7.2.1 Design Requirements

This design example has the following requirements:

• Supply voltage: 3.3V

• Input: –1A to +1A

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

7.2.2 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

= V

Gain

+ V

OUT

SHUNT

Diff_Amp

REF

´

(1)

where

• V

= I

R

SHUNT

LOAD

´

SHUNT

•

Gain

=

Diff_Amp

R

R

4

3

•

V

= V

REF

CC

´

R

R

R

6

5

6

+

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

100mV and maximum load current of 1A.

R

=

SHUNT(Max)

=

= 100 mW

V

I

SHUNT(Max)

LOAD(Max)

100 mV

1 A

(2)

The tolerance of RSHUNT is directly proportional to cost. For this design, a shunt resistor with a tolerance of 0.5%

is selected. If greater accuracy is required, select a 0.1% resistor or better.

The load current is bidirectional; therefore, the shunt voltage range is –100mV to +100mV. This voltage is

divided down by R1 and R2 before reaching the operational amplifier, U1A. Make sure that the voltage present

at the noninverting node of U1A is within the common-mode range of the device. Therefore, use an operational

amplifier, such as the OPA397, that has a common-mode range that extends below the negative supply voltage.

Finally, to minimize offset error, the OPA397 has a typical offset voltage of merely ±0.25µV (±5µV maximum).

Given a symmetric load current of –1A to +1A, the voltage divider resistors (R5 and R6) must be equal. To

be consistent with the shunt resistor, a tolerance of 0.5% is selected. To minimize power consumption, 10kΩ

resistors are used.

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SBOSA02A – AUGUST 2021 – REVISED APRIL 2025

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