amping a photodiode help - Page 2 - Electronic Circuits Projects Diagrams Free

Roff · 21st August 2008, 04:59 PM

Quote:

Originally Posted by blackbunny

yeah i do have common cathode, so u mean just switch the diode around in that diagram?

dark current is insignificant, other things like risetime etc are also insignificant from my knowledge.

Bandwidth/risetime requirements depend on the application. You didn't actually tell us what you plan to do with this, so we can't determine bandwidth requirements. The choice of op amps, diode biasing, feedback resistance, etc., all depend on bandwidth requirements.

blackbunny · 22nd August 2008, 01:17 AM

Quote:

Originally Posted by Roff

Bandwidth/risetime requirements depend on the application. You didn't actually tell us what you plan to do with this, so we can't determine bandwidth requirements. The choice of op amps, diode biasing, feedback resistance, etc., all depend on bandwidth requirements.

all i simply want is to have this diode amplified with separate channels, output some voltage that i can connect to the three adc pins on my avr chip to do something with it.

rezer · 22nd August 2008, 02:02 AM

The 3 amplifier circuit does not look like it will work. All three diodes are in parallel. What effects one branch will effect all branches. If this is what you want, just use one op-amp. It will do the same thing.

Why not use 3 seperate photo diodes with seperate op-amps?

Roff · 22nd August 2008, 02:34 AM

What power supply voltage(s) do you have available? Do you have a preferred op amp? It needs to have FET inputs.

Roff · 22nd August 2008, 02:37 AM

Quote:

Originally Posted by blackbunny

so what would be an appropriate amplifier circuit for this sensor?

datasheet

The datasheet says it is common anode.

Roff · 22nd August 2008, 02:37 AM

Quote:

Originally Posted by rezer

The 3 amplifier circuit does not look like it will work. All three diodes are in parallel. What effects one branch will effect all branches. If this is what you want, just use one op-amp. It will do the same thing.

Why not use 3 seperate photo diodes with seperate op-amps?

They are not in parallel.

rezer · 22nd August 2008, 03:16 AM

Quote:

Originally Posted by Roff

They are not in parallel.

The schematic blachbunny posted showing 3 amps does show the diodes to be in parallel.

Roff · 22nd August 2008, 03:54 AM

Quote:

Originally Posted by rezer

The schematic blachbunny posted showing 3 amps does show the diodes to be in parallel.

Notice he said,

Quote:

u mean something like this?

That was his idea of how they should be connected. The datasheet he linked to clearly says the three diodes have a common anode connection, with the cathodes on separate pins.

rezer · 22nd August 2008, 03:59 AM

Notice also what I said:

Quote:

Originally Posted by rezer

The 3 amplifier circuit does not look like it will work. All three diodes are in parallel. What effects one branch will effect all branches. If this is what you want, just use one op-amp. It will do the same thing.

Why not use 3 seperate photo diodes with seperate op-amps?

I indicated what I was talking about. I wasn't speaking about the datasheet. But no problem.

Roff · 22nd August 2008, 04:10 AM

Quote:

Originally Posted by rezer

Notice also what I said:

I indicated what I was talking about. I wasn't speaking about the datasheet. But no problem.

Sorry for the misunderstanding.

blackbunny · 22nd August 2008, 06:32 AM

Quote:

Originally Posted by Roff

What power supply voltage(s) do you have available? Do you have a preferred op amp? It needs to have FET inputs.

just usb bus power

Roff · 22nd August 2008, 05:06 PM

I misinterpreted the datasheet. The part is common cathode, as you stated.

Are you going to use AVCC, or the internal 1.1V reference, for the A/D?

blackbunny · 23rd August 2008, 12:40 AM

Quote:

Originally Posted by Roff

I misinterpreted the datasheet. The part is common cathode, as you stated.

Are you going to use AVCC, or the internal 1.1V reference, for the A/D?

avcc will be at 5v

Roff · 23rd August 2008, 04:44 AM

OK, here's a place to start. You will have to play with the feedback resistor and capacitor. I have a feeling the resistor value may need to be in the megohms.
CAVEAT: I have never used the ATmega88, the LTC6082, or your photodiode.

But it's better than what you have now.

canoo · 14th November 2008, 06:02 PM

Hello!

I'm working on a similar project. The components I'm using are as follows:

Photodiode
OpAmp

The results I'm getting with the solution posted here are sorta like on off (Any light at all and the output goes low. With no light the output goes high. Very hard to reach a happy medium). I have a feeling it has to do with the resistor and capacitor in the feedback?

I'm wondering how I should go about selecting the proper resistor/capacitor combination?

Thanks in advance!

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22nd August 2008, 02:02 AM	(permalink)
rezer	The 3 amplifier circuit does not look like it will work. All three diodes are in parallel. What effects one branch will effect all branches. If this is what you want, just use one op-amp. It will do the same thing. Why not use 3 seperate photo diodes with seperate op-amps? __________________ "Remember, you're special.....just like everyone else." Last edited by rezer; 22nd August 2008 at 02:04 AM.

22nd August 2008, 02:34 AM	(permalink)
Roff	What power supply voltage(s) do you have available? Do you have a preferred op amp? It needs to have FET inputs. __________________ Ron

22nd August 2008, 05:06 PM	(permalink)
Roff	I misinterpreted the datasheet. The part is common cathode, as you stated. Are you going to use AVCC, or the internal 1.1V reference, for the A/D? __________________ Ron

14th November 2008, 06:02 PM	(permalink)
canoo	Hello! I'm working on a similar project. The components I'm using are as follows: Photodiode OpAmp The results I'm getting with the solution posted here are sorta like on off (Any light at all and the output goes low. With no light the output goes high. Very hard to reach a happy medium). I have a feeling it has to do with the resistor and capacitor in the feedback? I'm wondering how I should go about selecting the proper resistor/capacitor combination? Thanks in advance!