Photodiode Circuit Help

Thread Starter

MrL

Joined Oct 21, 2009
46
Hi,

I'm current working on a photodiode circuit. My schematic for the circuit is below:

http://img829.imageshack.us/img829/33/transimpedanceamplifier.png

Below are the datasheets for the op-amp and photodiode used:

http://www.ti.com/lit/ds/symlink/lmv793.pdf
http://pdf1.alldatasheet.com/datasheet-pdf/view/45619/SIEMENS/SFH206K.html

The issue i have at the moment is the output reading doesn't seem to be what i'd expect. I've tried using different resistor values, and when i decrease the resistor value, the output voltage reading increases, which shouldn't be the case given that for this transimpedance layout, Vout/Iin = -Rf.

Also, when i place my laser diode (655nm wavelength) onto the sensitive area of the photodiode, the output voltage doesn't change.

Here are some photos of the circuit:

http://img812.imageshack.us/img812/9346/img0417ylf.jpg
http://img841.imageshack.us/img841/1977/img0414wk.jpg

Please note, i rotated the op-amp the other way round for convenience in the circuit layout. Also, the reason i'm using a surface mount component like this is because the other components are through hole.

Any help would be appreciated.

Thanks.
 

Audioguru

Joined Dec 20, 2007
11,248
Your circuit will work properly if you add a negative supply to the opamp.

A photo-diode conducts a little with dark current and conducts more when light shines on it. Then the opamp will try to make its output negative but it can't because it is missing a negative supply so its output stays at 0V. If the opamp has a dual polarity supply then its output will go negative when light shines on the photo-diode.
 

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Thread Starter

MrL

Joined Oct 21, 2009
46
Hi. I've tried making the changes you suggested by supplying -2.5V (to V-) and +2.5V (to V+), along with 2.5V bias on the photodiode, but i'm still getting no alteration in the voltage level when placing the laser diode near the photodiode. The voltage output is always 0V, and i've tried various resistor values. Could this indicate a faulty/broken photodiode?

Thanks.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,248
A photodiode can be a little solar cell where it generates a voltage when exposed to light. Then it has no bias voltage added.

Or a photodiode can have a reverse bias voltage which reduces its capacitance so it can switch faster. Then it leaks current when exposed to light.
 

Adjuster

Joined Dec 26, 2010
2,148
Hi. I've tried making the changes you suggested by supplying -2.5V (to V-) and +2.5V (to V+), along with 2.5V bias on the photodiode, but i'm still getting no alteration in the voltage level when placing the laser diode near the photodiode. The voltage output is always 0V, and i've tried various resistor values. Could this indicate a faulty/broken photodiode?

Thanks.
There may be more than one possible issue to be investigated here, beginning with whether you have any optical output from the laser, and whether this is being effectively coupled to the photodiode.

That said, if you have an output always very close to zero, with no sign of offsets changing with resistor values, perhaps a detector circuit issue is likely.

Bear in mind that all the circuit must be suspect for errors, or faulty parts, not just the photodiode. Check over the circuitvmethodically, do not just assume things are correct.

On a reverse-biased photodiode circuit, a suitable high-value resistor could be temporarily put in place of the photodiode to imitate the DC photocurrent. If that does not give what you expect, you would need to find out why.
Good luck.
 
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