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Floating Source on USB-6356

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I'm having a bit of trouble getting a consistent reading from a floating source signal.  The signal is coming directly from a hydrophone.

 

When I have the switch on my device (USB-6356) set to ground source the signal is all over the place and when I switch it to floating source the signal looks as it should, however it is not consistent some times I have to switch to ground source and then back to floating source for the signal to start reading and then sometimes it just stops reading.

 

But if I put a BNC tee on the signal and connect it to an oscilloscope I have no issues reading the signal with labview, regardless of which position the switch is in.

 

Does anyone have any tips so that I don't need the oscilloscope hooked up all the time?

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Message 1 of 14
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Could you please share a rough sketch of how everything is connected? (important about the GND)

For any floating signal, you have to reference it to the DAQ ground somehow as illustrated in the below image and article. I believe, when you connected the Tee and Scope, it looks like the Scope was doing the grounding for you.

 

santo_13_1-1619450159134.png
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019QRZSA2&l=en-US

-Santhosh
Semiconductor Validation & Production Test
Soliton Technologies
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Message 2 of 14
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Here is a rough sketch

 

rocksolidsr_0-1619450684560.png

I was under the impression that the USB-6356 would make that connection to AGND for me when I switched the switch to Floating Source

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Message 3 of 14
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Okay, so if you're using the BNC version, then it looks like you've just flip the FS/GS switch under the BNC jack to FS position.

santo_13_0-1619459824953.png

 

It could also be possible that the additional capacitance from the scope cable and frontend is helping somehow.

 

-Santhosh
Semiconductor Validation & Production Test
Soliton Technologies
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Message 4 of 14
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That is why I am concerned something isn't right b/c when I have the switch to the floating source it is not consistent. I will try another input and see if there is any difference.

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Message 5 of 14
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Are you sure that the 6356 has enough resolution to quantize the hydrophone signal? - I hope that is already taken care of because my simple google search says that the piezo hydrophones generate in the order of uV.

 

I have used the DSA cards for such low signals with reference microphones used for Acoustic testing.

-Santhosh
Semiconductor Validation & Production Test
Soliton Technologies
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Here is a capture of the signal, and that is on the lower size of what I've been reading and that is what matches my oscope.

 

rocksolidsr_0-1619460455063.png

 

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Message 7 of 14
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What is the output impedance of the hydrophone? Any links to its manual?

Perhaps the input impedance of the DAQ is too high to match- hence adding a scope input (1Mohm) to the circuit makes it work.

 

-AK2DM

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"It’s the questions that drive us.”
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Message 9 of 14
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Your model 8103 generates charges and this would need to be converted to voltage (in your case, the 1M input impedance of the scope was converting the charges to voltage) either using a load resistor or a high-tech charge amplifier.


Even the manufacturer is recommending a charge amplifier or a charge converter,

santo_13_1-1619538013960.png

 

8106 is the only model which is externally powered and has good (<30ohm) output impedance.

santo_13_0-1619537799039.png

 

-Santhosh
Semiconductor Validation & Production Test
Soliton Technologies
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