Differential and CM Voltage Confusion

Discussion in 'The Projects Forum' started by d_gelman, Jul 14, 2014.

  1. d_gelman

    Thread Starter New Member

    Oct 21, 2013

    I am trying to develop a multi-stage ACS for high voltage inputs. First stage is a differential circuit to determine the difference of the signal. They have a common-mode voltage of -46VDC. (I think)

    Please correct me if I am wrong, but I think common-mode voltage means that both inputs share a component of the signal. For example a reference of -46VDC and another input that starts at -46VDC and increases to a maximum of approximately -80VDC. If I am incorrectly using the term "common-mode voltage" - please let me know.

    I would like an output between 0 and 5V. So am I looking for an op-amp with high CM voltage range? Many data sheets only describe the op-amps CMRR - which I am assuming is not the same thing.

    Thank you in advanced for your help.
  2. crutschow


    Mar 14, 2008
    Your description of a common-mode signal sounds correct, but that is a very large signal voltage change. To handle such a large common-mode voltage a Difference Amplifier, designed to operate with large CM voltages, may work for you.
  3. DickCappels


    Aug 21, 2008
    It looks like you understand the meanings of "common mode" and "differential" voltages.

    Crutschow's link to difference amplifiers is a great place to start.

    You can also roll your own if your performance requirements are not very high.