Hi,
it would be helpful to see a scope picture...
C131 and R116 is a high pass filter with tau = 0.5ms. you could lower R116 to be faster.
Maybe extra delay is caused by saturated input stage of OPAMP. You could try two anti-serial 2.1V zeners across R119 to optimize.
R75 is useless.
Supply voltage of OPAMP and comparator is not shown. Also all VCC bypass and bulk capacitors are missing.
Klaus
I believe you'll understand why the circuit is behaving bad if you think a liitle bit about it's operation. Each photodiode will discharge while disconnected and has to be charged to 5V first when it's ctivated, causing the observed transient.
The lessest thing would be to place an individual bias resistor for each photodiode, better individual coupling capacitors to allow for different DC operation point of each diode if required.
There will be still considerable crosstalk form switching signal, but it may work for sufficient strong photodiode signals.
The over all design seems to be wrong to me. As the output stage is a schmidt trigger, then the infra red signal must be digital. So you have 18 diodes whose OFF and ON resistance must be matched, so that after Muxing and multiplied by 15 do not overload the schmidt.
The 4mS is this when you switch slowly from diode to diode, say every 100mS, or is it when you switch from an ON diode to an OFF diode.
Frank
Are you operating the diodes in photovoltaic mode?
I'm with FvM on this one. Each diode must be individually terminated with its own bias resistor.
from driver circuit and i think maybe it is better to multiplex diode's current rather than voltage. I don't know !
For this you need an extra input resistor.I put R75 for reducing noise in comparator output (Like Schmitt Trigger).
Current switching with zero bias will in fact involve less voltage swing, but still about 0.5 V between open and short circuit with respective transient currents. The solution may be feasible if photodiode and wiring capacitance is relative small compared to the common input node.About your photodiodes.... Do they need biasing? Try to operate them with 0V.
Hi,
Yes.
it also depends on multiplexing frequency. If it changes every minute, then the 4ms won´t be a problem.
If he likes to switch every 1ms, then it surely is a problem.
I little more info about the whole application could help to understand the function and to give better advice.
Klaus
Hi,
For this you need an extra input resistor.
About your photodiodes.... Do they need biasing? Try to operate them with 0V.
Klaus
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