Current can flow through the shunt in both positive and negative directions which will create both a positive and negative voltage that is fed back to the shunt amplifier circuit. Most cost-effective motor inverters do not have both positive and negative power supplies that can handle this bipolar signal. A bipolar current signal is brought into an amplifier that will only be effective from zero to the positive voltage supply. To allow the unipolar op-amp circuit to measure a bipolar signal, a voltage reference is summed into the non-inverting side of the current feedback op-amps. The AFE of the 2805x device contains a 6-bit DAC with a voltage follower for providing an output reference for this reason. A circuit configuration that can use a voltage reference to measure the bipolar current signal is shown in Figure 7-6.

Figure 6-6 Using the AFE's Built-In Voltage Reference For Measuring a Bipolar Signal

Equation 1 shows how to calculate the voltage at V_PGA. As an example, set the PGA gain = 3. V_ADCIN will be 2V_BIPOLAR + V_REF. Assume the system's V_REFHI is 3.3 V. To allow for maximum voltage swing in both directions, V_REF is set to 1.65 V. Now the maximum peak V_BIPOLAR voltage that can be measure is ±0.825 V.

Equation 1.

Suppose the same hardware is used and a higher resolution is required. The PGA gain = 6. V_ADCIN is 4V_BIPOLAR + 2V_REF. V_REF must be adjusted to be 0.825 V. The maximum peak V_BIPOLAR voltage that can be measured is ±0.4125 V.

The voltage reference output is adjusted by a 6-bit DAC. The VREFOUTCTL register controls the DAC's voltage output by Equation 2.

Equation 2.