10 bit ADC noise in Circuit.

[Sajjadkhan]

- some crosstalk can't be avoided in mixed-signal chips
- with your limited equipment, you'll face difficulties to determine for sure if the signal is actually present at the analog pin or just a measurement artefact. Let's assume for the time being that it's real.
- a low interference mixed signal design requires other techniques than a single layer PCB with cm long thin ground traces. You have to accept some level of "noise" with your kind of boards.

As a cross-check, it would be interesting to determine the ADC value variation with PWM off. There may be an addtional problem of unsuitable ADC operation parameters (clock, acquisition time). If the problem turns out to be mostly caused by external interferences, averaging and digital filtering can be still a way.

yep spikes gone when pwm is off. i guess the noise/spikes are generating internally so you cant do any thing from outside.
How many samples i have to average to have good filtering?

 

[FvM]

How many samples i have to average to have good filtering?

Uncertainty of averaged ADC value would be the main criterion. If the measurement fluctuations could be assumed as white noise (surely they aren't exactly, but the behaviour can be quite similar though), then the uncertainty will be reduced by a factor 1/sqrt(N), N being the number of averaged measurements.

It's not surprizing, that the spikes are gone without PWM, but do you get also full expected ADC accuracy?

 

[Sajjadkhan]

It's not surprizing, that the spikes are gone without PWM, but do you get also full expected ADC accuracy?

Yes, in fact if i remain under 2K frequency then i hardly get 1 bit error.

---------- Post added at 17:19 ---------- Previous post was at 17:18 ----------

One thing more, my bench sockets are not grounded i.e. i havent installed the third wire for earthing. can that be an issue?

 

[FvM]

One thing more, my bench sockets are not grounded i.e. i havent installed the third wire for earthing. can that be an issue?

High impedance circuit parts can pick up 50 Hz hum and enviromental elctrical noise, if not placed inside a metallic enclosure connected to the circuit ground. The effect may be increased with a floating (not connected to protective earth) power supply.

 

[Sajjadkhan]

Ok , i have made many experiments on PIC16f87x series. I have separated the Analog and Digital ground.All extra pins are set to digital input and are grounded.


When PWM is on then there is error even up to 4 -6 bits.
When PWM is off then the error is none.

I didn't believe myself and always thought that i am doing something wrong but accidentally when i visited the microchip site for this controller, its was written "Not Recommended". Well this chip have a problem and i will try another chip of another model in future for that. for now i have turned off the PWM.

If you guys faced the similar problem then do share. thanks.

 

[FvM]

That's a clear result, I think. I'm not sure however, if it's a particular problem of this chip, although there are surely some differences in detail. I rather fear it may be a problem of your circuit's ground schemes.

 

[Sajjadkhan]

That's a clear result, I think. I'm not sure however, if it's a particular problem of this chip, although there are surely some differences in detail. I rather fear it may be a problem of your circuit's ground schemes.

PIC16f886 has the same pin configuration as PIC16f876(the one i used). I'll replace it if it is available in my area and try to update the result.