ADC for a AC signal question

irmanao · Joined: 08 Apr 2015 Posts: 77

I have a ac signal from 0,1V to 3,1V that i want to process with a dspic. I would then like to do zero crossing (at 1,6V), my question is : are the 3,1V replaced by the value of 1023 (10bit) and the 0,1V by 0?

thanks

Ttelmah · Joined: 11 Mar 2010 Posts: 19498

Depends on where you set the voltage references....

If you set Vref- to 0.1v, and Vref+ to 3.1v, then the ADC will read from 0.1v to 3.1v. That's what the reference inputs are for....

irmanao · Joined: 08 Apr 2015 Posts: 77

I wasn't aware of that, thank you very much, i am new to programming.

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

Depends on your ADC, ac signal and what you want to do with the data.

Assuming your ac signal is swinging from 0V1 to 3V1, the ADC lower ref is 0V1 and upper ref is 3V1, then you are probably correct.
Depending on the nature of your ac signal you may not need to use zero crossing.

Mike

EDIT I see Mr T. got in as I typed.

temtronic · Posted: Wed Apr 08, 2015 11:10 am

It's NOT an 'ac ' signal if it transitions from +0V1 to +3V1, rather just a DC signal...

perhaps a 'moot' point but PIC ADC subsystems will NOT tolerate a real AC signal !!

jay

irmanao · Joined: 08 Apr 2015 Posts: 77

Yes i know the average is not 0 so it's not a ac signal
thanks

the signal is a sine wave (50 Ηz) and i need to trigger a pulse for two SCR's at the 1.6V point after a delay

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

have a look at this PDF selector matrix

http://www.microchip.com/stellent/groups/sitecomm_sg/documents/devicedoc/en570055.pdf

see the column ZCD under the gold heading Intelligent Analog
ZCD= Zero Crossing Detector

the pics designated are OPTIMIZED for just what you want to do only SO MUCH better than you may have envisioned.

check data for parts
16f161x
and
18fxx40/42

Ttelmah · Joined: 11 Mar 2010 Posts: 19498

Also, seriously, 'step back'. The poster needs to start by doing some basic C programming before trying to work with AC.

1) Do a basic 'hello world' output.
2) Read an ADC, and then experiment with how the numbers relate to the voltages etc..
3) Learn how to loop, test, one stage at a time.

Trying to jump in the deep end, rather than starting with basics, is a sure way of wasting a lot of time.

temtronic · Posted: Wed Apr 08, 2015 2:41 pm

hmm....
the signal is a sine wave (50 Ηz)

NOW you're telling us it really IS an AC waveform, so you CANNOT feed that signal into the PICs ADC. well you can but risk destroying it !
The PICs ADC section is NOT designed for -ve voltages, ONLY +ve ones.

jay

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

temtronic · Posted: Wed Apr 08, 2015 6:47 pm

hmmmmmmmm... perhaps the original poster can show us an example of his input waveform.
A 50Hz sine wave usually tells me it's 'mains voltage' from across the pond.
Having a 'zero cross' at 1.6V when the signal goes from .1 to 3.1 does sound like superimposed AC on a DC level.

now I'm more confused....
a picture would be worth 1000 words ( or bytes !)

Jay

irmanao · Joined: 08 Apr 2015 Posts: 77

It is superimposed on a DC offset