Part Details for AD8275 by Analog Devices Inc

This circuit provides a fully programmable universal analog front end (AFE) for process control applications supporting 2-, 3-, and 4-wire RTD configurations, thermocouple inputs with cold junction compensation, unipolar and bipolar input voltages, and 4 mA-to-20 mA inputs as shown in the configuration diagram of Figure 2.

This circuit is comprised of an AD8226 rail-to-rail output inamp, connected to the positive input of the AD8275 G = 0.2 difference amplifier, whose output is then connected to the input of the AD7685, a 16-bit, 250 kSPS PulSAR ADC in MSOP/QFN. The AD8226 is set for a gain-of-one mode (high voltage/current inputs), and its output is referenced to ground. Single-ended or differential inputs can be used. The output of the AD8226 is a bipolar signal, which drives the AD8275 input. The AD8275 acts to attenuate and level shift the bipolar input, providing for a gain of 0.2. Hence, differential inputs of 20 V p-p at its inputs will yield a 4 V p-p single-ended range on the output. The ADR439 precision 4.5 V reference is used to provide both the internal common-mode bias voltage for the AD8275 (VREF/2 = 2.25V), as well as the external reference voltage for the AD7685 ADC. Under these conditions, the output of the AD8275 swings from +0.25 V to +4.25 V, which is within the range of the AD7685, 0 V to +4.5 V.

The circuit is comprised of the AD8295 and AD8275 as the analog front end circuit, an AD7687 ADC with an ADR431 reference, and only a few other external components for decoupling, etc.

The 8-channel battery testing system (EVAL-CN0352-EB1Z) contains eight input/output boards (EVAL-CN0352-EB1Z_IO) and one MCU board (EVAL-CN0352-EB1Z_MCU) that plug into one base board (EVAL-CN0352-EB1Z_BAS). This circuit is the input/output board.