Hi,
Then most probably there is noise at at the input at first place.
I see no noise filtering in the whole circuit.
Noise is something random.
But in your case it is not really random: It repeats, thus it´s an oscillation.
I often say: I circuit without capacitors isn´t a reliably working circuit.
Capacitors used as noise filters but also used for power supply stabilisation.
None I see.
Also we don´t see the PCB layout. Wrong GND routing may also cause oscillation problems.
And we don´t see what your scope pictures show (just one channel is named), and we don´t see where you connected the according scope_GND to the PCB_GND.
This is why many simulations don't show the realit (..and thus I call useless). They expect perfect PCB layout, expect perfect power supplies, expect no noise...
Klaus
Thanks for the quick reply KlausST.
There is a bypass cap on the board at the supply of the comparator LM2903YPT.
VDD5 is generated from an onboard LDO and its quite stable. There is cap of >10uF at VDD5 net.
There are provisions to add caps to filter the noise at U4 pin 3, U4 pin 2 and U3 pin 3. But these caps were not populated initially.
So, I have tried adding filter cap at one of these pins, but it didnt solve the issue.
I didnt want to solve this issue by trial and error. but rather understand what is reason for these oscillations.
So, stopped trying to add caps.
Regarding the screenshot from the scope,
Green channel is waveform at U4 pin 2.
Yellow channel is waveform at VDD12.
The scope GND is connected to the PCB GND, which is close to LM2903 GND.
The other signals uses the same GND reference and no oscillations were observed in the GND or in other signals.
Regarding the issues in the PCB, Yes, This is under evaluation.
But, the same circuit was prepared with a general purpose PCB after this issue was observed and got same result.
So, I think it may not be PCB specific.
Regarding simulation, I learnt the hard way. I will be careful next time.
One question: Can we short the output of LM2903YPT as done in this circuit? Is there any impact on the LM2903 functionality if two outputs are shorted?
Thanks,
Navbp