OPA328 datasheet(16/23 Pages) TI

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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 –100 mV to +100 mV. 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 OPAx328, that has a common-mode range that extends below the negative supply

voltage. Finally, to minimize offset error, the OPAx328 has a typical offset voltage of merely ±3 µV (±25 µV

maximum).

Given a symmetric load current of –1 A to +1 A, 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, 10

‑kΩ

resistors are used.

To set the gain of the difference amplifier, the common-mode range and output swing of the OPAx328 must

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

respectively, of the OPAx328 given a 3.3-V supply.

–100 mV < VCM < 3.4 V

(3)

100 mV < VOUT < 3.2 V

(4)

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

Gain

Diff_Amp

= 15.5

)

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 1 kΩ. A value of 15.4 kΩ is selected for R2 and R4 because this

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

8.2.1.3 Application Curve

1.65

-1.0

-0.5

0.5

1.0

3.30

Input Current (A)

Figure 8-3. Bidirectional Current-Sensing Circuit Performance: Output Voltage vs Input Current

8.2.2 Transimpedance Amplifier

Wide gain bandwidth, low input bias current, low input voltage, and low current noise make the OPAx328 an

excellent wideband photodiode transimpedance amplifier. Low-voltage noise is important because photodiode

capacitance causes the effective noise gain of the circuit to increase at high frequency.

OPA2328

SBOS957 – FEBRUARY 2022

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