Figure 6-1. Basic Circuit Connections

6.1.4 Offset Trim

The INA146 is laser-trimmed for low offset voltage and drift. Most applications require no external offset

adjustment. Figure 6-2 shows an optional circuit for trimming the offset voltage. A voltage applied to the Ref

terminal is summed with the output signal. This feature can be used to null offset voltage. To maintain good

common-mode rejection, the source impedance of a signal applied to the Ref terminal must be less than 10 Ω

and a resistor added to the positive input terminal must be 10 times that, or 100 Ω. Alternatively, the trim voltage

can be buffered with an operational amplifier such as the OPA277.

6.1.5 Input Impedance

The input impedance of the INA146 is determined by the input resistor network and is approximately 100

kΩ. The source impedance at the two input terminals must be nearly equal to maintain good common-mode

rejection. A 12-Ω mismatch in impedance between the two inputs causes the typical common-mode rejection to

be degraded to approximately 72 dB. Figure 6-7 shows a common application measuring power supply current

through a shunt resistor. The source impedance of the shunt resistor, RS, is balanced by an equal compensation

resistor, RC.

Source impedances greater than 800 Ω are not recommended, even if the source impedances are perfectly

matched. Internal resistors are laser trimmed for accurate ratios, not to absolute values. Adding equal resistors

greater than 800 Ω can cause a mismatch in the total resistor ratios, degrading CMR.

INA146

SBOS109A – SEPTEMBER 1999 – REVISED NOVEMBER 2023

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