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.

7.1.4 Noise Performance

The noise performance of the INA117 is dominated by the internal resistor network. The thermal or Johnson

noise of these resistors produces approximately 550nV/√Hz noise. The internal op amp contributes virtually no

excess noise at frequencies above 100Hz.

Many applications can be satisfied with less than the full 200kHz bandwidth of the INA117. In these cases,

the noise can be reduced with a low-pass filter on the output. The two- pole filter shown in Figure 7-9 limits

bandwidth to 1kHz and reduces noise by more than 15:1. Since the INA117 has a 1/f noise corner frequency of

approximately 100Hz, a cutoff frequency below 100Hz does not further reduce noise.

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INA117

SBOS154B – SEPTEMBER 2000 – REVISED APRIL 2024

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