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double range amplification issue
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sdouble:
Dear all,
I am currently fighting with something that does look very simple but behaves strangely.
My idea is to amplify the output of a photomultiplier and slightly filter those signals.
I basically have 2 parallel amplification stages with different gains. I also low pass filter following the sallen key topology. this is a 45 MHz bessel filter to avoid any overshoot.
sdouble:
the outputs are VF1 and VF2.
if i put nothing at the input or a small signal, the circuit behaves. If I try to send a large input pulse of, say 0.5V, both channels start to oscillate at 500 MHz roughly.
the oscillation can never stop, making the circuit totally useless.
any clue from someone who ever faced such a situation ?
Audioguru:
A Sallen-Key lowpass filter has a series input resistor. Your circuit has none, it needs a series input resistor for each filer.
Did you build it on a solderless breadboard with its massive amounts of stray capacitance?
Do the messy wires all over the place on the breadboard pickup Wifi and other 500MHz interference?
duak:
In addition to the missing input resistors, are there any bypass capacitors on the power supply pins of the opamps? Section 10 of the data sheet gives quite specific recomendations for the proper layout: http://www.ti.com/lit/ds/symlink/lmh6702.pdf
sdouble:
I just showed the scheme. a 2 layer PCB was routed and all decoupling caps are included in the actual design.
thanks for your help.
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