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I have a signal coming from a photodiode detector, the pulse is really short, so the integration time is very small, and the total power measured is very small (very close to the noise of the detector). I want to take the signal detected by photodiode (a short square pulse with variable amplitude) and stretch the duty cycle from about 1% to 50% at the same amplitude of the pulse being stretched (which changes from pulse to pulse).

Normally i would use a monostable 555 timer, but i'm not sure how to get the amplitude issue worked out.

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  • \$\begingroup\$ Can you show us an example of the waveform? \$\endgroup\$
    – Bruce Abbott
    Nov 19, 2022 at 17:37

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One approach - use the trigger pulse to trigger both a monostable and a sample-and-hold (S/H) circuit. The photodiode pulse triggers the S/H to sample its own trigger signal amplitude. This creates a buffered output voltage scaled to equal trigger pulse amplitude. When the monostable circuit times out, use that trailing edge to reset the S/H, driving its output voltage to near 0 V.

Analog Devices (ADI), Linear Technology (LTI, now a part of ADI), and National Semiconductor (now a part of TI) have many S/H parts.

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  • \$\begingroup\$ Be aware that choosing a trigger level is difficult: Pulse amplitude is not detectable below this trigger level, so you'd prefer a low trigger level. Some noise may reach the trigger level and be falsely detected, especially if the trigger level is set too low. \$\endgroup\$
    – glen_geek
    Nov 19, 2022 at 14:29

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