Instrumentation Amplifier Adjustment

Discussion in 'The Projects Forum' started by Mark_varney, Jan 15, 2015.

  1. Mark_varney

    Thread Starter New Member

    Dec 1, 2013
    Part of a project I'm working on i need to build a traditional 3 op amp instrumentation amplifier with dc offset adjustment and low frequency CMRR adjustment.

    The questions i have would be what Opamp would be best suited for this? i appreciate there are ready made instrumentation amps but i need to use a 3 op amp version.

    To adjust the dc off set is it as simple as connecting a pot to the last op amps offset null connections?

    and lastly how does one adjust the CMRR?
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
  3. Mark_varney

    Thread Starter New Member

    Dec 1, 2013
    Thanks Eric, just had a skim through the latter document and see that it has a cmr adjustment which is just what im after.

    Many thanks
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Choosing the opamp depends on the requirements of the circuit: gain, bandwidth, noise, distortion, drift, etc.

    If your chosen amp doesn't have an offset pin, you can inject a very small DC current into any of the inverting inputs.

    CMRR is determined by the value matching tolerance of the feedback resistors. To fine tune CMRR, replace one of the resistors with a slightly smaller value plus a trimpot in series. Connect a signal to both inputs (common mode), and trim for minimum output (rejection).