Figure 7-6. Current to Voltage Converter

In all cases, the sense resistor imbalances the input resistor matching of the INA117, degrading the CMR. Also,

the input impedance of the INA117 loads RS, causing gain error in the voltage-to-current conversion. Both of

these errors can be easily corrected.

The CMR error can be corrected with the addition of a compensation resistor, RC, equal in value to RS as shown

in Figure 7-4, Figure 7-5, and Figure 7-6. If RS is less than 20Ω, the degradation in CMR is negligible and RC can

be omitted. If RS is larger than approximately 2kΩ, trimming RC can be required to achieve greater than 86dB

CMR. This trim is because the actual INA117 input impedances have 1% typical mismatch. If RS is more than

approximately 100Ω, the gain error is greater than the 0.05% specification of the INA117. This gain error can be

corrected by slightly increasing the value of RS. The corrected value, RS', can be calculated by:

RS′=RS×380kΩ

380kΩ−RS 

(1)

Example: For a 1V/mA transfer function, the nominal, uncorrected value for RS is 1kΩ. A slightly larger value, RS'

= 1002.6Ω, compensates for the gain error due to loading.

The 380kΩ term in the equation for RS' has a tolerance of ±25%, so sense resistors above approximately 400Ω

can require trimming to achieve gain accuracy better than 0.05%.

Of course, if a buffer amplifier is added as shown in Figure 7-7, both inputs see a low source impedance, and

the sense resistor is not loaded. As a result, there is no gain error or CMR degradation. The buffer amplifier

INA117

SBOS154B – SEPTEMBER 2000 – REVISED APRIL 2024

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