Electronics > Projects, Designs, and Technical Stuff

3rd order Sallen Key Unity Gain LPF - stability question

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Someone:

--- Quote from: JDW on April 26, 2020, 01:20:56 am ---Thank you also for showing that so long as I use 5% tolerance capacitors (e.g., C0G) all will be well.
--- End quote ---
C0G is only the temperature stability/coefficient tolerance, it is a separate rating to the capacitor tolerance, and capacitor tolerance is usually specified in an extremely specific measurement with regard to excitation signal, frequency, DC bias. The tolerance of a capacitor in circuit is the multiplicative result of all its variances. EIA Class I ceramic capacitors are the exception to the rule with their very low sensitivities, almost every other capacitor type varies outside its nominal "tolerance" in real use.

JDW:

--- Quote from: Someone on April 26, 2020, 02:55:01 am ---
--- Quote from: JDW on April 26, 2020, 01:20:56 am ---Thank you also for showing that so long as I use 5% tolerance capacitors (e.g., C0G) all will be well.
--- End quote ---
C0G is only the temperature stability/coefficient tolerance, it is a separate rating to the capacitor tolerance, and capacitor tolerance is usually specified in an extremely specific measurement with regard to excitation signal, frequency, DC bias. The tolerance of a capacitor in circuit is the multiplicative result of all its variances. EIA Class I ceramic capacitors are the exception to the rule with their very low sensitivities, almost every other capacitor type varies outside its nominal "tolerance" in real use.

--- End quote ---
I appreciate the explanation, but if you could extend that explanation to Jay's LTSpice simulation, where he used 5% tolerance capacitors, that would be even more specific and helpful.  But from what I can see in my breadboard test and in Jay's very kind simulation, the schematic put forth in my opening post seems to be a solid design, despite the fact it's not possible to simulate the Microchip Op-Amp in LTSpice.

Jay_Diddy_B:
Hi,

You can work out what all the effects like temperature, tolerance, dc bias add up to and run the simulation with the tolerance set to accommodate all of the parameters.

If I run the Monte Carlo analysis again with

10% resistors

25% capacitors, 5% for initial accuracy and 20% for other effects

I get:




and



No hint of any instability

Regards,
Jay_Diddy_B

Someone:

--- Quote from: JDW on April 26, 2020, 03:03:03 am ---despite the fact it's not possible to simulate the Microchip Op-Amp in LTSpice.
--- End quote ---
The MCP601 can be simulated in LTspice, but that requires a person with the motivation to do your work for you.

Siwastaja:
Note the extra RC filter referred to by nctnico is required due to special input characteristics of SAR ADCs. If you don't like the idea, you can think about it being a part of the ADC, not part of your filter. Though, this extra RC filter can participate in your filter response, as well. See ADC datasheets and appnotes for more information. Typically, R is small like 10-50 ohms, and C is small as well, some hundreds of pF.

Often, with the suggested values (by the ADC manufacturer), the corner frequency of the RC is far above of the required Nyquist frequency, so it does not work for anti-aliasing. It's to filter large current spikes only.

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