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coupling between oscilloscope probes

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fanshuo

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Hi all,
I found if I connect one probe to a DC on a PCB, another to a high frequency signal on the same PCB, the DC pin shows big coupling

if only use one probe to test the DC, no coupling is shown

any idea?
Thanks
 

What frequency? Have you used separate ground clips on each probe? What oscilloscope? x10 probes?

Keith
 

It could be the probes. One thing to try is using the spring tip type probes rather than the ground wire with a crocodile clip. Also, make sure the board isn't also earthed through the power supply.

Keith
 

Thanks for your reply. I will try to check it.
But it is difficult for me to understand why the probe could cause the coupling
 

The ground wires with a crocodile clip are notorious for pickup- they are like little aerials. Actually, not so little as some are around 9" long.

Keith
 

The grounds for the DC and high frequency signal on the PCB are the same, are they? Not, say, separate digital and analogue grounds?
 

FoxyRick said:
The grounds for the DC and high frequency signal on the PCB are the same, are they? Not, say, separate digital and analogue grounds?
Click to expand...

they are separate on the PCB, but they are the same on the IC
 

Loading a 100 MHz signal with 10 pF probe input capacitance causes a considerable load current. Due to the inductive probe ground clip, part of th current is flowing through probe cable ground and back to the other probe, generating a voltage drop that shows in the other channel.
 

FvM said:
Loading a 100 MHz signal with 10 pF probe input capacitance causes a considerable load current. Due to the inductive probe ground clip, part of th current is flowing through probe cable ground and back to the other probe, generating a voltage drop that shows in the other channel.
Click to expand...

thanks for your reply.
make sense to me.
 

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