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I am wondering if it is possible to create a hybrid PID circuit using opamps with key components whose values are programmable. Programmable resistors - no problem. But capacitors?

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    \$\begingroup\$ I regularly encounter programmable capacitors. On a chip that is. It is simple actually, just a bunch of capacitors with a series switch (usually a CMOS pass gate). Add a simple decoder and there's your programmable capacitor. And there's no reason why you could not do the same outside a chip. \$\endgroup\$ Mar 8, 2016 at 13:42

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You only need to adjust the RC product to tune a PID controller, so there should be no problem if you have fixed (or discretely switched) capacitor values and adjustable resistors. You might need to switch capacitors of you have no idea of the plant characteristics and have to accommodate a huge range of possible parameters.

But more and more this kind of thing is done entirely in the digital domain. In particular, slower loops are excellent candidates,

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  • \$\begingroup\$ Thanks. My problem is that it's a PID in a loop with sub microvolt sensitivity, so it needs precision components and very low noise opamps. Not sure if it can be done all digitally with the necessary noise constraints. \$\endgroup\$ Mar 8, 2016 at 13:40
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    \$\begingroup\$ As long as you can translate the sub microvolt in a representative digital form, it is possible. It all depends on the ADC/DAC. It is not that digital is by definition less accurate or unsuitable for small signals. Actually analog should be less accurate and more noisy \$\endgroup\$ Mar 8, 2016 at 13:46
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    \$\begingroup\$ As long as the ADC has a resolution below the noise level of the analog signal your digital implementation can be no worse, and probably much better, than an analog implementation. Of course you have to make sure you don't throw away resolution in intermediate calculations by doing silly things. \$\endgroup\$ Mar 8, 2016 at 14:00
  • \$\begingroup\$ The fundamental frequency of oscillation of the system is around 20Hz, and we are using OPA227 to keep it all under control. I really need to do a serious noise analysis of any potentially fully digital PID. \$\endgroup\$ Mar 8, 2016 at 14:06
  • \$\begingroup\$ At 20Hz I would certainly consider an analog solution. \$\endgroup\$ Mar 8, 2016 at 15:14

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