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I am designing the circuit that will read the current in a DC motor. The PWM on the H bridge will be running at 10/15 KHz with an ADC sampling frequency and a control loop running at ~ 5KHz. I will have to check the R/L constants of the motors to make sure the control frequency is suitable. For now the proposed the circuit is showed below.

Initially I was planning on using a low pass filter before the ADC pin of the PIC. However I have read a few contradicting threads on whether i should or should not use a low pass before the ADC of the PIC. I read a few Application notes all promoting the use of the filters. Should I use an anti aliasing filter here?

If the filter is needed would you able able to comment on the cut-off frequency to use? The cut off frequency of the Sallen Key below is around 2.5 KHz. (1/2 the intended sampling rate of the ADC).

I would also be curious to see if someone could comment on C9 below. I saw a few schematics using a small ceramic capacitor placed there so i decided to use one too. Not sure about its real purpose to be honest.

Schematics

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    \$\begingroup\$ you need another resistor equal in value to R7 from +ve amp input to GND to complete your attempt at a differntial amplifier \$\endgroup\$
    – Neil_UK
    Sep 6, 2016 at 12:22
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    \$\begingroup\$ The question you have to ask yourself is: do you want digitized aliased high frequency noise or not? \$\endgroup\$
    – PlasmaHH
    Sep 6, 2016 at 12:24
  • \$\begingroup\$ Your schematic uses an unreadable font, I honestly can't tell which one is C9. Is it the 22µF or 10nF? \$\endgroup\$
    – Dmitry Grigoryev
    Sep 6, 2016 at 12:41
  • \$\begingroup\$ 27pF but i saw values ranging from ~20pF to ~60pF \$\endgroup\$
    – user122764
    Sep 7, 2016 at 9:57

2 Answers 2

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Should I use an anti aliasing filter here?

I think you should. In some applications they are omitted because the system is intentionally trying to sample signals beyond the Nyquist rate of the ADC. I can't think of a reason not to use one. If you know of a good reason, please share.

I would also be curious to see if someone could comment on C9 below.

It seems to be nothing more than a low-pass filter (along wtih R5 and R6) to reject high frequency noise coming from the source before it gets amplified by the first stage amplifier. I think it is a good idea to put the cutoff of this filter at a reasonable frequency. If you are using the ADC as part of a feedback system, that might be 10x the sample rate.

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C9 appears to be nothing more than a filter used to reduce high frequency noise due to the motor from being passed along and amplified/processed with the signal.

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