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Selecting the right switch for an opamp feedback network

Giulio Zoppi
Intellectual 645 points

Hello ,

I am working with an opamp-based audio amplifier for musical instruments.

In particular I need to switch the feedback network on the flight and I was oriented towards the replacement relays with solid-state switches (better if click-less)

I am a bit confused about what to use .

the opamp supply is 9-12V single ended.

I need  both SPST and SPDT devices, AN I2C controllable device would be a plus.

Best Regards,

GZ

  • 0
    Guru 317320 points

    Use the search function to find switches. (None of them are controlled through I²C.)

    Solid-state switches have a rather high resistance, compared to relays. There are ways of minimizing the effect; see SLYT612 and MT-088.

  • 0
    TI__Guru 52432 points

    Hi Giulio,

    For your application you would search for 2:1 and 1:1 configurations of switches. We spec our parts for single power supplies at 5V and 12V, so to meet your power range of 9V - 12V the part should be rated at both of these voltages to ensure proper operation across the possible VDD range. This also unfortunately limits the number of parts you can select. I did a parametric search and filtered it out in an excel file to show you the possible options:

    There is a bit more design flexibility if the power supply can be at 12V consistently. That being said what Clemens had indicated solid state switches do have a higher On Resistance, the picture above shows the typical Ron values for the parts. Along with that there is always a stability concern putting a MUX in the feedback path of an Op-Amp because of the On Capacitance of the switch. This app note goes over some details on how to help prevent unwanted operation of the Op-Amp do the capacitance in the feedback path. 

    If you have any other questions on helping you select the right MUX for your application please don't hesitate to reach out again!

    Best Regards,

    Parker Dodson