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Help in impedance matching

Discussion in 'General Electronics Discussion' started by lizbeth0188, Oct 5, 2013.

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  1. lizbeth0188


    Oct 5, 2013
    I connected 16 instrumentation amplifiers for a EEG amplifier design.I have shorted all the reference and all amplifier have a common reference.But while doing this I find that the noise picked up by the amplifier has increased.One possible reason for this is since I have shorted all the reference together ,the impedance in the reference input is dropping.As a result the CMRR of the amplifier is unable to provide suitable attenuation for the common noise signals.Is there any way to match the impedance between the reference input and active input in the all instrumentation amplifiers?Is there any other solution you could provide for this issue?Tried providing a unity gain op-amp circuit to the reference input.But didn't help much.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Have you checked that is is indeed common mode noise? Short-circuit the inputs of the amplifiers and disconnect them from any long wires or traces to minimize noise at the input. What happens to the output noise?

    How low-noise is your reference voltage? Try adding a bypass capacitor directly to each instrumentation amplifier's ref-in pin to decrease the impedance of the ref voltage as seen by the amplifier.

    Are we alking about the same type of circuit like this? Then the answer is no. The reference input has a low impedance (R3 in the schematic) whereas the active inputs have a very high input resistance (limited by the operational amplifier within the instrumentation amplifier), You can artificially reduce the impedance at the active inputs but that turns the whole act of using instrumentation amplifiers moot.
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