The INA117 can be used to measure a current by sensing the voltage drop across a series resistor, R_S. Figure 7-4 shows the INA117 used to measure the supply currents of a device under test. The circuit in Figure 7-5 measures the output current of a power supply. If the power supply has a sense connection, the power supply can be connected to the output side of R_S to eliminate the voltage-drop error. Another common application is current-to-voltage conversion, as shown in Figure 7-6.

Figure 7-3 CMR Trim Circuit

Figure 7-4 Measuring Supply Currents of Device Under Test

*RC not needed if RS is less than 20Ω – see text.

Figure 7-5 Measuring Power Supply Output Current

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 R_S, 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, R_C, equal in value to R_S as shown in Figure 7-4, Figure 7-5, and Figure 7-6. If R_S is less than 20Ω, the degradation in CMR is negligible and R_C can be omitted. If R_S is larger than approximately 2kΩ, trimming R_C can be required to achieve greater than 86dB CMR. This trim is because the actual INA117 input impedances have 1% typical mismatch. If R_S 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 R_S. The corrected value, R_S', can be calculated by:

Example: For a 1V/mA transfer function, the nominal, uncorrected value for R_S is 1kΩ. A slightly larger value, R_S' = 1002.6Ω, compensates for the gain error due to loading.

The 380kΩ term in the equation for R_S' 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 can operate as a unity gain buffer or as an amplifier with non-inverting gain. Gain added ahead of the INA117 improves both CMR and signal-to-noise. Added gain also allows a lower voltage drop across the sense resistor. The OPA1013 is a good choice for the buffer amplifier since both the input and output can swing close to the negative power supply.

Figure 7-7 Current Sensing With Input Buffer

Figure 7-8 shows very high input impedance buffer used to measure low leakage currents. Here, the buffer operational amplifier is powered with an isolated, split-voltage power supply. Using an isolated power supply allows full ±200V common-mode input range.