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.

 

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.

 

Yes it is. haha.

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

Thanks.

 

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.

 

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.

 

why would the PCB common not be tied to the vehicle common?

 

Make everything differential, i.e. use differential amplifier(s) and signal path(s).

 

differential or isolated?

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