Figure 19 depicts a simplified schematic for this design showing the MSP430 ADC inputs and full input conditioning circuitry. The ADC is configured for a bipolar measurement where final conversion result is the differential voltage, VDIFF, between the positive and negative ADC inputs, A1+ and A1–. The bipolar, ground-referenced input signal must be level-shifted and attenuated by the op amp so that the output is biased to VREF / 2 and has a differential voltage that is within the ±VREF / 2 input range of the ADC. The transfer function for the op-amp circuit simplifies to Equation 4.
The voltage applied to the negative ADC input, A1–, is based on the resistor divider formed by R6 and R7 and is set to VREF / 2 by setting R6 equal to R7, as shown in Equation 5.