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Can the Rg pins of the AD8421 be driven by the AD5245 digital potentiometer?

Product Number: AD8421

Hi,

I would like to be able to change the gain of the AD8421 through a potentiometer. Can I use the AD5245 (10k version) in rehostat mode? Are there any drawbacks?

  • Hi,

    I'll look into this and will get back to you.

    Regards,

    Kristine

  • Hi Topspin,

    One drawback when using digipot is it has parasitic capacitances that could affect the performance of AD8421. Lastly, AD5245 has a 30% nominal tolerance, which is a drawback as well because this produces gain error. You can refer to this application note for more information: AN-1578 (Rev. B) (analog.com)

    Best Regards,
    Yokki

  • Hi,

    In addition to Yokki's answer, there are a few other points worth to consider.

    The AD5245 can only handle voltages between its supply voltages, so about 5V span maximum. The AD8421's minimum supply voltage is almost the same, -2.5V to 2.5V, but in this case the input and output voltage ranges are so much limited that makes it almost useless.

    The wiper resistance of the AD5245 limits the gain range, as you can see in the application note (Equation 1)

    To illustrate the effect of the capacitances Yokki mentioned, see this simulation plot:

    Transient response

    Frequency response:

    The capacitors add poles in the feedback loops of the input amplifiers, so they can cause peaking at high frequencies and increased overshoot, especially at low gains (higher value of the gain setting resistor).