Hello, first of all I am brand new to the forum. I did ask this question on another forum and was not given any advice, useful information, or really even a relevant avenue to look down. Hoping this place will be a bigger help. I'll start off by saying I am pretty new to electronics but am not asking to be spoon-fed answers.
Essentially here is what I have going on. I am shining a laser pointer on a photodiode(BPW21). By checking the voltage coming off of the diode I get a nice 0.00V(no light) and with the light(0.250V). I will spare you the detail but the intensity of the light acting on the photodiode does change for my application. Meaning I can get 'intensity'/'irradiance' measures of 0-0.250V.
Now, the goal is to turn this range into a larger but still accurate analog signal so it can be read by a microcontroller. Ideally I would like to see 0.0-5.0V but at this point will happily settle for 0.0-2.5V. Now I am aware, it is the current of photodiodes which has a linear response to light intensity not the voltage. So I selected a transimpedance operational amplifier to achieve this goal(current->voltage+gain).
I have attached an image of the circuit diagram I am currently using(hopefully the photo diode isn't backwards in the diagram it's not in the actual circuit ). I will create a diagram of what is exactly on the bread board by request.
The problem. Without the transimpedance op-amp I get potentials of 0-0.250V. With amplification I get 0-0.330V even after a megaohm load resistor. Although, for the range of around 0.400-0.500V(not amplified) when amplified I see 4.00-5.00V after amplification. In my application I will only be seeing the 0-0.250V (as well as the associated current response), so having the gain so large after the 0.400V is not useful to me. I think there is something wrong with my circuit, but am not sure what. Any input, advice, things to look into, etc, would be greatly appreciated. IE should I be doing two stage amplification, is something blatantly wrong with my circuit, should I be doing more of an instrumentation amplifier circuit, should I use a bigger supply voltage on the op-amp(the circuit will be shielded from light when operated), etc. Noise tuning, all the finalized stuff I can handle on my own, right now though, getting an appropriate/useful gain is what I am stuck on.
Interestingly I was obtaining more useable results with a junky LM358N op-amp on a single supply and the same photodiode. I only bought the LTC1050's because they have significantly better specifications, but I must be doing something wrong here.
Here are the data sheets for the components I am using:
LTC1050 Datasheet - http://cds.linear.com/docs/en/datasheet/1050fb.pdf
BPW21 Datasheet - http://www.vishay.com/docs/81519/bpw21r.pdf
Sometimes I test the circuit with a BPW34 but it's not the ultimate photodiode that I want - http://www.vishay.com/docs/81521/bpw34.pdf
Essentially here is what I have going on. I am shining a laser pointer on a photodiode(BPW21). By checking the voltage coming off of the diode I get a nice 0.00V(no light) and with the light(0.250V). I will spare you the detail but the intensity of the light acting on the photodiode does change for my application. Meaning I can get 'intensity'/'irradiance' measures of 0-0.250V.
Now, the goal is to turn this range into a larger but still accurate analog signal so it can be read by a microcontroller. Ideally I would like to see 0.0-5.0V but at this point will happily settle for 0.0-2.5V. Now I am aware, it is the current of photodiodes which has a linear response to light intensity not the voltage. So I selected a transimpedance operational amplifier to achieve this goal(current->voltage+gain).
I have attached an image of the circuit diagram I am currently using(hopefully the photo diode isn't backwards in the diagram it's not in the actual circuit ). I will create a diagram of what is exactly on the bread board by request.
The problem. Without the transimpedance op-amp I get potentials of 0-0.250V. With amplification I get 0-0.330V even after a megaohm load resistor. Although, for the range of around 0.400-0.500V(not amplified) when amplified I see 4.00-5.00V after amplification. In my application I will only be seeing the 0-0.250V (as well as the associated current response), so having the gain so large after the 0.400V is not useful to me. I think there is something wrong with my circuit, but am not sure what. Any input, advice, things to look into, etc, would be greatly appreciated. IE should I be doing two stage amplification, is something blatantly wrong with my circuit, should I be doing more of an instrumentation amplifier circuit, should I use a bigger supply voltage on the op-amp(the circuit will be shielded from light when operated), etc. Noise tuning, all the finalized stuff I can handle on my own, right now though, getting an appropriate/useful gain is what I am stuck on.
Interestingly I was obtaining more useable results with a junky LM358N op-amp on a single supply and the same photodiode. I only bought the LTC1050's because they have significantly better specifications, but I must be doing something wrong here.
Here are the data sheets for the components I am using:
LTC1050 Datasheet - http://cds.linear.com/docs/en/datasheet/1050fb.pdf
BPW21 Datasheet - http://www.vishay.com/docs/81519/bpw21r.pdf
Sometimes I test the circuit with a BPW34 but it's not the ultimate photodiode that I want - http://www.vishay.com/docs/81521/bpw34.pdf
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