Optical probe for oscilloscope

[Dave92F1]

Back in January Elektor published a "Optical Probe for Oscilloscopes" project: https://www.elektormagazine.com/articles/optical-probe-for-oscilloscopes

It's a photodiode with a couple of op-amps.

My question - how is this better than just hooking a photodiode (or small solar cell) directly to an oscilloscope?

I've successfully measured video latency with small solar cells and an oscilloscope - CH1: solar cell and camera, CH2: solar cell on display; flash a light at the camera, measure time between Ch1 and Ch2 spikes.

I don't think the result is quantitative in either case. So why bother with the opamps?

[FenTiger]

Impedance matching? The photodiode sees a roughly zero Ω load due to the virtual ground created by IC3A, while IC3B is configured to drive a 50Ω cable.

[Dave92F1]

Impedance matching?

Maybe. Would that improve the frequency response?

I mean, why bother impedance matching? A solar cell (or I presume a photodiode) seems to drive an oscilloscope directly just fine.

[FenTiger]

Would that improve the frequency response?

I don't know. I'm just guessing, and hoping somebody will tell me why I'm wrong.

[JohnPi]

Bandwidth.

A scope may have ~ 20 pF or input capacitance in parallel with 1 MΩ -- giving a time constant of about 20p*1M = 20 us. The photodiode (which has additional capacitance, further reducing the BW) generates a current and this takes some time to charge the 20 pF capacitance. When the light turns off, it takes some time for the 1 MΩ to discharge the capacitance. The final voltage is the photocurrent * 1 MΩ, or the VF of the photodiode itself (about 0.6 V). An opamp appears to short the photodiode current to the virtual ground -- thus that current doesn't have to charge any capacitance -- the opamp does the work of driving the output and also the virtual ground through the feedback resistor, giving an output voltage of Iphotodiode* Rfeedback.

You can see this yourself -- touch and remove a scope probe to a battery or power supply -- you will see a fast rising edge when you make contact, but an exponential decay when removed (it may be quite noisy when you do this manually, but you will be able to get a few clean traces) 

[tggzzz]

Search terms that will help: photovoltaic mode, photoconductive mode, PIN photodiode, transimpedance amp.

Which type of design is most appropriate depends on objectives and constraints.

[tanveerriaz]

response time of solar cell and photodiodes are difference .