Author Topic: double range amplification issue  (Read 712 times)

0 Members and 1 Guest are viewing this topic.

Offline sdoubleTopic starter

  • Frequent Contributor
  • **
  • Posts: 280
  • Country: fr
double range amplification issue
« on: June 23, 2019, 10:06:06 pm »
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.
« Last Edit: June 23, 2019, 10:09:28 pm by sdouble »
 

Offline sdoubleTopic starter

  • Frequent Contributor
  • **
  • Posts: 280
  • Country: fr
Re: double range amplification issue
« Reply #1 on: June 23, 2019, 10:12:43 pm »
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 ?
 

Offline Audioguru

  • Super Contributor
  • ***
  • Posts: 1507
  • Country: ca
Re: double range amplification issue
« Reply #2 on: June 23, 2019, 10:54:51 pm »
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?
 

Offline duak

  • Super Contributor
  • ***
  • Posts: 1048
  • Country: ca
Re: double range amplification issue
« Reply #3 on: June 23, 2019, 11:29:57 pm »
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

 

Offline sdoubleTopic starter

  • Frequent Contributor
  • **
  • Posts: 280
  • Country: fr
Re: double range amplification issue
« Reply #4 on: June 24, 2019, 04:16:50 am »
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.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf