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 8-6. 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 = R4 and R2
= R3, Equation 6 is simplified to Equation 7.
Equation 7. VOUT – VCM = VSHUNT · (1 + R2 / R1) / (1 + R4 / R3)