This might be a bit long to explain but actually a power supply noise is affecting a system's measurements. I will briefly try to explain the system and the noise issue with several pics. This is directly related to my previous question: How to verify observe or check the source of AC line noise
I'm using this [system][1] together with this [module][2](SECTION 4 shows ELECTRONIC COMPONENTS). This is a pressure scanner system. Basically the ZOC modules amplifies and multiplexes 64 pressure channels(500Hz sampling freq. for each channel) and sends the particular channel's analog voltage output(chosen by a decoder code sent from the system) to the RAD_unit which then converts this voltage to a digital by its ADCs and sends to a PC.
Here is the basic illustration of the system setup:
[![enter image description here][3]][3]
All system is fed from one AC mains outlet using a power strip.
Depending on the day, time of the day or powering the system through different AC mains outlets I was getting different magnitude of spiky noises in my measurements. Below shows this difference. Both voltage is on vertical axis represents sensor data and scaled with factor 100.(For example 10 in vertical pressure axis is 0.1 Volt). Here is a comparison of two measurements where the bottom is when there is spiky noise:
[![enter image description here][4]][4]
Later on I checked the +-15VDC power supply outputs(RPM100 in above illustartion) which powers the module and sensors, and I found out there is noise similar to the one in measurements.
Here is how the power supply connects to the module:
[![enter image description here][5]][5]
And here are some screenshots of the power supply's +15V to GND measurements from scope in AC coupling:
A very noisy situation:
In FFT view:
A less spiky/noisy situation:
[![enter image description here][8]][8]
And here when there is almost no significant noise(the gap is scope's artifact):
Question:
It seems to me there is a big correlation between the measurement noise and the power supply noise here. Power supply is very old at least a decade old but a special one and powers all the units with its special connector.
1-) What kind of power supply noise can it be? It is obviously effected by the AC lines)
2-) For a quick solution, Im planning to cut the cable and place 100nF caps between +15V to GND and -15V to GND. Do you think it can be a solution? And if so would 100nF be okay?
Answer
Your oscilloscope scans are helpful. I see groups of noise spikes repeating at 10 millisecond intervals. This suggests a brushed motor running at 6000 rpm. Your oscilloscope time base is accurate enough that the 10 ms. rep rate is certainly not related to 60 Hz. line frequency.
But from the FFT plot, those noise spikes have consistent frequency content at about 68 kHz. There are many spikes on that FFT plot because the noise is discontinuous. As suggested by Ali Chen, noise sources at this frequency might emanate from switching supplies, or possibly a faulty light ballast.
Perhaps a combination of the two?...A motor driven by a Pulse-width-modulated power source (PWM @ 68 KHz.)?
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