Quick question about IA and DA

Discussion in 'General Electronics Chat' started by Dritech, Apr 28, 2018.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    Hi all,

    1) If the inputs of an Instrumentation Amplifier are left floating while the circuit is powered, can this cause any damage to the IA ? Such a scenario can happen when the inputs are connected to removable terminals (example alligator clips).
    NOTE: By floating I mean disconnected and not without reference to ground.

    2) If the negative input voltage of a Differential Amplifier is higher than the positive input, can this somehow cause damage to the DA ?
  2. ebp

    Well-Known Member

    Feb 8, 2018
    Neither situation should cause damage.

    Case 2 is a common operating condition and simply means that that the output of the amplifier would go to a negative voltage, providing it had a negative power supply.

    If the inputs are taken above the positive supply rail or below the negative rail, excessive and possibly damaging current can flow into or out of the input(s). Typically 5 mA is a safe limit for current, but some devices can handle 10 mA or so, while some specify 2 mA maximum. Some differential amplifiers have attenuation resistor networks at the inputs and can handle voltages well beyond the supply rails, but this will be made very clear in the datasheet.

    Open inputs are likely to be more susceptible to damage by static discharge simply because there is no other path for the current associated with the ESD event. The ability of inputs to withstand ESD varies greatly according to the amplifier design. Unless the datasheet specifies ESD withstanding ability, assume that any ESD into any pin is a bad thing.
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    Thanks for the reply.

    For case 2, I am using a single supple DA. So in cases where the negative input voltage of a differential amplifier is higher than the positive input, will the output stay at 0V ?

    As for case 1, I will be using the AD8237 instrumentation amplifier. There is no mention of any ESD protection what may be used to protect the IA, but the datasheet states that one must include resistors in series with the inputs in cases where the input voltages may exceed the supply rails (which is the case with ESD). Should these resistors be included to protect the IA from being damaged by ESD ?