Cross talk after changing Picoscope models

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joshua_morris
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Cross talk after changing Picoscope models

Post by joshua_morris » Thu Oct 18, 2018 9:53 pm

Hello all. I have been using a Picoscope 3425 for some time now and recently picked up a Picoscope 4444. I am trying to conduct the same test on the 4444 that I have always done with the 3425 and am getting some different results. Essentially I have 4 pairs of strain gauges connected to the scope through 4 separate Whetstone bridges. I can adjust the bridges to balance my voltages to 0V before I collect my data. With the 3425 I am able to balance one bridge at a time. When I try to balance one bridge on the 4444, all four of my signals shift in voltage making it impossible to balance them. The voltages also appear to be higher than usual (1.5V vs 100mV) although this is tough to say for certain without being able to balance them properly. They do have higher voltages if I just swap Picoscopes without changing anything else. I am using the same cables and grounding configuration in both cases. The 4444 does use D-sub to BNC adapters whereas the 3425 just has BNC connectors. Otherwise I can't see any differences. I must be overlooking something in these two models that would cause this sort of a "cross-talk". If anyone could share their advice or their resolution to a similar issue it would be greatly appreciated.

Martyn
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Re: Cross talk after changing Picoscope models

Post by Martyn » Fri Oct 19, 2018 9:56 am

This may be the way you are connecting to the 4444 using the adapters, the following drawings may help
Attachments
ta299.PNG
ta271.PNG
Martyn
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joshua_morris
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Re: Cross talk after changing Picoscope models

Post by joshua_morris » Fri Oct 19, 2018 1:46 pm

I am using the TA271 single BNC adapters which we bought from Picoscope. Is the D-sub on the 4444 scope/adapter grounded differently than the BNC connector on the 3425? The 3425 says that it has differential channels so I would assume it looks similar to the BNC side of the drawings you shared.

Martyn
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Re: Cross talk after changing Picoscope models

Post by Martyn » Fri Oct 19, 2018 8:49 pm

So you are coupling one side of your differential signal through ground to the other channels. For differential measurements you need the TA299 BNC connector, or use the Pico Connect 441 lead.
Martyn
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