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AD8421 performance considerations

Dear all,

designing with AD8421 is really a nice thing, because this device has superb performance characteristics making it a very good part. However looking at following questions related to it shows it has some hidden pit falls, too.

This is similar to its faster and more current hungry equivalent AD8429.




AD8429 gain-switching


What to make sure, that such things won't happen?

  • Both inputs (+IN, -IN) are sensitive to RF signals > 1MHz
    • Limit possible input frequencies with proper input filtering -> well described in datasheet
  • RG inputs are sensitive to RF signals similar to the inputs (+IN, -IN), however here it is hard to filter, because any capacitance at these nodes change the gain and frequency response
    • don't add much parasitic capacitance to these nodes, if you want a smooth frequency response without excessive gain peaks
    • keep any high voltage AC-signals away from this signal traces
    • that is true even for the output signal, as this introduces a unintended/parasitic feedback loop
    • thus short traces keep things simple here, the longer they get, the easier is it for other signals to couple into RG
  • As RF signals don't need large structures to crosscouple -> even short copper traces in parallel or crossing each other on adjacent pcb layers can introduce trouble here
  • Same is true on bread boards and so on
  • Tuning frequency response of this part is a hard thing (?), filtering with additional circuits is much easier

How about you do you found any other point for these InAmps, or are some of my hints not hitting the right point?

Best regards,


Format correction
[edited by: tomspe at 1:01 PM (GMT -4) on 25 Oct 2018]
  • Hi Tomspe,

    If this is general design consideration to optimized the AD8421 or any inamp performance please add my inputs below:

    • I would not recommend to use breadboard for it has a lot of parasitics that could degrade the CMRR performance as well as cause oscillation on the parts
    • Make the connections of the RG pins as short as possible
    • Do not introduce any capacitance across the RG nodes cause it can cause peaking and instability.
    • In dealing with differential signal, anything you put on the input side of the inamp should be well matched so that it will not degrade the AC CMRR performance of the circuit.
    • Cap loading can make the inamp oscillate. You can fix it by putting series resistor at the output of the inamp.
    • Please be aware that AD8421 is not rail to rail out inamp, if you're applications requires single supply of 5V, I would prefer to use the AD8422.

    What is your intended applications? Just let us know if you need any assistance.

    Best regards,


  • Hi Emman,

    thank you for the good advice and hints.

    The application is just a analog signal conditioning front end for a measurement system. As there are several switchable gain setting stages, we tried to use the AD8421 as wide band programmable gain amplifier.

    It works, but currently has some gain peaking issues in range around 2MHz to 8 MHz, due to a not as optimal layout as it could be. This increased system noise to a higher level then expected, because RF rectification and similar things have side effects into measured signal range. (Well it is not really noise but it looks quite similar.)

    An improved pcb layout might solve these issues, however that's currently a pure trial and error approach, as there're few measures to track down the origin of noise and gain peaking. (hard to say if simulation models are accurate even down to such parasitic levels, and pcb modifications are time consuming and hard to do).

    Anyway your hints give good points to look out for.

    Best regards,


  • Note: AD8422 has even a much more flat frequency response then AD8421 -> If smaller bandwidth of AD8422 is OK -> it will be the better solution.