Inner workings of adc

[Michael Wakileh]

Hi! I've been cathing up on theory, and have questions regarding the inner/electrical workings of the ADC module and Chold (Sample and Hold Capacitor).

reading through various documents (for example ww1.microchip.com/downloads/en/AppNotes/01298A.pdf)
I'm left with some uncertainties.

It is currently my understanding that simply pointing to the correct ADC Channel (via ANSEL Analog Select bits) will connect and charge/discharge CHOLD to the level at that pin. In the beforementioned reference that pin is
a driven digital output so Chold is charged to Vdd....

I'm guessing the Analog Select Bits directly control the Sampling Switch Rss. So its also possible to sample....and then disconnect CHOLD via ANSEL and still start a conversion via the godone bit?
I also assume disconnecting CHOLD only works as long as the go/done bit hasen't been set yet..

"Sampling and Conversion: After a new channel
is selected, a minimum amount of sampling time
must be allowed before the GO/DONE bit in
ADCON0 is set to begin conversion. Once
conversion begins, it is OK to select the next
channel, but sampling does not begin until current conversion is complete!
....."

I'd be greatful for any pointers...
(unfortuntely I don't have my electronics nearby to gain insight... would have been a quick test of a Vref...)

greets mike

[mackrackit]

This thread might help clear things up. Then again it might not.
http://www.picbasic.co.uk/forum/showthread.php?t=11947
let us know.

[Michael Wakileh]

Thanks you for replying. I'm not having problems on the software side... I've used the ADC module quite often without really grasping some of the inner workings...(for that I simply select the channel, wait the sampling time, set go/done)


I guess the questions I have is:

Can the CHold capacitor be charged (by selecting the appropriate channel and waiting the sample time), then be disconnected by deselecting the channel, and still be converted by immediately setting the GoDone bit after disconnection?
regards,
mike

[mackrackit]

Not sure what you are wanting???
Selecting the channel just tells what channel the go/done bit converts if the ADC is enabled.

[Michael Wakileh]

Sorry, I've been quite busy and never got back to this... I had wondered why, in the above mentioned appnote (ww1.microchip.com/downloads/en/AppNotes/01298A.pdf), 2 channels are needed (and if it's possible to get by with just 1)... I plan to do some testing one of these days, but I'm busy with other projects at the moment... anyway, that's where I was going with my post...

regards
Michael

[Acetronics2]

Hi, Michael

2 Channel required , why ???

SIMPLY because it is needed to charge CHold to Vcc ... through one channel. ( How else ? )

AND the second channel is used for measure...

sooo ...

2 "buttons" need 3 channels , 3 buttons ... 4 channels, etc, etc ...

Capisce ???

Alain