I have a band pass filter with a center frequency of 100kHz, a bandwidth of 10kHz, and a gain close to 1. Input signal is 100mV-150mV peak to peak with frequencies varying from 80kHz to 120kHz. I am using an OPA355 op-amp.
I then use the output of this filter as the input to my comparator. The comparator works perfectly in simulation and when tested alone. I am using LM339:
As soon as I connect the two, the filter output immediately changes due to an added noise component. On close inspection, the amplitude of the filter output decreases and has visible noise spikes.
This messes up VTH, VTL and hysteresis settings of the comparator. Hence the expected output is not attained.
and at 1ms/D:
I noticed that:
- If I remove the 2.5V from the potential divider from pin 4 of the comparator, the band-pass filter output goes back to it's original correct state.
- if I remove the wire connecting the output of the filter to pin 5 of the comparator, the bandpass filter output also goes back to normal.
I have tried, adding a decoupling capacitor across R1, changing resistors used for the voltage divider, using different LM339 comparators, and also using different channels of the comparator. None of these methods have worked. I am really stuck here, any suggestions on how to reduce/eliminate this noise would be appreciated.
How is noise being generated on the output of the bandpass filter, and not on the comparator output?
Please do let me know if I need to provide further information. I also have pictures of the (crude) setup on my breadboard if those would help identify a possible culprit. Thanks