Hey,
I was wondering if anyone out there can help me with a problem im having with 'ringing' on a comparator output. As the output changes from logic level '0' to logic level '1' there is a short burst of oscillations on the output. This is really undesirable as the comparator output is used as an input to a quadrature waveform generation circuit that ultimately inputs encoded signals to a digital tachometer. The ringing on the comparator output causes spikes to occur on the velocity profile of the digital tach.
At the moment I am using positive feedback on a LM311N comparator. The positive feedback has reduced the ringing considerably but it is still affecting the velocity profile on the digital tachometer.
If anyone has any idea how to get rid of the comparator ringing entirely I would be extremely grateful as this is all part of a final year undergrad electronics degree project and is due for assessment in 4 weeks.
Thanks,
Kieran.

Originally Posted by kieran.french

Hey,
I was wondering if anyone out there can help me with a problem im having with 'ringing' on a comparator output. As the output changes from logic level '0' to logic level '1' there is a short burst of oscillations on the output. This is really undesirable as the comparator output is used as an input to a quadrature waveform generation circuit that ultimately inputs encoded signals to a digital tachometer. The ringing on the comparator output causes spikes to occur on the velocity profile of the digital tach.
At the moment I am using positive feedback on a LM311N comparator. The positive feedback has reduced the ringing considerably but it is still affecting the velocity profile on the digital tachometer.
If anyone has any idea how to get rid of the comparator ringing entirely I would be extremely grateful as this is all part of a final year undergrad electronics degree project and is due for assessment in 4 weeks.
Thanks,
Kieran.

Good power supply decoupling, a good ground plane, and a little hysteresis will generally clean up a comparator output unless you have a lot of nonmonotonicity (noise or ringing) on the input. The hysteresis needs to exceed the amplitude of the noise to be effective.
Edit:
I guess I just restated what Styx said.

thanks guys, the ringing is there because of multiple crossings of positive to negative voltage, i guess ill just use hysteresis and hopefully that will sort it out as i cannot afford any ringing on the output whatsoever.
Thanks again

Originally Posted by kieran.french

thanks guys, the ringing is there because of multiple crossings of positive to negative voltage, i guess ill just use hysteresis and hopefully that will sort it out as i cannot afford any ringing on the output whatsoever.
Thanks again

Do you know why the input transition is nonmonotonic? That is the best place to tackle the problem.

hey,
I have bypassed the supply voltages using 0.1uF capacitors, it makes a slight improvement but still not quite good enough. I have tried adding additional hysteresis but again to no avail. Iv attached a scope shot of the ringing. The top waveform is the comparator outptut, and the bottom two waveforms are a set of quadrature waveforms for which the comparator output is used to generate.