Circuit Note CN-0355 Devices Connected/Referenced 3-Channel, Low Noise, Low Power, AD7793 24-Bit, Sigma-Delta ADC Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve todays Wide Supply Range, Micropower, AD8420 analog, mixed-signal, and RF design challenges. For more Rail-to-Rail In-Amp information and/or support, visit www.analog.com/CN0355 30 V, MicroPower, Overvoltage ADA4096-2 Protection, Rail-to-Rail Input/Output (RRIO), Dual Op Amp Low Power, Temperature Compensated Bridge Signal Conditioner and Driver The circuit can process full-scale signals from approximately EVALUATION AND DESIGN SUPPORT 10 mV to 1 V, using the internal programmable gain amplifier Circuit Evaluation Boards (PGA) of the 24-bit, sigma-delta (-) ADC, making it suitable CN-0355 Evaluation Board (EVAL-CN0355-PMDZ) for a wide variety of pressure sensors. System Demonstration Platform (EVAL-SDP-CB1Z) SDP to PMOD Interposer Board (SDP-PMD-IB1Z) The entire circuit uses only three ICs and requires only 1 mA Design and Integration Files (excluding the bridge current). A ratiometric technique ensures Schematics, Layout Files, Bill of Materials that the accuracy and stability of the system does not depend on a voltage reference. CIRCUIT FUNCTION AND BENEFITS The circuit in Figure 1 is a complete, low power signal conditioner for a bridge type sensor and includes a temperature compensation channel. This circuit is ideal for a variety of industrial pressure sensors and load cells that operate with drive voltages of between 5 V and 15 V. +3.3V V = +24V CC +24V 100 1/2 AV DV Q1 DD DD ADA4096-2 10k MMBTA06-7-F AIN1(+) 40.2k 100nF CS CS P5 0.1% V DRIVE SCLK SCLK 91k 210A R WIRE P2-1 P6 DIN DIN 0.1% IOUT1 R P3 10k WIRE P2-2 DOUT/RDY DOUT/RDY BRIDGE TYPE AIN2(+) 140k TRANSDUCER P7 0.1% 100nF MFG NSCSANN600MGUNV P9 P1-3 RTD R WIRE P2-3 10k AIN2() 10k R1 R4 R P4 WIRE P2-4 100nF 10k P1-4 + P1-2 +24V AD7793 V O 1/2 100nF R2 R3 FB ADA4096-2 10k 1F AD8420 IN-AMP AIN1() P1-1 5k 10k REF 100nF 100pF 0.1% 25ppm 100nF V = 1.05V REF RFIN(+)/AIN3(+) RFIN()/AIN3() GND Figure 1. Differential Bridge Type Transducer Monitor with Temperature Compensation (Simplified Schematic: all Connections and Decoupling not Shown) Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 2014 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 12041-001CN-0355 Circuit Note The transfer function can be computed as follows: CIRCUIT DESCRIPTION The circuit shown in Figure 1 is based on the AD7793, 24-bit, R F V V 1 DRIVE REF -. It has three differential analog inputs and has an on-chip, R8 low noise PGA with gain that ranges from unity gain to 128, where R can be 40.2 k, 91 k, or 140 k, and R8 = 10 k. F making it ideal for multiple sensor interfaces. The AD7793 An NPN transistor is use to boost the current needed to drive the consumes a maximum of only 500 A and is therefore suitable bridge sensor. Feedback to the inverting input of the ADA4096-2 for low power applications. It has a low noise, low drift internal makes the inverting input voltage equal to the noninverting band gap reference and can accept an external differential input voltage, thereby ensuring a constant voltage of the voltage reference. The output data rate is software programmable from drive across the bridge circuit. 4.17 Hz to 470 Hz. Transistor Q1 is a BJT that has maximum breakdown voltage of The AD8420 low power in-amp with a supply current of 80 A 80 V, capable of dissipating 0.35 W at 25C. The maximum maximum, can operate up to 36 V single-supply and is used to collector current is 500 mA. remove the common voltage at the bridge transducer. It can also provide gain, if needed, to the small differential signal output of Instrumentation Amplifier the transducer. The AD8420 rejects the common-mode voltage generated at the The ADA4096-2, dual channel op amp, with a typical supply bridge and only amplifies the differential bridge voltage, as shown current of 60 A per amp and a wide operating input voltage in Figure 3. The AD8420 has rail-to-rail output voltage swing range of up to 30 V, drives the sensor bridge. The other half of that is completely independent of the input common-mode the ADA4096-2 is used as a buffer for the reference voltage. voltage. This feature exempts the AD8420 from the restrictions caused by the interaction between the common-mode input and There are a wide variety of pressure sensors requiring a voltage output voltages associated with most conventional instrumentation drive between 5 V and 15 V. The circuit shown in Figure 1 amplifier architectures. The gain of the in-amp is set at unity. provides a complete solution for bridge type transducers and +V has four critical sections the transducer voltage drive, the in- DRIVE amp, the reference buffer, and the ADC. BRIDGE TRANSDUCER P1-3 Bridge Type Transducer Voltage Drive V CC R1 R4 +24V The ADA4096-2 is configured as a noninverting amplifier with + 10k P1-4 P1-2 V O configurable gain set by the feedback resistor, as shown in AD8420 R2 R3 100nF Figure 2. 1F IN-AMP V OUT P1-1 R12 FB V = +24V 10k CC R10 +24V 100nF 100 +V REF REF V REF 1/2 FOR G = 1: ADA4096-2 R10 = DNI, R12 = 0 R19 Figure 3. AD8420 Instrumentation Amplifier P5 40.2k +V DRIVE The input to the AD8420 has a differential mode noise filter R20 P6 91k (20 k/1 F/100 nF) with a 7.6 Hz bandwidth and a common- BRIDGE mode noise filter (10 k/100 nF) with a 150 Hz bandwidth. TRANSDUCER R21 P1-3 P7 140k Traditional instrumentation amplifier architectures require the R1 R4 R8 + P1-4 P1-2 10k reference pin to be driven with a low impedance source. Any +V IN V O AD8420 impedance at the reference pin degrades both common-mode R2 R3 rejection ratio (CMRR) and gain accuracy. With the AD8420 P1-1 architecture, resistance at the reference pin has no effect on V IN AD8420 CMRR. The transfer function of the AD8420 is Figure 2. Transducer Voltage Drive V = G (V V ) + V OUT IN+ IN REF The gain is set by configuring the jumpers indicated in Table 1. where: VREF = 1.05 V Table 1. Pin Configuration for Specific Voltage Drive G = 1 + (R12/R10) Gain R19/R20/R21 RF Feedback P5 P6 P7 5.02 40.2 k Short Open Open The AD8420 differential input voltage is internally limited with 10.1 91 k Open Short Open diodes at 1 V from 40C to +85C. If the input voltage exceeds 15 140 k Open Open Short the limit, the internal diodes start to conduct and draw current. The current is limited internally to a value that is safe for the AD8420. Rev. 0 Page 2 of 6 12041-002 12041-003