Pico 4425 Channel overrange & Common-mode overrange

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Pico 4425 Channel overrange & Common-mode overrange

Postby joesauto98@cox.net » Tue Jun 06, 2017 8:42 pm

Hello, I've been using my Pico 4425 for a couple years now and absolutely love it. Every now and then when I try to connect my ground lead I get a couple warnings: Channel overrange & Common-mode overrange and no matter where I move the ground to I get this warning & I try to connect my signal wire and it just continues to show these warnings and wont give me a waveform if I disconnect the ground from the vehicle the warnings go away but as soon as I connect the ground the warning comes right back. This doesn't happen most of the times but every now and then a car will come in the shop and this happens and makes the scope unusable. Any help would be greatly appreciated. Thanks
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Re: Pico 4425 Channel overrange & Common-mode overrange

Postby Steve Smith » Wed Jun 07, 2017 2:50 pm

Hello and thank you for your post

The common mode range error warning (Yellow exclamation mark icon) is displayed when the ground reference for a BNC shell exceeds +- 30 V.
When this warning is visible, no data is drawn across the screen and the scope will appear to be unresponsive until the common mode range error is removed.

Typically, each measurement taken via our test leads are referenced to chassis ground and with the 4425 scope each channel requires its own reference.

Can I ask:

1. On each occasion when connecting your test lead ground wire, was this to chassis ground or to a voltage above 30 V?
2. When the common mode range error occurs is your Laptop powered via a mains charger?

One possible issue you may have experienced (that can generate a common mode range error) is "noise" from Mains/DC Power supply. In this case the solution is simply to connect the scope unit ground pin (adjacent to the USB socket) to the vehicle ground using a suitable ground jumper lead.

In my experience of working with a variety of laptops you may find the DC power supply (from the lap top charger) is exceptionally noisy when the laptop battery is discharged. As the lap top battery recovers the noise from the charger is reduced and the common mode range error clears. This may explain the intermittent nature of this warning message.

I hope this helps, take care........Steve
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