Will big impedance on ADC influence result on it ?(photores)

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bigboss59400
Posts: 30
Joined: 13 Jun 2010 17:49

Will big impedance on ADC influence result on it ?(photores)

#1 Post by bigboss59400 » 21 Jan 2013 10:21

Hello,

I want to use a photoresistor with voltage divider on to detect the presence of the sun (like night or day), however photoresistor have big impedance regardless the luminosity.
Moreover I saw in datasheet that the ADC wouldn't be load with an impedance >10kohms but I saw some project which exceed this limit...

Does a big impedance influence a lot the result on ADC ?(I don't want to make a luxmeter)

Thanks

Sparky1039
Posts: 1179
Joined: 24 Nov 2005 20:07
Location: Colorado, USA

Re: Will big impedance on ADC influence result on it ?(photo

#2 Post by Sparky1039 » 21 Jan 2013 19:32

Does a big impedance influence a lot the result on ADC ?
Yes, very much so, why do you think they publish the input impedance requirement if it wasn't important? :wink:
How to get around your problem is to use an external op-amp as a unity gain buffer. The output impedance of the op-amp is very low (few ohms) which can drive the ADC input correctly. The very high input impedance of the op-amp is well suited for connection to the photo-resistor divider circuit.
However, if you only require a go-no-go type of indication vs. actual ADC measurement value, it would far simpler to make use of the on-board voltage comparator(s) found on many PIC devices. With a comparator you can set a voltage "trip" point to tell the PIC if it's dark or light based upon whether the photo-resistor voltage drop is above or below this threshold.

bigboss59400
Posts: 30
Joined: 13 Jun 2010 17:49

Re: Will big impedance on ADC influence result on it ?(photo

#3 Post by bigboss59400 » 23 Jan 2013 18:22

Thanks man for your reply,
Yes, very much so, why do you think they publish the input impedance requirement if it wasn't important? :wink:
I was thinking this remark was just here for high precision application but it isn't seem like that :lol:

Yeah op-amp could be a good option but I didn't want to put other component on this board... I don't know why I haven't thinking to pic comparators

Thanks for your helpful reply

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