9.2.2 Detailed Design Procedure
The first stage of the application circuit consists of an OPA333-Q1 in a noninverting summing amplifier configuration and serves two purposes:
- To level-shift the ground-referenced signal to allow bidirectional current measurements while running off a unipolar supply. The voltage across the shunt resistor, VSHUNT, is level-shifted by a common-mode voltage, VCM, as shown in Figure 29. The level-shifted voltage, VINX, at the noninverting input is given by Equation 3.
Equation 3. VINX = (VCM · R3 + VSHUNT · R4) / (R3 + R4)
- To amplify the level-shifted voltage (VINX). The OPA333-Q1 is configured in a noninverting gain configuration with the output voltage, VOUT, given by Equation 4.
Equation 4. VOUT = VINX · (1 + R2 / R1)
Using Equation 3 and Equation 4, VOUT is given as a function of VSHUNT and VCM by Equation 5.
Equation 5. VOUT = (VCM · R3 + VSHUNT · R4) / (R3 + R4) · (1 + R2 / R1)
Using Equation 5 the ADC differential input voltage, before the first-order RC filter, is given by Equation 6.
Equation 6. VOUT – VCM = VSHUNT · (1 + R2 / R1) / (1 + R4 / R3) + VCM · (R2 / R1 – R3 / R4) / (1 + R3 / R4)
If R1 = R3 and R2 = R4, Equation 6 is simplified to Equation 7.
Equation 7. VOUT – VCM = VSHUNT · (1 + R2 / R1) / (1 + R4 / R3)