Capacitor couples noise?

[seyyah]

I am trying to do pulse measurement. i.e pulse count.

I have the following circuit. I add a small rc filter which has a cut off frequency higher than the pulse frequency. I'm observing the signal from the scope i don't see any noise or pulse deformity. But when i compare my results with an industrial counter my results are wrong when the capacitor is in the circuit. When i take it out the results are identical. This means that capacitor couples some noise into the circuit which i cannot see on the scope. I add that to prevent the noise. Should i remove it and why capacitor affects the result? Did you propose other circuit than this to interface with a 24v pulse to 5v microcontroller, thanks.

 

[melc]

I am trying to do pulse measurement. i.e pulse count.

I have the following circuit. I add a small rc filter which has a cut off frequency higher than the pulse frequency. I'm observing the signal from the scope i don't see any noise or pulse deformity. But when i compare my results with an industrial counter my results are wrong when the capacitor is in the circuit. When i take it out the results are identical. This means that capacitor couples some noise into the circuit which i cannot see on the scope. I add that to prevent the noise. Should i remove it and why capacitor affects the result? Did you propose other circuit than this to interface with a 24v pulse to 5v microcontroller, thanks.

The capacitor is killing the pulse shape too (the rising edge), increasing the jitter at the comparator output. You need a comparator with schmitd trigger (with positive feedback). The simplest way to interface a 24V pulse to a 5v microcontroller is to use an optocoupler with internal schmitd trigger like H11L1.
https://www.fairchildsemi.com/ds/H1/H11L2-M.pdf

 

[seyyah]

The input signal is slow and as i have said before, i can't see any delay at the output (of course there's but lower than significant) and the input and output signals are identical except the amplitude. But the results propose that there's noise. Also when i enable the digital filter that prevents rapid, narrow pulse changes it also helps to recover the problem. This result also conforms with the previous conclusion. But it can't be seen on a 100Mhz/1Gs scope.

So you say there may be jitter at the output. But does it affect the counting? It should only affect the phase of the output signal. It may rise at different times at each periods but i think the frequency should not change. Also i think hysteresis or schmitt triggering should not affect the result since the signal or noise or jitter goes from the max value to min value which discards hysteresis and schmitt triggering.

 

[seyyah]

I was able to capture the 2 situations with capacitor and without capacitor. Pictures clearly show that capacitor worsens the transition of pulse states. I also tried to add some hystresis which helped but not completely solved the problem. Even small capacitors have the same problem but least. So isn't it a good idea to add low pass filter before these type of pulse measurement circuits? Or should i change the capacitor type? Also what causes this phenomenon? Non-ideal capacitor? Thanks.

 

[jason99pan]

I am trying to do pulse measurement. i.e pulse count.

I have the following circuit. I add a small rc filter which has a cut off frequency higher than the pulse frequency. I'm observing the signal from the scope i don't see any noise or pulse deformity. But when i compare my results with an industrial counter my results are wrong when the capacitor is in the circuit. When i take it out the results are identical. This means that capacitor couples some noise into the circuit which i cannot see on the scope. I add that to prevent the noise. Should i remove it and why capacitor affects the result? Did you propose other circuit than this to interface with a 24v pulse to 5v microcontroller, thanks.

I agree the "melc", the capacitor will kill the pulse if the pulse frequency are grater than 5Hz(approximately).

Maybe you can decrease the value of the capacitor, or you can use the melc's suggestion (use the optocoupler) .

 

[Siniša]

You should add hysteresis to your circuit, add high value resistor from output of comparator to + input and additional resistor from + input to rest of the input circuit. What happens is that your capacitor slows input and at the moment when gets close to transition noise is large enough to toggle state of comparator. Adding hysteresis will solve this problem.