Pcsu1000 - ground/earth noise

Instruments PC Oscilloscopes and Recorders
1

Hello,

I purchase Few days ago a PCSU1000, an began to do measurements.
I observe something “Strange” but it is possibly normal … can you help me….

I try the following :
Connect the portable PC with the power cord connected to the 230V OR working on batteries OR Working on Batteries and the ground of the computer connected to the Earth.
When my probe is not connected to nothing, I’ve a marginal signal 0.5mv using the 5mv Scale. I suppose this is ok.
When I connect my probe to the earth Or on the “ground of the computer” I observe this kind of thing (AC mode)

Here are few snapshots that illustrates what I see :

mode GND :
img37.imageshack.us/img37/5056/gndmode.gif

Probe in the air (not connected)
img42.imageshack.us/img42/3083/a … nected.gif

Probe connected to me :
img98.imageshack.us/img98/6972/a … tedtom.gif

Probe connected to PC Ground (PC Using External PSU)
I’ve the same kind of noise when I connect the Probe to the Earth of my house
img707.imageshack.us/img707/2307 … tedtot.gif

Probe connected to PC Groung (PC working on batteries and External PSU not connected !)
img237.imageshack.us/img237/2307 … tedtot.gif

A zoom of this noise using 1µS scale :
img694.imageshack.us/img694/2307 … tedtot.gif

Is this normal ?
Did I miss something ?

Thanks in advance for your answer, or pointing me another post where this problem has been solved.

thanks and best regards. Gilles.

2

It seems that this is normal. The noise seems to be mainly the voltage drop on the ground lead of the USB cable.
I got nearly similar result with my laptop. There is no difference if the external PSU is connected or not.
First I connected the probe tip to the USB cable connector GND (shield) at the rear of the PCSU1000 oscilloscope. Here is the result:

Then I connected the probe to the GND of the USB cable at the PC end. This looks quite similar as your observation:

Even higher noise is seen on the GND of the connectors on the opposite side of the laptop. All the mother board’s noise is there too.

3

Thank you. This confirms what I thought : computers are NOISY !
I tryed on another laptop, and got the same kind of thing.
I will try with another probe very soon (we never know …)

Best regards gilles.