EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Vindhyachal.takniki on February 23, 2016, 04:59:03 am
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I have a system in which pin is toggled. Pin toggle is viewed on CRO with 10x setting & peak detect. There are two cases:
Case1: MCU board is powered by laptop usb power. CRO earth is connected to MCU board ground. laptop is powered from internal battery & no external supply is connected to laptop. SO CRO earth is connected to MCU ground which in turn connected to laptop ground via usb connector shield.
In attaced pdf, page 1 shows the pin toggle waveform. It is nice & clean, no noise or whatever.
Case1: Now external supply is connected to laptop also.
Now suddenly there is noise in system. Page 2, shows the noise in pin toggle. Pin toggle time is still the same. (However pic shows it larger, I think its due to camera speed). But noise is present in system as shown.
Then I connected CRO probe to ground of MCU where CRO earth probe is connected. Page 3 shows the noise in MCU ground also.
1. What is this noise? One reason could be that now laptop is connected to earth, which is now connected to MCU ground. But this happened in first case also where CRO earth is connected to CMU ground?
2. Is this actually a noise? Because noise is present both in pwer & ground of MCU, so I think if it is measured by differential probe then there will ne no noise in case2.
3. is this noise dangerous for circuit, i.e it affects analog circuitry. If yes, how to get rid of this noise.
PS: I have long wire coming thorugh CRO i.e separte earth probe & separte CRO probe. Another erason could be this is due to large CRO probe wire length. But if that would be case, it should happen in case 1 also.
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I had this exact thing happen to me along with an erratic touchpad. The laptop was given to me because of the touchpad and it turned out to be the cheap POS replacement power supply they bought was incredibly noisy. Hook your scope up to the laptop power supply output and you might find your problem.