AD8421 AD8429 gain selector switch drawbacks?

We have been using AD8421 AD8429 in a fixed gain layout with good success. We would like to add some flexibility to our design via gain selection switch.  Possible configuration would be on-off-on SPDT PCB-mounted slider switch (for example DigiKey P/N CKN5051-ND) with common on one of the gain pins and completing gain circuit with either gain 100 or gain 10 SMT resistor, or leaving it open for gain 1. Are there any drawbacks in the amplifier performance with this design?

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    •  Analog Employees 
    on Jun 12, 2014 10:24 PM

    Hi Fedor,

    Thanks for using Analog products in your circuit! One of the things you should be aware of is that the Rg nodes are sensitive to impedances. The mechanical switch will likely introduce some capacitance to the Rg pins, which in turn may cause some peaking in the frequency domain. You can still switch the gains in through the mechanical switch, but it is important to avoid that extra capacitance. You can do this by implementing the circuit below:

    Notice that by implementing this circuit, the capacitance due to the switch is not seen at the Rg terminals. You can just vary this circuit depending on the switch you want to use.

    Also, if you really need gains of 1, 10 and 100, you could take a look at the AD8253. It is a programmable gain amplifier with gains of 1, 10, 100 and 1000. The AD8228 is also available if you can do with fixed gains of 10 and 100.

    I hope this helps!

    Kris

  • Kriss, thanks for the insight!

    Would this modification of your advice work as well? :

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    •  Analog Employees 
    on Jun 16, 2014 12:09 AM

    Hi Fedor,

    You should be fine with that circuit. If you do want a cheaper solution, you could also try Harry's suggestion.

    Thanks!

    Kris