High impedance high CMR, ±10V analog front end signal conditioning circuit diagram

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Circuit function and advantage

The circuit shown in Figure 1 is a complete analog front end that uses a 16-bit differential input PulSAR® ADC to digitally convert ±10 V industrial grade signals. The circuit utilizes only two analog devices to provide a high-impedance instrumentation amplifier input, level shifting, attenuation, and differential conversion with high common-mode rejection (CMR) performance. Thanks to its high level of integration, this circuit saves printed circuit board space and provides a cost-effective solution for common industrial applications.

Typical signal levels are up to ±10 V in process control and industrial automation systems. While the signal input from sensors such as thermocouples and load cells is small, large common-mode voltage swings are often encountered, which requires flexible analog inputs that can handle large and small differential signals with high common-mode rejection. It also has a high impedance input.

Figure 1. High-performance analog front end for industrial process control applications (schematic diagram: all connections and decoupling are not shown)

Attenuation and level shifting must be performed when processing industrial grade signals with modern low voltage ADCs. In addition, fully differential input ADCs offer the following advantages: good common-mode rejection, fewer second-order distortion products, and a simplified DC adjustment algorithm. Therefore, industrial signals need to be further conditioned to properly interface with differential input ADCs.

The circuit shown in Figure 1 is a complete and highly integrated analog front-end industrial-grade signal conditioner that uses only two active devices to drive the differential input 16-bit PulSAR ADC AD7687: Precision Instrumentation Amplifier (on-chip integration of two auxiliary operations Amplifier) ​​AD8295 precision in-amp (with two on-chip auxiliary op amps) and level shifter / ADC driver AD8275. The low noise 2.5V XFET® reference ADR431 provides a reference for the ADC.

The AD8295 is a precision instrumentation amplifier with two non-dedicated signal processing amplifiers on-chip and two precision matched 20 kΩ resistors in a 4 mm &TImes; 4 mm package.

The AD8275 is a G = 0.2 differential amplifier that can be used to attenuate ±10 V industrial signals. The attenuated signal can be easily interfaced to a single-supply, low-voltage ADC. The AD8275 performs attenuation and level shifting in this circuit to maintain a good CMR without any external components.

The AD7687 is a 16-bit successive approximation ADC that operates from a single 2.3 V to 5.5 V supply. It uses a differential input with good CMR and simplifies the use of SAR ADCs.