I am attempting to create a photodidoe amplifier circuit used for detecting fluorescence from an atomic source and cannot determine the type of buffer/follower OP AMP I should use when trying to convert the high impedance from the photodiode circuit to the low impedance necessary for the 50 Ohm BNC cable. I am currently considering BUF602 or BUF 634, but am open to other suggestions. I have attached a drawing of the circuit.
It's not clear that you need a buffer in your situation. What is the bandwidth required for your measurement? How long is the cable that connects to your measurement system? What type of instrument are you connecting to (what is its input impedance)? Answers to these questions will help determine whether you need a buffer.
We also need to know the capacitance of the photodiode and its expected signal current. You will probably need a transimpedance amplifier for your application.
Regards, Neil P. Albaugh ex-Burr-Brown
Hello and thank you for the quick reply. I have attached the specifications of the photodiode I will be using (Thorlabs FDS100). I believe this has the relevant information you have asked for on the photodiode (i.e. Bandwidth=35 MHz, Capacitance=40 pF). The BNC cable I will be using is roughly 10 feet long and is connected to an oscilliscope (Tektronix TDS 380) with a 1 M Ohm input impedance.
Your requirements are not going to be trivial--- A photodiode with 40 pF @ -10V will be difficult to deal with if you need much sensitivity from your fast photodetector circuit. A much smaller PIN photodiode will be far easier to deal with than one with an active area of 13 sq mm. Is it possible to use a much smaller active area device? If you chose the large photodiode because you need to capture more photons, consider using a very small detector with a lens or mirror to focus the large collecting area on a small active area. Optical gain is noise-free, so take advantage of it wherever you can.
You did not say how much photocurrent you expect from your detector.
Sorry about not including the expected photocurrent earlier, I must have
missed that point. We expect a photocurrent somewhere between roughly 100 nA
and 100 microA depending on the measurement. Unfortunately, using a smaller photodiode would severely hamper
the measurements I will be taking. I will be using a lens in the set-up, but
the measurements technique I will use requires that the image (the diameter of
the image will be about 1 inch) and the object size be equivalent, so I will
already be greatly reducing the amount I will image. Thus, using a smaller
photodiode further enhances this issue. Hopefully there is a way to
design the circuit while still using the larger photodiode. Thanks once again
for all the help.
No optics, eh? Well, we will "endeavour to persevere" as Chief Dan George said.
I have attached a circuit that uses a fast current feedback amplifier in a transimpedance amplifier configuration with your 40pF Thor photodiode. I've placed a 5-pole passive L-C low pass filter on the amplifier output to eliminate the out-of-band noise to improve your SNR. You might want to add a second amplifier just after the transimpedance amplifier to bump up the signal level but that depends on your scope sensitivity and resolution. High-speed digital scopes have a pretty poor noise floor and you want to be limited by the detector noise, not the scope.
Thank you Neil very much for the help. I truly appreciate it.
You're welcome, Sean.
hello, i dont understund your op choice (the ths3201), for what reason you dont used some opa that have fet input?
I was weeing your amplifier circuit, but I should you to have more low gain steps than use high value resistors, they must cause noise...
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