Photodiode choice and transimpedence amplifier design for measuring light pulses - Page 1
Forum section: electronics / projects

author: amaschas
author_rank: Regular Contributor
timestamp: September 26, 2024, 10:35:58 pm
content: I'm current working on a project where being able to measure the length of light pulses on my oscilloscope would be extremely helpful, and the research I've done so far has lead my to believe that a fast photodiode with a transimpedance amplifier would be a good approach. I have two cases that I want to verify:1) Observing the rise time, fall time and length of a light pulse ranging from a few hundred µs to several seconds. The light pulse is triggered by a flash hot shoe, which sends a signal to a microcontroller to pull up the enable pin on an LED driver circuit that I've designed. I want to verify how closely the timing of the light pulse matches the timing of the voltage pulse that pulls the enable pin high.2) Observing the light pulses resulting from a 30khz/10bit PWM signal to the aforementioned driver circuit. The rise and fall times can get very fast here, on the order a single ns, but I'm mostly in there in the top 80% of the range, and less concerned with absolute rise times here that characterizing the pulse forms in the upper 80% of the duty cycle range. I think I'd be happy with response times in the 10s of µs here.I took a look at this project:https://www.eevblog.com/forum/projects/visible-light-oscilloscope-probe/but from what I understand I believe I can actually achieve what I want with a single stage amplifier, and that a 3-stage amplifier would in fact reduce the overall bandwidth of the circuit.I've also been playing around with this photodiode circuit design wizard:https://tools.analog.com/en/photodiode/The SFH-213 seems like a good choice in that its significantly faster than I need, with a 5ns response time, covers the range of wavelengths I care about, and is reasonably cheap at about $1/per.Where I'm struggling is with the selection of an op-amp. This choice seems very sensitive to the peak light intensity in w/m^2 of the signal I want to measure, and there's quite a bit of variability there, the actual value varies with wavelength, and there are several scenarios that I'm hoping to measure where intensity could end up varying quite a bit. To that end I'm hoping to build something that's adjustable over a wide range of intensity.I'm also a bit confused by the peaking parameter in the circuit design wizard, as it seems to indicate that the longer my pulse is, the more constrained my choice of op-amp. If anyone could provide some insight into what's going on there it would be helpful.

author: moffy
author_rank: Super Contributor
timestamp: September 26, 2024, 11:12:16 pm
content: A discussion on this topic which might help:https://www.eevblog.com/forum/projects/high-frequency-photodiode-transimpedance-amplifier-design/

author: tggzzz
author_rank: Super Contributor
timestamp: September 26, 2024, 11:45:49 pm
content: TIAs are discussed in The Art of Electronics 3, and in Phil Hobbs' bookhttps://electrooptical.net/Building_ElectroOptical_Systems/"Building electro-optical systems : making it all work" is directly relevant since "[it is] intended in the first instance for use by oppressed graduate students".Phil Hobbs is referred to several times in TAoE.