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INA121: Use of Trim Pot in INA Gain resistance

Joseph Kitching
Prodigy 70 points
Part Number: INA121

Hello

Is it acceptable to use a trim pot as part of the Gain resistance of an INA used in a differential op amp application? Are there 1% tolerance trim pots available?

Thanks

  • +1
    TI__Genius 10850 points

    Hi Joseph,

    Thanks for your post. You can use a variable resistor to set the gain of the INA. You may find high accuracy potentiometers online. Digikey and Vishay are two places you can start your search. 

  • 0
    Guru 140200 points

    Hi Joseph,

    1% toleranced trim pots are extremely expensive and hard to get, if at all. And they come in huge dimensions.

    5% toleranced trim pots are available. They are wirewound or made of a special precision resistance foil. Unfortunately, they are also extremely expensive and cost more than 10€ each.

    Because of this the most practicable way is to take a small multi-turn cermet trimmer with +/-100ppm/K and +/-10% manufacturing tolerance and to put this trimmer in series to a fixed resistor which is at least five times higher than the trimmer resistance (from end to end). By this the 100ppm temperature coefficient goes in the total temperature coefficient by only one fifth. For the fixed resistor take a precision metal film (thin film !) resistor with <50ppm/K and <+/-1% manufacturing tolerance.

    Another issue is stray capacitance. The -input of any OPAamp is the most vulnerable place in an OPAmp circuit. It's good design practise to add as least stray capacitance here as possible. Because of this, take tiny components and mount them as close to the two -inputs of INA121 (pins 1 and 8) as possible. Mount them not too close to the solid ground plane, if existing. Remove the ground plane under two resistors, if necessary.

    Kai