Would this work - using ADC to take readings from photodiode?

[Laura]

Hello, I was looking for some advice.

I am looking to use some hardware to take readings from a photodiode and convert them to voltage for display on a graph. I would need readings at least every microsecond. Is this possible with any NI hardware?

Thanks.

[Buechsenschuetz]

I am afraid NI does not offer hardware to connect the signal of a photodiode to one of their DAQ boards.

Photodiodes always need some kind of amplifier as 'front end' between the diode and the ADC. There is an excellent article on this issue: 'Photodiode front ends - the real story'. You can find a link to it on

http://users.bestweb.net/~hobbs/

In most cases, a transimpedance amplifier with a single op-amp can do the job.

[Laura]

Thank you for your reply.

So if I connected up an op-amp and then read that into an ADC - would that work?

I noticed the PIC6014 has sampling rate 200 kS/s - that would give me around the number of measurements I would be looking for.

Does that sound feasible?

[Buechsenschuetz]

I think it will work. Of course you have to respect the input range of the ADC - and use it as completely as possible, allowing for approx. 10 p.c. headroom for signal overshoots. You should design the op amp stage accordingly. Most common op amps will have an output swing to 1 or 2 volts below the supply voltages, e.g. if you use +12V and -12V as supply voltage the op amp will not be able to swing beyond +10..11 and -10...11V. The bandwidth of your op amp arrangement should at least equal the sampling rate.

We are using a similar arrangement, with a photomultiplier which needs a transimpedance amplifier, too, and a ADC board (although not a NI product).