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gain resistor selection guide

arun
Intellectual 450 points
Other Parts Discussed in Thread: LM324, INA149, INA117, INA146

Dear sir,

I need to measure the lead acid battery voltage. it connected in series of 5 battery.each battery has 12v,20Ah.

we measure individual battery voltage using differential amplifier.how to i get 10mV accuracy of output from amplifier.

what are parameters to be considered while designing.

  • 0
    Guru 140200 points
    Hi Arun,

    it depends a bit only your actual design, but usually a high CMRR (common mode rejection ratio) is important and a low input offset voltage.

    If you plan using a differential amplifier, keep in mind that a design with discrete resistors will usually not allow for high CMRR because the resistors show manufacturing tolerances, temperature drift and long term drift. The resistors must not vary more than 0.01%!

    So, it's a good idea to use a chip having these resistor integrated on the die. TI has some very nice chips providing a brilliant CMRR.

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 30600 points

    Hi Arun,

    Are you looking at any particular product? Which differential amplifier are you using right now in your design? If you are designing a discrete instrumentation amplifier, then Kai is correct with his feedback. If your instrumentation amplifier is an INA product and takes an external resistor, the table specifying the resistor values should be given in the datasheet.

    -Paul

  • 0 in reply to kai klaas69
    Intellectual 450 points
    thank you for your valuable reply kai
  • 0 in reply to Paul Goedeke
    Intellectual 450 points
    Dear paul,
    I am using LM324 op amp for my differential amplifier
  • 0 in reply to arun
    Guru 140200 points
    Hi Arun,

    you could use the INA117, the INA149 or a similar INA for your application.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 450 points

    Thank you for your valuable reply.

    i need four channel input and output.this IC has only 1 channel and also refer normal 4 channel op amp with same specification of differential amplifier

  • 0 in reply to arun
    Guru 140200 points

    Hi Arun,

    there's no INA containing four channels. But you could take the INA146. It's less expensive and worth the money.

    The following simulation shall show how difficult it is to build such a circuit with discretes. An idal OPAmp was chosen with no input bias current and no input offset voltage. If the voltage diving resistors don't show any manufacturing tolerances, everything is fine:

    The following simulation shows what happens if the resistors have up to +/-1% tolerances:

    This is an error of nearly 200mV!

    But even resistors with only +/-0.1% tolerances will not be adequate:

    This is still an error of nearly 20mV.

    The INA146, on the other hand, will generate an error of less than 10mV.

    Kai