How to convert analog output of photodiode into digital intensity chart?

Thread Starter

Mischa Johal

Joined Aug 29, 2018
3
I am working on a project for measuring the fluorescence intensity coming off tagged DNA. These fluorescent tags are exited by lasers, and the fluorescent signal is read using a photodiode. The only problem; I don't know how to read the information collected by the photodiode (how to convert it from analog to digital). Any explanation or comments would help greatly, and would be much appreciated!
 

dendad

Joined Feb 20, 2016
3,535
There are lots of example circuits around, like this simple one .....
http://www.bristolwatch.com/ele2/pdiode_op_amp.htm
An amplifier will be needed to increase the signal, then that can be fed to an analog to digital converter.
You will need to decide on what precision is required. Like, is 8bits ok? That will give you 0 to 255 reading.
10 bits is 0 to 1023, and so on.
As the precision increases, so does the difficulty of the circuit to keep the readings stable.
8 to 12 bits is not that hard but going for something like 18 bits requires precision components and careful layout.
If you are after calibrated readings it will be best to purchase a proper commercial instrument.

That said, most micro controllers have A/D converters built in. Even the humble Arduino, so it could be worth getting one and having a play to just get an idea of what you need.
 

Thread Starter

Mischa Johal

Joined Aug 29, 2018
3
I am looking to use fibre optic and coaxial cables to send the output from a photodiode to a computer. How can I do this, and, once the information has been transferred to a computer, how do I then interpret that information as a wavelength graph.
 

MrChips

Joined Oct 2, 2009
21,148
If one is to assume that the light intensity is extremely low, then it would be better to measure the intensity at the photo-diode first before transmitting to the computer. Fibre optic may not be the best option unless there is an issue with accessibility to the source of the fluorescence.

What information are you expected to record? Photo-intensity alone?
Is the fluorescence static?
 

shteii01

Joined Feb 19, 2010
4,644
I am working on a project for measuring the fluorescence intensity coming off tagged DNA. These fluorescent tags are exited by lasers, and the fluorescent signal is read using a photodiode. The only problem; I don't know how to read the information collected by the photodiode (how to convert it from analog to digital). Any explanation or comments would help greatly, and would be much appreciated!
Normal people convert analog to digital using Analgo to Digital Converter, commonly abbreviated as ADC.

What are the minimum and maximum outputs of photodiode?
(The answer should be two numbers.)
 

danadak

Joined Mar 10, 2018
4,057
I am looking to use fibre optic and coaxial cables to send the output from a photodiode to a computer. How can I do this, and, once the information has been transferred to a computer, how do I then interpret that information as a wavelength graph.
1) Digitize PHOTODIODE
2) UART data over fiber optic link or Bluetooth(PSOC 4 BLE) or wireless (ESP8266 or ESP32)
3) Python app to process the data

First two would look like this, one chip, for UART Fiber approach -


The SAR digitizes the photodiode and using DMA UART is stuffed and transmitted.

If you need trimming of Vref to maximize dynamic range of TIA then use a VDAC vs Vref.

If 12 bits not enough use DelSig, will do up to 20 bits.

This is all onchip.

upload_2018-8-31_7-13-43.png


To plot data use Stampplot Pro vs write Python plotter app.

http://www.selmaware.com/stampplot/


Regards, Dana.
 

MrChips

Joined Oct 2, 2009
21,148
Molecular fluorescence is a highly specialized and advanced field with new techniques constantly emerging.

Is your project related to university research at the undergraduate, masters, or PhD level?
Are you interested in measuring optical wavelength as well as intensity?

Are you able to modulate the exciting laser?
Are you able to modulate the photo input or output with a Kerr Cell?

What ADC resolution and conversion time is required for this application?
 

Thread Starter

Mischa Johal

Joined Aug 29, 2018
3
Molecular fluorescence is a highly specialized and advanced field with new techniques constantly emerging.

Is your project related to university research at the undergraduate, masters, or PhD level?
Are you interested in measuring optical wavelength as well as intensity?

Are you able to modulate the exciting laser?
Are you able to modulate the photo input or output with a Kerr Cell?

What ADC resolution and conversion time is required for this application?
Thanks for your reply! My project is on capillary electrophoresis (a type of DNA analysis), I am interested in measuring optical wavelength and intensity, and I am unable to module the laser. I am not sure what your next question was (I am pretty new to electronics), and I would most likely need a pretty high resolution ADC.
 

MrChips

Joined Oct 2, 2009
21,148
Optical spectroscopy is not as simple as digitizing the signal from a photodiode with an ADC.
I suggest you investigate optical spectrometers. Depending on the acquisition time requirements, you will either need a diffraction grating or multi-spectral sensors.
 
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