This design features a highly linear, low-noise signal chain intended to measure dc voltages. Offset and gain errors are easily corrected with a simple two- or three-point calibration, but noise and linearity cannot be easily calibrated. Choosing low-noise and highly linear components is crucial. The 1/f noise, or flicker noise, is a much greater concern than broadband noise because this signal chain is intended to measure low-frequency signals.

The THP210 device, a fully differential amplifier shown in Figure 3-2, converts the single-ended input signals to differential signals for measurement with ADS127L21B.

Figure 3-2 TIDA-010970 ADC Block Diagram

A single-ended input measurement allows the DMM to measure the load relative to a common ground. Converting the signal to a differential signal provides better signal integrity and noise immunity for the remaining signal chain. The THP210 uses external precision thin-film resistors, RES21A, with a 1:2.5 ratio to achieve a gain of 0.4V/V. This stage attenuates the OPA828 output from a 10V range to a 4V range. The ADC reference is 4.096V, so the maximum ADC input signal uses 98% of the ADC full-scale range. This maximizes the ADC resolution.

The RES21A resistor divider is 10kΩ-based. RES21A has low nonlinearity because the large resistances reduce self-heating. These larger resistances add broadband noise to this system; however, the linearity improvement from RES21A outweighs the additional noise since most DMM measurements are taken at slower speeds.