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INA188: Floating Inputs

Marco DAlessio
Prodigy 25 points

Part Number: INA188

Dear all,

I'm designing a circuit that uses INA188 as instrumentation amplifiers to measure some strain gauges. The strain gauges are not always connected to the INA188 amplifiers, so when there's no input they saturate either towards the positive or the negative supply rail (they are powered with a differential voltage). The saturation and the potential higher power consumption is not an issue for me, but I'm wondering if the op-amps could get damaged in this condition and thus I should connect them to a reference voltage. If so, would it be ok to connect both inputs to GND via pull-down resistors?

  • +1
    TI__Guru 72851 points

    INA188 will not be damaged by leaving the inputs floating but be aware that the output voltage will also slam against one of the rails resulting in 'noisy' output due to chopper attempting to correct the large offset voltage developed under such conditions. Thus, it is much better to use large pull-up resistors to bias the inputs at the ground potential (assuming dual supply), which should also result in the output staying within its linear range close to Vref.

  • 0 in reply to Marek Lis
    Prodigy 25 points

    Hi Marek, thank you very much for the swift reply! I agree it's indeed better to have them pulled up to ground to reduce the noise in the system. Is this what you had in mind?

  • 0 in reply to Marco DAlessio
    TI__Guru 72851 points

    Yes, this should do it. 

    Having said that, connecting Vref to negative rail, dramatically reduces linear output voltage range of INA188 with input common-mode voltage, Vcm, moved away from 0 - see below.  Also, do you really want the output to be referenced to negative rail instead of ground?  Vout = Vref +Vin_dif * Gain 

    You did not specify what supplies you use but the potential issue is the same for any dual supply.

    A better solution might be to reference the output to ground - see below unless you need input range of Vin_diff up to 300mV

  • 0 in reply to Marek Lis
    Prodigy 25 points

    Hi Marek, thanks for your input. I agree with you, I've indeed connected the Ref pin to GND, whereas the EP is connected to V-.

  • 0 in reply to Marco DAlessio
    TI__Guru 72851 points

    Glad I could help.  Good luck.