Interfacing photodiodes to PIC ADC

[vsmguy]

The PIC ADC requires that the input impedance be max 2.5k for fast operation.

Then, how I do go about interfacing photodiodes to the ADC without using something like opamps, because as far as I know photodiodes have very high impedance.

The circuit I have at mind is :

http://www.geocities.com/vimalsshankar/sch.png

Should it be that I have to adjust the value of R1 to be within specification ?

Please comment.

[asmallri]

The simplest way of interfacing this sensor to the PIC is to use an opamp.
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[vsmguy]

[SherpaDoug]

What are you trying to measure? If you are measuring full sunlight or some other bright constant light source you may get away with the single resistor design. But if you are looking for high speed or high sensitivity you are going to need an amplifier.
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[Ttelmah]

SherpaDoug, has 'put his finger' on the critical question.
The photodiode, acts as a current source, with the amount of current, dependant on the quantum efficiency, and the amount of light involved.
Now this diode, will itself have leakage, as does the ADC input on the PIC (quoted as up to 100nA). The current flowing must be large enough, to draw the ADC input, to the required voltage for sampling, in a reasonable timescale. The longer the timescale, the more significant the leakage terms, and other things like thermal noise in the diode become. Now just how small the current involved is, is perhaps best illustrated by a typical 'high sensitivity' photo detector, which will typically use a sampling resistor, in the order of 1000MR. However, with a larger diode (normally bad for frequency response, because it gives a larger junction capacitance, but 'better' in terms of how many photons are collected), for bright illumination levels, the diode may well be useable without an op-amp, especially if the sampling time is kept reasonably long (allowing the internal capacitor to charge). However for lower light levels an amplifier will be needed. It doesn't have to be an op-amp. A simple FET source-follower can of course be used.

Best Wishes

[vsmguy]

am sorry I was not very specific .

The setup will be used for hobby robotics in very dimly lit environment for line followers.

The environment should not saturate the diode though - typically classroom flouroscent lighting. I am on the lookout for pointers regarding that.

White poster paper with black poster paper/scotch tape will be the targets to detect.

I don't want digital output using comparator because I want to use dynamic calibration using intensity values.

It's not very convenient to keep on adjusting presets for triggering threshold.

Will a simple LM324 work ?

[asmallri]

[vsmguy]

The LM324 can be run from a single supply too...

Will it work ?

What is the specs of the opamp I am looking at to interface with photodiode ?

[kender]

[vsmguy]

Kender,

after I came back here from EDABoard, I had a sense of Deja Vu ;)

Will a LF351 fit the bill ?