To set the gain of the difference amplifier, the common-mode range and output swing of the OPA397 must be

considered. Equation 3 and Equation 4 depict the typical common-mode range and maximum output swing,

respectively, of the OPA397 given a 3.3V supply.

–100mV < VCM < 3.4V

(3)

100mV < VOUT < 3.2V

(4)

The gain of the difference amplifier can now be calculated as shown in Equation 5:

Gain

=

Diff_Amp

V

V

= 15.5

V

V

R

(I

I

)

OUT_Max

OUT_Min

SHUNT

MAX

MIN

-

-

´

3.2 V

100 mV

100 m

-

W ´

- -

[1 A

( 1A)]

=

(5)

The resistor value selected for R1 and R3 is 1kΩ. 15.4kΩ is selected for R2 and R4 because this number is the

nearest standard value. Therefore, the calculated gain of the difference amplifier is 15.4V/V.

The gain error of the circuit primarily depends on R1 through R4. As a result of this dependence, 0.1% resistors

are selected. This configuration reduces the likelihood that the design requires a two-point calibration. A simple

one-point calibration, if desired, removes the offset errors introduced by the 0.5% resistors.

7.2.3 Application Curve

1.65

-1.0

-0.5

0

0.5

1.0

0

3.30

Input Current (A)

Figure 7-2. Bidirectional Current-Sensing Circuit Performance: Output Voltage vs Input Current

OPA397, OPA2397, OPA4397

SBOSA02A – AUGUST 2021 – REVISED APRIL 2025

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