Stable ADC readings

[Acetronics2]

Hi, Sayzer

I am testing it using a simple voltage divider with two resistors of the same value so that I get a value around 512 (ADC in 10 bit mode).

That means he works in ratiometric mode and supply changes do not interfere here ...
so, problem is elsewhere ...

Alain

[sayzer]

Hi, Sayzer



That means he works in ratiometric mode and supply changes do not interfere here ...
so, problem is elsewhere ...

Alain

Yes Alain, I missed that.

Then, may be he should pause ~2ms after each read.

Also, for example, reading lower voltages and reading higher voltages may behave differently.
If reading higher voltages are accurate, but lower voltages are not, then there is something else, too.




---------------------------

[mister_e]

Input impedance too high, too short of ADC aquisition time, noisy PSU, unstable PSU, no averaging routine, multiplexed function on the ADC pin, loose/dirty bread board contacts, poor wiring, poor oscillator, poor decoupling, etc... quite a few possibilities...

[dream-]

As mentioned I am decoupling, using averaging, and testing the ratio of two resistors hooked up to the same battery after the regulator.

Let me draw the schematic and I will post.

But before that, let me say that I switched from ADCIN to doing a manual ADCON0.1 = 1 reading, and now I get a stable value. So my problem is an erroneous use of the ADCIN command or a problem in the logic of my routine.

BEFORE:

DEFINE ADC_BITS 10 ' Set number of bits in result (8, 10 or 12)
DEFINE ADC_CLOCK 3 ' Set clock source (rc = 3)
DEFINE ADC_SAMPLEUS 50 ' Set sampling time in microseconds
...
WSum = 0
Pause 100

For i = 0 to 49
ADCIn PinProbe, W0
Pause 10
WSum = WSum + W0
Next i

W0 = WSum / 50

NOW:

Pauseus 50 'Stabilize
ADCON0.1 = 1  'start the ADC
WHILE ADCON0.1 : WEND
  
W0.HighByte = ADRESH
W0.LowByte = ADRESL

It works, now the reading stays fixed and it is much faster. (I even didn't have to do any averaging)

[Acetronics2]

Hi,

What about reinitialising your Wsum value to 0 @ each 50 samples batch ??? don't you run this loop in another continuous loop ... ??? " par hasard " ...

BTW " PinProbe " is the alias for which pin ???

Alain

[dream-]

Hi,

What about reinitialise your Wsum value to 0 @ each 50 samples batch ??? don't you run this loop in another continuous loop ... ???

Alain

Oh I do. I left that part out of the code I pasted, but it is indeed running in an outside continuous loop.

PinProbe VAR PORTA.2
...
myLoop:

WSum = 0
Pause 100

For i = 0 to 49
ADCIn PinProbe, W0
Pause 10
WSum = WSum + W0
Next i

W0 = WSum / 50
...
LCDout $fe, 1 'Clear Screen
Lcdout $fe, $80
Lcdout "Reading: ", DEC W0

Goto myLoop

The readings I get with the ADCIN method are in the ballpark but move around too much. With the "manual" ADC method I get a stable value.

[Acetronics2]

ADCIN also polls for the ADCON0.1 bit ...

sooo .... aheum, how to tell it ...

Alain