ADC input signal filter selection
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  1. #1
    Join Date
    Jan 2019

    Default ADC input signal filter selection

    HI Folks, I'm brand new to the forum and really appreciate your support- Thank You!. I have a question; I recently designed an A/D circuit which worked wonderfully on my breadboard and also when I transitioned/soldered the hardware to a small prototype board. When I moved the same prototype board out to the truck and put it into service, the display values were jittery and jumped around quite a bit. The degree of variation was its worst at the very lowest end of the values and almost completely disappeared at the upper ranges. This seemed liked "signal noise" to me. Very unscientific conclusion. After doing some research I decided to add a larger capacitor (from 47 picofarad to 22 microfarad) on the signal line.

    My question is- is there a simple way to size a series resistor and capacitor for a typical A/D input for folks without an advanced degree in circuit design? Random selection seemed to work in this case but I'm hoping for some more scientific help. I read through Microchip's AN693 white paper and also looked at guidance on a low pass RC filters and downloaded Microchip's Filter Lab App. But none of that helped much. One of my goals is also to minimize space and components. Is there an easier or simpler starting point or some basic guidance on ways to select Resistor & Capacitor values for signal conditioning?

    Thanks in advance for any help and guidance and for your general support-

  2. #2
    Join Date
    Mar 2003
    Commerce Michigan USA

    Default Re: ADC input signal filter selection

    Well Mark, Its hard to say where your noise is coming from. You said it was working quite well on the bench but now you are having a/d display problems. For starters, what reference are you using for the a/d? Internal band gap or VCC/VDD? Also, what does the input network look like going to the PIC? I would like to know what kind of response you are looking for in the readout and what you are trying to measure.
    Dave Purola,

  3. #3
    Join Date
    Jan 2006

    Default Re: ADC input signal filter selection

    In addition to and similar to Dave's reply, there may be a Vref issue.
    Since the circuit is in a truck, I guess that the power supply may not be filtering the sparks and thus if the Vref is Vdd, then even if the signal is clear, ADC reading will jump up and down.

    A series resistor like 100R to Vdd pin and then a cap like 100uf to GND may lower this issue.
    An inductor from Vss pin of PIC to GND may also be a good filtering.
    Sampling the signal reading like 100 times may also lower the issue.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

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