Using two AtoD channels - they're interacting?!

[HankMcSpank]

Thanks for taking the time to reply.

I had a look, here's what I found...

' ADCIN1X4.BAS
'  For PIC12F683 and melabs PICBASIC PRO Compiler
'
' Display result of 10-bit A/D conversion on LCD

Include "modedefs.bas"		' Mode definitions for Serout

' Define ADCIN parameters
Define	ADC_BITS	10	' Set number of bits in result
Define	ADC_CLOCK	3	' Set clock source (3=rc)
Define	ADC_SAMPLEUS	50	' Set sampling time in uS


LCD	Var	GPIO.1		' LCD TX pin

adval	Var	Word		' Create adval to store result


	ADCON0.7 = 1		' Right justify result
	ANSEL = %00111000	' Set AN3 analog, rest digital
	CMCON0 = 7		' Analog comparators off

	Pause 500		' Wait .5 second for LCD to init

mainloop:	ADCIN 3, adval		' Read channel 3 to adval

	Serout LCD, T2400, [$fe, 1]	' Clear screen
	Serout LCD, T2400, ["Value: ", #adval]	' Display the decimal value  

	Pause 100		' Wait .1 second

	Goto mainloop		' Do it forever

	End

Unfortunately, that example only uses one channel.

Pretty much after I posted, I sorted my problem (how does that always work then?!).

Two things were the root of my issue...

1. I'm using a sig gen, rectifying a sine wave from it & feeding into & low pass filter (the resulting extracted DC level feeds into one AtoD channel) ....I was getting over zealous with the sig gen output amplitude - and the rectified DC level breached 5V - which sends the PICs AtoD results a bit whacky (which is hardly surprising) ...one channels starts seriously impacting the sample from a totally different channel! So Hank makes amental note - be careful with the input voltage magnitude fed into an AtoD pin!

2. The other problem was that I wasn't allowing sufficient time between using successive AtoD channels - I'm finding that in my setup, it needs at least 30ms between successive AtoD channels 'samples' - hey ho.

[tenaja]

I'm finding that in my setup, it needs at least 30ms between successive AtoD channels 'samples' - hey ho.

This is the case with every PIC, and for each and every a/d reading. It is called the "charge time" for the sampling capacitor, and is most often the bottleneck for sampling rates. (Some PICs charge it faster.) With a single channel it is often not noticed because the sampling cap is charged while doing other things, or because you get an "old" reading from the same channel.