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I want to design op amp integrator circuit with variable time constant using a simple switch. What is the typical shunt capacitance of a switch? I assume Ron of switch to be less than 5 Ohms. I want switch to minimally effect overall feedback capacitance. I am not worried about transient response when capacitor are switched. Any suggestions on the design?

Thank you all for your suggestions.

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  • \$\begingroup\$ I would think "not enough to worry about" \$\endgroup\$ Jun 26, 2018 at 15:54

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It appears sound. I'll just assume you calculated your time constants correctly, as you don't list any design goals.

Just to avoid noise, though, I'd suggest moving the switch to where the caps join the output (the switch is larger, and you may not want the noise it picks up on your virtual ground).

You also might want to eliminate the "switch glitch" where both caps are switched out, by hardwiring the 47pF and switching the 680pF in and out. This has the additional advantage of requiring only an SPST switch and thus reducing routing (and as such noise).

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Depending on the switch you select, you may have an issue with inductance in conjunction with the larger capacitance. Switches add series inductance, which is something to watch out for.

Since your corner frequency is around 1kHz and the RLC pole will be most likely greater than 10MHz (counting less than 100nH of inductance for the switch). It's a good idea to estimate parasitics and see if they have an effect on your design.

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