ADC signal sampling with different grounds

Discussion in 'General Electronics Chat' started by surfline, Aug 25, 2010.

  1. surfline

    Thread Starter Active Member

    Aug 12, 2009
    30
    0
    Hi,

    I built a PCB using a PIC to do PWM motor control on the fuel pump motor in a car. This is working despite the PCB ground being noisy as Motor - is being switched on and off.

    I'm trying to sample a 0-5V analog input from a car sensor and feed that into an ADC pin on the PIC. The problem is that the Car GND and PCB ground are different as shown in the pictures below. The voltage of the signal with respect to car ground is fine, but the voltage of the signal with respect to PCB ground has the problems. This is apparent from the voltage waveform of PCB ground with respect to car GND.

    I tried using inductors / chokes. I tried Capacitors from signal to GND but I am having problems because the signal current is so weak, but I have a feeling that it needs some sort of decoupling between the signal to gnd or between the car ground and PCB ground.

    Any suggestions?

    Thanks.
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Woah, is that a ScopeMeter? I'm jealous.

    Anyway, the question. Microchip recommends in some datasheets to connect AVdd to Vdd through a 10 ohm resistor and to decouple that with a 100n capacitor. See if doing the same with your PCB ground helps.
     
  3. surfline

    Thread Starter Active Member

    Aug 12, 2009
    30
    0
    Yes it is. haha.

    Do you mean between AVss and Vss ? Or did you actually mean AVdd and Vdd?

    Thanks.
     
  4. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Since it's the ground that's the problem, connect the resistor between PCB ground and Battery-. A decoupling capacitor will also help for high frequency noise. If you still have noise, use a large-ish filter cap (~100µF) across the power supply rails. Remember not to connect your motor through this resistor, or you will get smoke.
     
  5. surfline

    Thread Starter Active Member

    Aug 12, 2009
    30
    0
    Resistor between PCB ground and Battery -, in parallel with cap didn't work. Large uF caps (I'm using 20Hz PWM frequency) on the supply rail help the noise on the supply rail, but doesn't fix the ground issue. The ground is still bouncing up and down causing the analog Input to correspondingly follow that pattern.

    Any other suggestions?

    Thanks.
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    why would the PCB common not be tied to the vehicle common?
     
  7. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Make everything differential, i.e. use differential amplifier(s) and signal path(s).
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    differential or isolated?

    Differential inputs produce a signal valid in reference to a common node.
     
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