Analog Devices: CN0225 Reference Design for High Impedance, High CMR, ±10 V Analog Front End Signal Conditioner for Industrial Process Control and Automation

atomar — Sun, 27 Oct 2013 06:00:54 GMT

Current Revision posted to Documents by atomar on 10/27/2013 6:00:54 AM

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Kit Overview

The reference circuit EVAL-CN0225-SDPZEVAL-CN0225-SDPZ is a complete analog front end for digitizing ±10 V industrial level signals with a 16-bit differential input PulSAR® ADC. The circuit provides a high impedance instrumentation amplifier input with high CMR, level shifting, attenuation, and differential conversion, with only two analog components. Because of the high level of integration, the circuit saves printed circuit board space and offers a cost effective solution for a popular industrial application.

Signal levels of up to ±10 V are typical in process control and industrial automation systems. With smaller signal inputs from sensors such as thermocouples and load cells, large commonmode voltage swings are often encountered. This requires a flexible analog input that handles both large and small differential signals with high common-mode rejection and also has a high impedance input.

Attenuation and level shifting are necessary to process industrial level signals with modern low voltage ADCs. In addition, fully differential input ADCs offer the advantages of good common-mode rejection, reduction in second-order distortion products, and simplified dc trim algorithms. Industrial signals, therefore, need further conditioning to properly interface with differential input ADCs.

The Circuit is a complete and highly integrated analog front end industrial level signal conditioner that uses only two active components to drive an AD7687AD7687 differential input 16-bit PulSAR ADC: the AD8295AD8295 precision in-amp (with two on-chip auxiliary op amps) and the AD8275AD8275 level translator/ADC driver. An ADR431ADR431 low noise 2.5V XFET® reference supplies the voltage reference for the ADC.

The AD8275AD8275 is a G = 0.2 difference amplifier that can be used to attenuate ±10 V industrial signals, and the attenuated signal can be easily interfaced to a single supply low voltage ADC. The AD8275 performs the attenuation and level shifting function in the circuit, maintaining good CMR without any need for external components.

The AD7687AD7687 is a 16-bit, successive approximation ADC that operates from a single power supply between 2.3 V and 5.5 V. It has a differential input for good CMR and also offers the ease of use associated with SAR ADCs.

The circuit uses the EVAL-CN0225-SDPZEVAL-CN0225-SDPZ circuit board and the EVAL-SDP-CB1ZEVAL-SDP-CB1Z System Demonstration Platform (SDP-B) evaluation board. The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the circuit’s performance. The EVAL-CN0225-SDPZ board contains the circuit to be evaluated and the SDP evaluation board is used with CN0225 evaluation software to capture the data from the EVAL-CN0225-SDPZ circuit board.

Key Applications: Electronic Test & Measurement

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Development Tools

PC with a USB port and Windows XP or Vista (32-bit), or Windows 7 (32-bit)
EVAL-CN0225-SDPZEVAL-CN0225-SDPZ circuit evaluation board
EVAL- SDP-CB1ZEVAL- SDP-CB1Z SDP evaluation board
- Blackfin Processor ADSP-BF527
- USB 2.0 Interface to PC
- JTAG header footprint
- 32MB SDRAM
- 2 x Small Footprint 120 Pin Connectors
- Configurable Peripheral Interfaces - SPI - SPORT - I2C - GPIOs - Parallel - Timers
DC Supply: +15 V, –15 V, and +6 V
Low distortion single-ended or differential signal source, such as Agilent 81150A-00281150A-002 or Audio Precision System Two 2322

Technical Documents

Learning Center

Type	Description
User Manual	ADI: User Manual for CN-0225 High Impedance, High CMR, +/-10V Analog Front End Signal Conditioner for Industrial Process Control and Automation
User Guide	ADI: User Guide for EVAL-SDP-CB1Z
Datasheet	ADI: Datasheet for AD8295: Precision Instrumentation Amplifierwith Signal Processing Amplifiers (Rev A)
Datasheet	ADI: Datasheet for AD7687: 16-Bit, 1.5 LSB INL, 250 kSPS PulSAR Differential ADC in MSOP/QFN (Rev B)
Datasheet	ADI: Datasheet for ADR431-EP/ADR434-EP/ADR435-EP: Low Noise XFET Voltage References with Current Sink and Source Capability (Rev A)
Datasheet	ADI: Datasheet for AD8275: G = 0.2, Level Translation, 16-Bit ADC Driver (Rev A)

Design Elements

Type	Description
Schematics	ADI: Schematics for EVAL-CN0225-SDPZ Evaluation Board
Layout	ADI: Layout for EVAL-CN0225-SDPZ
BOM	ADI: Bill Of Material (BOM) for EVAL-CN0225-SDPZ Evaluation Board

Video

Video 1	Video 2
www.youtube.com/watch	www.youtube.com/watch

Kit Features

Key Features for EVAL-CN0225-SDPZEVAL-CN0225-SDPZ Evaluation Board are as below:

Complete analog front end for digitizing ±10 V industrial level signals
AD7687AD7687 16-bit differential input PulSAR® ADC
- 16 Bits Resolution with No Missing 16-Bit Codes
- Throughput: 250 kSPS
- INL: ±0.4 LSB typ, ±1.5 LSB max (±23 ppm of FSR)
- Dynamic range: 96.5 dB
- S/(N + D): 95.5 dB @ 20 kHz
- THD: −118 dB @ 20 kHz
High impedance instrumentation amplifier input
- with high CMR, level shifting, attenuation, and differential conversion, with only two analog components
Two active components to drive an AD7687 differential input 16-bit PulSAR ADC:
- The AD8295AD8295 precision in-amp (with two on-chip auxiliary op amps)
  - Saves board space
  - Includes precision in-amp, 2 op amps, and 2 matched resistors
  - No heat slug for more routing room
  - Differential output fully specified
  - Wide power supply range: ±2.3 V to ±18 V
- The AD8275AD8275 level translator/ADC driver
  - Translates ±10 V to +4 V
  - Drives 16-bit SAR ADCs
  - Input overvoltage: +40 V to -35 V (VS = 5 V)
  - Fast settling time: 450 ns to 0.001%
  - Rail-to-rail output
  - Wide supply operation: +3.3 V to +15 V
  - High CMRR: 80 dB
An ADR431ADR431 low noise 2.5V XFET® reference supplies the voltage reference for the ADC.
- Low noise (0.1 Hz to 10.0 Hz): 3.5 µV p-p @ 2.5 V output
- No external capacitor required
- Low temperature coefficient
- Load regulation: 15 ppm/mA
- Line regulation: 20 ppm/V
- Wide operating range: 4.5 V to 18 V

Kit Contents

The Analog Devices EVAL-CN0225-SDPZEVAL-CN0225-SDPZ kit is supplied with below contents:

Evaluation Board

Tags: ad7687, adr431, ad8295, ad8275, analog_devices, eval-cn0225-sdpz