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INA116: low input impedance?

Part Number: INA116


INA116 instrumentation amplifier shows unexpectedly low input impedance (<1Mohm)

I put 2 V pp signal to VL through 1Mohm resistor and only get 1 V on the input, as I'd get from resistor divider with 1Mohm INA116 resistance to ground.

This value of input impedance is indirectly confirmed by other measurements taken on other parts of the circuit (not shown here) 

Datasheet calls for >10^15 Ohmi input impedance, both differential and common mode.

Any thoughts are greatly appreciated

  • Hello Ruslan,

    A properly functioning INA116 should have extremely high input resistance as the datasheet Electrical Specification table indicates. Therefore, something doesn't seem quite right when you observe such low input resistance.

    The INA116 inputs are ESD protected, but it is still possible to damage the input circuit/ESD cell if exposed to an excessively high ESD voltage event. In that case the input circuit may become leaky and the high input impedance becomes compromised. If all of the INA116 devices are exhibiting the behavior, then ESD likely isn't the cause.

    You mention the 1 Megohm resistor in series with the INA116 pin 6 input. If the probe placed at the input has considerable capacitance, then a passive low-pass filter is formed by the R and C. For example, if the series R is 1 Megohm, and probe capacitance is 15 pF, the filter has a 10 kHz cutoff frequency. If a frequency not too much greater than 10 kHz is applied to the input of the filter, the gain will be down -6 dB (0.5 V/V) at some point.  If the frequency is further increased the gain will continue to drop.

    Have you checked the output of the INA116 with a DSO when the input is being probed and producing the voltage drop? Make sure that the measurement isn't injecting noise into the circuit, or that the INA116 isn't becoming unstable when the input is being probed.

    Lastly, make sure the INA116 was obtained from a qualified TI distributor. That way you know you are working with product that has been fully tested and is warranted by TI.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thank you very much, Thomas, the chip seems to have been malfunctioning, probably due to ESD damage.

    I replaced it with new one and the impedance went to >50MOhm values, probably more, but this is the maximum that I can measure in my setup.

    I also noticed that there is an input offset of several decades of millivolts which depends on supply voltage and positive/negative supply mismatch.

    Does it make any sense to measure the offset as voltage between inverting and non-inverting inputs with digital multimeter?

    Thanks again for your help!

  • Hi Ruslan,

    It sounds like you are on the right track for solving the mystery of the low INA116 input impedance!

    Since the inputs of the INA116 present an extremely high input impedances connecting a low impedance 1-Megohm, or 10-Megohm, voltmeter across them will likely load them and corrupt the true offset level. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering