Circuit Note CN-0225 Devices Connected/Referenced Precision Instrumentation Amplifier with AD8295 Circuits from the Lab reference circuits are engineered and Signal Processing Amplifiers tested for quick and easy system integration to help solve todays AD8275 G = 0.2, Level Translator, 16-Bit ADC Driver analog, mixed-signal, and RF design challenges. For more AD7687 16-Bit, 250 kSPS PulSAR Differential ADC information and/or support, visit www.analog.com/CN0225. Ultralow Noise XFET Voltage Reference ADR431 with Current Sink and Source Capability High Impedance, High CMR, 10 V Analog Front End Signal Conditioner for Industrial Process Control and Automation level shifting, attenuation, and differential conversion, with only EVALUATION AND DESIGN SUPPORT two analog components. Because of the high level of integration, Circuit Evaluation Boards CN-0225 Circuit Evaluation Board (EVAL-CN0225-SDPZ) the circuit saves printed circuit board space and offers a cost System Demonstration Platform (EVAL-SDP-CB1Z) effective solution for a popular industrial application. Design and Integration Files Signal levels of up to 10 V are typical in process control and Schematics, Layout Files, Bill of Materials industrial automation systems. With smaller signal inputs from CIRCUIT FUNCTION AND BENEFITS sensors such as thermocouples and load cells, large common- mode voltage swings are often encountered. This requires The circuit, shown in Figure 1, is a complete analog front end a flexible analog input that handles both large and small for digitizing 10 V industrial level signals with a 16-bit differential signals with high common-mode rejection and differential input PulSAR ADC. The circuit provides a high also has a high impedance input. impedance instrumentation amplifier input with high CMR, +15V +15V U2 7 U1-C 11 +VS AD8275 AD8295 +IN1 VREF ADR431 +15V +2.5V 10k 2 IN 5 A1R1 SENSE 10 14 13 U1-A +IN2 IN2 16 50k 10k 20k OUT +VS AD8295 6 VCOM 9 IN1 A2 VINP 4 +IN +1.25V VIN 3 RG1 20k OUT +IN REF2 20k 15 3 8 OUT2 A1 10k VIN 2 RG2 IA 12 U1-B 50k 1 IN 20k REF1 1 A1R2 8 REF 6 AD8295 OUT1 VINN VS VS 7 22F 5 4 VOUTP 1 2 33 10 3 15V REF VDD VIO 3.3V 9 IN+ SDI SDI U3 8 VOUTN 1.5nF SCK SCK AD7687 7 33 SDO SDO IN 6 GND CNV CNV 4 +10V 5 +2.25V +2.25V 0V +1.25V +1.25V +0.25V +0.25V 10V Figure 1. High Performance Analog Front for Industrial Process Control (Simplified Schematic: All Connections and Decoupling Not Shown) Rev.A Circuits from the Lab circuits 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 determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2011 Analog Devices, Inc. All rights reserved. 10058-001CN-0225 Circuit Note Attenuation and level shifting are necessary to process output to drive the ADC. The AD8275 (G = 0.2) level translator industrial level signals with modern low voltage ADCs. In is a difference amplifier with matched on-chip precision laser- addition, fully differential input ADCs offer the advantages of trimmed thin film resistors to ensure low gain error, low gain good common-mode rejection, reduction in second-order drift (1 ppm/ maximum), and high common-mode rejection distortion products, and simplified dc trim algorithms. (80 dB). The AD8275 has a wide power supply range from Industrial signals, therefore, need further conditioning to +3.3 V to +15 V, as well as a large input voltage range from properly interface with differential input ADCs. 12.3 V to +12 V when operating on a single +5 V power supply. The circuit in Figure 1 is a complete and highly integrated Driving the Differential Input ADC analog front end industrial level signal conditioner that uses The circuit in Figure 1 uses a balanced difference amplifier only two active components to drive an AD7687 differential composed of the AD8275 (U2) and one of the uncommitted op input 16-bit PulSAR ADC: the AD8295 precision in-amp (with amps (U1-C) in the AD8295. This op amp (U1-C) is used to two on-chip auxiliary op amps) and the AD8275 level invert the positive output of the AD8275 (thereby providing a translator/ADC driver. An ADR431 low noise 2.5V XFET complementary negative output) and drive the REF1 and REF2 reference supplies the voltage reference for the ADC. pins of the AD8275. The output common-mode voltage of the differential output (VCOM = 1.25 V) is developed from the The AD8295 is a precision instrumentation amplifier with two 10 k external resistor divider connected to the 2.5 V reference uncommitted on-chip signal processing amplifiers and two precisely matched 20 k resistors in a small 4 mm 4 mm and is applied to the noninverting input of U1-C. The equations package. describing the circuit operation are as follows: The AD8275 is a G = 0.2 difference amplifier that can be used to VOUTP + VOUTN = 2 VCOM attenuate 10 V industrial signals, and the attenuated signal can VOUTP = VOUTN + 0.2 VIN be easily interfaced to a single supply low voltage ADC. The VOUTP = VCOM + 0.1 VIN AD8275 performs the attenuation and level shifting function in the circuit, maintaining good CMR without any need for VOUTN = VCOM 0.1 VIN external components. From the equations, with a 10 V input voltage, the voltages to the ADC (VOPTP and VOUTN) will each swing between The AD7687 is a 16-bit, successive approximation ADC that 0.25 V and 2.25 V, 180 out of phase with respect to each other, operates from a single power supply between 2.3 V and 5.5 V. It with a common-mode voltage of 1.25 V. The differential signal, has a differential input for good CMR and also offers the ease of use associated with SAR ADCs. therefore, utilizes 4 V out of the 5 V available differential input range of the ADC. CIRCUIT DESCRIPTION The ADR431 is a 2.5 V reference in a family of XFET voltage The circuit is comprised of the AD8295 and AD8275 as the references featuring low noise, high accuracy, and low analog front end circuit, an AD7687 ADC with an ADR431 temperature drift performance. The ADR431 drives the resistor reference, and only a few other external components for divider and the reference input of the AD7687 ADC. The decoupling, etc. ADR431output is buffered by the second uncommitted op amp (U1-B) in the AD8295 and drives the power supply (VDD) of Instrumentation Amplifier (Integrated into the AD8295) the AD7687. A single-pole RC filter composed of two 33 The instrumentation amplifier (IA) integrated into the resistors and a 1.5 nF capacitor serves as a 3 MHz cutoff AD8295 is set for a gain-of-one as the operating condition. antialiasing and noise reduction filter for the AD7687. For applications where higher gain is required, an appropriate Layout Considerations external gain resistor can be added. The power supply of the AD8295 is 15 V, which allows for the full 10 V industrial The performance of this or any other high speed or high input signal level. The reference pin of the in-amp connects to resolution circuit is highly dependent on proper PCB layout. ground, thereby making the output of the AD8295 ground This includes, but is not limited to, power supply bypassing, referenced. signal routing, and proper power planes and ground planes. See Tutorial MT-031, Tutorial MT-101, and the article A Practical Difference Amplifier/Attenuator (AD8275) Guide to High-Speed Printed-Circuit-Board Layout for more The signal at the output of the AD8295 in-amp is single-ended detailed information regarding PCB layout. with a maximum amplitude of 10 V. This signal must be attenuated and level shifted to the proper level to drive the AD7687 ADC. A simple resistive level attenuator stage directly on the output of AD8295 would not provide a differential Rev. A Page 2 of 5