Voltage fluctuation with ground

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Murugesh_89

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Hi,
When i measure the voltage at the output of 7805,7808 by keeping the CRO's negative probe in negative of SMPS, i am getting some spikes in the output voltage. I am using 1 sq.mm wire to connect the circuit with the SMPS.


When i measure the voltage at the output of 7805,7808 by keeping the CRO's negative probe in ground of 7805, i am getting clear output voltage.


These 8v and 5v are then going to micro controller and some other components. My doubt is whether these micro controller and components are getting clear voltage as per Noiseless image or getting spikes in their supply as per Noisy image.

Can anybody please clarify me on where i am wrong?

Thanks,
Murugesh
 

A wise man once said to me: "Ground is where you plant tomatoes." In other words, just because schematically the two points are "ground" doesn't mean they have the same current (i.e., noise) flowing at those points.

I would look at several issues here:

1) Look at the pcb layout with respect to those two ground points.
2) Is the scope probe ground lead picking up radiated noise when it's near the SMPS?
3) Look at the voltage right at the components in question; use as short a ground lead as possible on your scope probe. (One of those springy things that fits right over the end of the probe is good)
 

To see if you are picking up common-mode ground noise, connect the oscilloscope ground pin and probe to the same point on the ground.
 

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