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LOAD

BUS

OUT

REF

SHUNT

U1A

U1B

SHUNT

Figure 8-2. Bidirectional Current-Sensing Schematic

8.2.1.1 Design Requirements

This solution has the following requirements:

• Supply voltage: 3.3 V

• Input: –1 A to +1 A

• Output: 1.65 V ±1.54 V (110 mV to 3.19 V)

8.2.1.2 Detailed Design Procedure

The load current, ILOAD, flows through the shunt resistor, RSHUNT, to develop the shunt voltage, VSHUNT. The

shunt voltage is then amplified by the difference amplifier consisting of U1A and R1 through R4. The gain of the

difference amplifier is set by the ratio of R4 to R3. To minimize errors, set R2 = R4 and R1 = R3. The reference

voltage, VREF, is supplied by buffering a resistor divider using U1B. The transfer function is given by Equation 1.

= V

Gain

+ V

OUT

SHUNT

Diff_Amp

REF

(1)

where

• V

= I

SHUNT

LOAD

SHUNT

•

Gain

Diff_Amp

•

= V

REF

There are two types of errors in this design: offset and gain. Gain errors are introduced by the tolerance of the

shunt resistor and the ratios of R4 to R3 and, similarly, R2 to R1. Offset errors are introduced by the voltage

divider (R5 and R6) and how closely the ratio of R4 / R3 matches R2 / R1. The latter value affects the CMRR of

the difference amplifier, ultimately translating to an offset error.

The value of VSHUNT is the ground potential for the system load because VSHUNT is a low-side measurement.

Therefore, a maximum value must be placed on VSHUNT. In this design, the maximum value for VSHUNT is set

to 100 mV. Equation 2 calculates the maximum value of the shunt resistor given a maximum shunt voltage of

100 mV and maximum load current of 1 A.

SHUNT(Max)

= 100 mW

SHUNT(Max)

LOAD(Max)

100 mV

1 A

(2)

OPA2328

SBOS957A – FEBRUARY 2022 – REVISED JUNE 2022

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