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VCA822: VCA822 nulling circuit

Dennis Brown
Intellectual 260 points

Part Number: VCA822

For the VCA822 variable gain amplifier datasheet (on page 34, figure 91) there is mention of a couple offset nulling circuits.   Shown are a couple 0.1uF caps shown for decoupling purposes.  Is this value ok for lower frequencies?  As an example at 10kHz that 0.1uF cap has only 159ohms of impedance.  Does increasing the value help the purpose of the decoupling cap at lower frequencies?

There are also a couple analog potentiometers shown, is there any reason that digital potentiometers could or should not be used?  Given that "digipots" typically have a limited BW, making certain that any noise that may come for those compensation pins is/are there any other provision(s) that could be suggested?    

  • 0
    TI__Expert 4070 points

    Hi Dennis,

    Let me look at this, I will get back to you by tomorrow.

    Regards

    Anant

  • 0 in reply to Anant Sinha
    TI__Expert 4070 points

    Hi Dennis,

    Offset nulling circuit is mainly to correct the DC offset and this decoupling cap filters out any high frequency noise. I don't see a reason why just 0.1uF should be a problem and using only 0.1uF should do the job. . But yes to have even better decoupling, you can have multiple capacitor of different values in parallel.

     Also, digipot can be used on place of the analog potentiometer. The bandwidth here is anyway limited RC formed by the potentiometer resistance and the decoupling capacitor.

    Regards

    Anant

  • 0 in reply to Anant Sinha
    Intellectual 260 points

    Hello Anant

    Thank you for the reply.  Looks like I should be ok to go. 

    I may add some more decoupling caps but otherwise I will follow the data sheet.

    dennis brown

  • 0
    Prodigy 10 points

    Need to add that he should be careful paralleling additional caps without CAREFUL consideration and evaluation - it's possible to add undesired/unexpected resonances that can make things worse than no decoupling at all! There was a good app note discussing this for some 3 terminal regulators; I'll repost if I can find it