Using the AD771x Family of 24-Bit Sigma-Delta A/D Converters Application Note (AN-406)

The thermocouple is grounded at AIN1(–) in order to provide a return path for the input leakage currents of the differential inputs that must flow at all times. If the voltage coming from the thermocouple has a commonmode voltage component, this short may be replaced by a resistor to ground. Due to the Johnson noise which this resistor generates, it should not be any larger than about 100 kΩ. In applications where the thermocouple is electrically connected to a chassis whose ground is referred to the local ground, this connection is unnecessary. INTRODUCTION The AD771x Series is a family of ADCs that are designed specifically for low frequency, high accuracy industrial transducer applications. Figure 1 shows a block diagram of one of these devices. The analog front end, which in some cases also includes excitation sources for sensors, accepts low level signals that can be amplified by the internal programmable gain amplifier before being applied to a sigma-delta modulator. The pulse stream appearing at the output of this modulator is applied to a digital filter which produces a low-pass filtered, high resolution, serial output.