Bit CH2 to Bit CH0 equal 1, the voltage on the AVDD pin is internally attenuated by 6, and the resultant voltage is applied to the ∑-Δ modulator using an internal 1.17 V reference for analog-todigital conversion. This is useful, because variations in the power supply voltage can be monitored. CALIBRATION The AD7785 provides four calibration modes that can be programmed via the mode bits in the mode register. These are internal zero-scale calibration, internal full-scale calibration, system zero-scale calibration, and system full-scale calibration, which effectively reduces the offset error and full-scale error to the order of the noise. After each conversion, the ADC conversion result is scaled using the ADC calibration registers before being written to the data register. The offset calibration coefficient is subtracted from the result prior to multiplication by the full-scale coefficient. To start a calibration, write the relevant value to the MD2 to MD0 bits in the mode register. After the calibration is complete, the contents of the corresponding calibration registers are updated, the RDY bit in the status register is set, the DOUT/RDY pin goes low (if CS is low), and the AD7785 reverts to idle mode. During an internal zero-scale or full-scale calibration, the respective zero input and full-scale input are automatically connected internally to the ADC input pins. A system calibration, however, expects the system zero-scale and system full-scale voltages to be applied to the ADC pins before the calibration mode is initiated. In this way, external ADC errors are removed. From an operational point of view, a calibration should be treated like another ADC conversion. A zero-scale calibration (if required) should always be performed before a full-scale calibration. System software should monitor the RDY bit in the status register or the DOUT/RDY pin to determine the end of calibration via a polling sequence or an interrupt-driven routine. Both an internal offset calibration and a system offset calibration take two conversion cycles. An internal offset calibration is not needed, as the ADC itself removes the offset continuously. To perform an internal full-scale calibration, a full-scale input voltage is automatically connected to the selected analog input for this calibration. When the gain equals 1, a calibration takes 2 conversion cycles to complete. For higher gains, 4 conversion cycles are required to perform the full-scale calibration. DOUT/RDY goes high when the calibration is initiated and returns low when the calibration is complete. The ADC is placed in idle mode following a calibration. The measured full-scale coefficient is placed in the full-scale register of the selected channel. Internal full-scale calibrations cannot be performed when the gain equals 128. With this gain setting, a system full-scale calibration can be performed. A full-scale calibration is required each time the gain of a channel is changed to minimize the full-scale error. An internal full-scale calibration can be performed at specified update rates only. For gains of 1, 2, and 4, an internal full-scale calibration can be performed at any update rate. However, for higher gains, internal full-scale calibrations can be performed when the update rate is less than or equal to 16.7 Hz, 33.2 Hz, and 50 Hz only. However, the full-scale error does not vary with the update rate, so a calibration at one update rate is valid for all update rates (assuming the gain or reference source is not changed). A system full-scale calibration takes 2 conversion cycles to complete, irrespective of the gain setting. A system full-scale calibration can be performed at all gains and all update rates. If system offset calibrations are being performed along with system full-scale calibrations, the offset calibration should be performed before the system full-scale calibration is initiated. GROUNDING AND LAYOUT Because the analog inputs and reference inputs of the ADC are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection of the part removes common-mode noise on these inputs. The digital filter provides rejection of broadband noise on the power supply, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs, provided that these noise sources do not saturate the analog modulator. As a result, the AD7785 is more immune to noise interference than a conventional high resolution converter. However, because the resolution of the AD7785 is so high, and the noise levels from the AD7785 is so low, care must be taken with regard to grounding and layout. The printed circuit board that houses the AD7785 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. A minimum etch technique is generally best for ground planes because it provides the best shielding. It is recommended that the GND pins of the AD7785 be tied to the AGND plane of the system. In any layout, it is important to keep in mind the flow of currents in the system, ensuring that the return paths for all currents are as close as possible to the paths the currents took to reach their destinations. Avoid forcing digital currents to flow through the AGND sections of the layout. Rev. 0 | Page 27 of 32 AD7785 The ground planes of the AD7785 should be allowed to run under the device to prevent noise coupling. The power supply lines to the AD7785 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals such as clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This reduces the effects of feedthrough through the board. A microstrip technique is by far the best, but it is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes, and signals are placed on the solder side. Good decoupling is important when using high resolution ADCs. AVDD should be decoupled with 10 μF tantalum in parallel with 0.1 μF capacitors to GND. DVDD should be decoupled with 10 μF tantalum in parallel with 0.1 μF capacitors to the system’s DGND plane, with the system’s AGND to DGND connection being close to the AD7785. To achieve the best from these decoupling components, they should be placed as close as possible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 μF ceramic capacitors to DGND. Rev. 0 | Page 28 of 32 AD7785 APPLICATIONS INFORMATION The AD7785 provides a low cost, high resolution analog-todigital function. Because the analog-to-digital function is provided by a ∑-Δ architecture, the part is more immune to noisy environments, making it ideal for use in sensor measurement and industrial and process control applications. decoupling capacitors can be placed on the front end to eliminate any noise pickup that may be present in the thermocouple leads. The AD7785 has a reduced commonmode range with the in-amp enabled, so the bias voltage generator provides a common-mode voltage so that the voltage generated by the thermocouple is biased up to AVDD/2. TEMPERATURE MEASUREMENT USING A THERMOCOUPLE The cold junction compensation is performed using a thermistor. The on-chip excitation current supplies the thermistor. In addition, the reference voltage for the cold junction measurement is derived from a precision resistor in series with the thermistor. This allows a ratiometric measure-ment so that variation of the excitation current has no effect on the measurement (it is the ratio of the precision reference resistance to the thermistor resistance that is measured). Figure 20 outlines a connection from a thermocouple to the AD7785. In a thermocouple application, the voltage generated by the thermocouple is measured with respect to an absolute reference, so the internal reference is used for this conversion. The cold junction measurement uses a ratiometric configuration, so the reference is provided externally. Because the signal from the thermocouple is small, the AD7785 is operated with the in-amp enabled to amplify the signal from the thermocouple. As the input channel is buffered, large GND AVDD VBIAS BAND GAP REFERENCE AIN1(+) AIN1(–) GND AVDD R C AIN2(+) MUX BUF AIN2(–) Σ-Δ ADC IN-AMP SERIAL INTERFACE AND CONTROL LOGIC REFIN(+) RREF GND REFIN(–) AVDD INTERNAL CLOCK IOUT2 CLK Figure 20. Thermocouple Measurement Using the AD7785 Rev. 0 | Page 29 of 32 DOUT/RDY DIN SCLK CS DVDD AD7785 06721-020 THERMOCOUPLE JUNCTION R REFIN(+) REFIN(–) AD7785 length), and IOUT1 and IOUT2 match, the error voltage across RL2 equals the error voltage across RL1, and no error voltage is developed between AIN1(+) and AIN1(–). The voltage is developed twice across RL3. However, because this is a common- mode voltage, it does not introduce errors. The reference voltage for the AD7785 is also generated using one of these matched current sources. It is developed using a precision resistor and applied to the differential reference pins of the ADC. This scheme ensures that the analog input voltage span remains ratiometric to the reference voltage. Any errors in the analog input voltage due to the temperature drift of the excitation current are compensated by the variation of the reference voltage. TEMPERATURE MEASUREMENT USING AN RTD To optimize a 3-wire RTD configuration, two identically matched current sources are required. The AD7785, which contains two well-matched current sources, is ideally suited to these applications. One possible 3-wire configuration is shown in Figure 21. In this 3-wire configuration, the lead resistances result in errors if only one current is used, as the excitation current flows through RL1, developing a voltage error between AIN1(+) and AIN1(–). In the scheme outlined, the second RTD current source is used to compensate for the error introduced by the excitation current flowing through RL1. The second RTD current flows through RL2. Assuming RL1 and RL2 are equal (the leads would normally be of the same material and of equal GND AVDD REFIN(+) REFIN(–) BAND GAP REFERENCE IOUT1 RTD RL2 RL3 AIN1(+) GND AVDD AIN1(–) BUF IOUT2 Σ-Δ ADC IN-AMP SERIAL INTERFACE AND CONTROL LOGIC DOUT/RDY DIN SCLK CS REFIN(+) GND RREF REFIN(–) INTERNAL CLOCK CLK Figure 21. RTD Application Using the AD7785 Rev. 0 | Page 30 of 32 AD7785 DVDD 06721-021 RL1 AD7785 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 0.65 BSC 0.30 0.19 COPLANARITY 0.10 0.20 0.09 8° 0° SEATING PLANE 0.75 0.60 0.45 COMPLIANT TO JEDEC STANDARDS MO-153-AB Figure 22. 16-Lead Thin Shrink Small Outline Package [TSSOP] (RU-16) Dimensions shown in millimeters ORDERING GUIDE Model AD7785BRUZ 1 AD7785BRUZ-REEL1 EVAL-AD7785EBZ1 1 Temperature Range –40°C to +105°C –40°C to +105°C Package Description 16-Lead TSSOP 16-Lead TSSOP Evaluation Board Z = RoHS Compliant Part. Rev. 0 | Page 31 of 32 Package Option RU-16 RU-16 AD7785 NOTES ©2007 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D06721-0-4/07(0) Rev. 0 | Page 32 of 32