Why does my DAQ counter count more when a capacitor is used to eliminate noise

Question

Hi,
In my application i need to measure the speed of the motor using an encoder. The output of the encoder is a bit noisy (around 200 mV of noise.).Actually we are using a Siemens variable speed drive and it is the switching noise even after perfect shielding. Also the ouput of the encoder has a DC shift in waveform fr 383mV.When i connect to the counter channel (source :to count the pulses or gate :to measure frequency) the readings are high than actual. Then we conected a 0.1 microfarad capacitor across the counter channel and the DIO Ground. Then checked the waveform using a 60 mHz tecktronic oscilloscope. The waveform was without noise but the shape was a bit distorted, meaning there was no sharp rising edges and has a slow rise.But still there was a dc level shift for 383 mv.When i measured using counter the count was abnormal.Actual reading should show 1000 pulses /second, but with a capacitor it was counting 4789,5000, 356 etc.
Please let me know where the problem is. IS there any specific input signal rise time/fall time etc that the counter requires. Also will the dc level shift of 383 mV affect the counting.
I am using DAQ mx example vi(namely low frequency count as my freq range is 0 to 4000 hz)and also tried with simple digital count. Also please tell me if there is any problem in the connections
Thanks,
Sudha

Answer 1

While putting the capacitor between your signal and ground will git rid of any transients, it will also "round" out your signal. Your square wave provided by the encoder can be approximated by the sum:
y(t) = a_0 + sum(k=0...infinity, a_k*cos(k*f*t)) + sum(k=0...infinity, b_k*sin(k*f*t))
The capacitor has a frequency-depenent resitant (called reactance) which decreases with increasing frequency.
The reactance is:
X(f) = 1 / 2*PI*f*C
Where f is your frequency and C=1uF. What happens is the capacitor shorts the higher frequency harmonics to ground and makes your square waveform "ring" at the edges.
If this ringing causes you to swing high-and-low close enough to the switching point, then you could count pulses that really aren't there. What you really need is some sort of hysteresis to prevent this false switching. You should put this after the bypass capacitor and before the counter.
Also, another question is: What are the output levels of your encoder? The 383mV DC offset shouldn't affect a counter, but if your logic "high" voltage doesn't exceed about 2V then you might be hitting a region of metastability: where the counter randomly switches high/low.
Regards,
Chris_B