Initial decay increasing with sampling rate

[lkmaciel]

Hello everyone,

I'm experiencing an initial decay in my signal that increases with increasing sampling rates. I've included my acquisition code and examples of acquiring the signal with 1 kHZ and 10 kHz sampling rates.

I'm acquiring three physical channels using a USB-6003 DAQ board. By the time I run my code, the amplifier is already working and sending data to another computer through a RS232 cable.

I'd love to know what could be causing this, since the code is very simple. Could it be a hardware issue?

Thanks in advance,

Lucas

[Blokk]

There can be several causes, some explained here: http://digital.ni.com/public.nsf/allkb/72FEC45B7EAA8E2586257E26006E6E21

[M_Stas]

Hello lkmaciel,

Is the issue occur when you acquiring one physical channel?

[lkmaciel]

I tried that now, and it doesn't occur. Only if I acquired a different channel before.

For example, acquiring 3 channels simultaneously causes the initial decay in all three.

If I then acquire only channel 1, I have a decay on the first run but not in any subsequent run.

If I try to acquire channel 2 then, again it will show decay only on the first run.

So this is only happening when I run my VI for the first time with the channel.

[crossrulz]

What amplifier are you using?  How did you wire up the signals?  Is the signal ground connected to AI_GND?

[lkmaciel]

I'm using a Kistler 5073 charge amplifier (https://www.kistler.com/?type=669&fid=61429&model=document).

The wires go straight from the amplifier to the DAQ board (ground and channels 0, 1 and 2).

[M_Stas · Accepted solution]

In your code example you leave "input terminal configuration" terminal unwired. For USB-6003 default value for channels 0-4 is "differential". So if you connect wires as RSE this configuration can lead some issues in your measurements

[Blokk]

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@M_Stas wrote:

In your code example you leave "input terminal configuration" terminal unwired. For USB-6003 default value for channels 0-4 is "differential". So if you connect wires as RSE this configuration can lead some issues in your measurements

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Hmm, interesting point. But actually then how the OP can even see signals measured, if the terminal is set as Differential (as default if this is correct)? The pinout is totally different....

[lkmaciel]

Thanks, M_Stas! Changing it to RSE solved the problem. I just need to get my head around understanding it now.

[crossrulz]

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@Blokk wrote:

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@M_Stas wrote:

In your code example you leave "input terminal configuration" terminal unwired. For USB-6003 default value for channels 0-4 is "differential". So if you connect wires as RSE this configuration can lead some issues in your measurements

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Hmm, interesting point. But actually then how the OP can even see signals measured, if the terminal is set as Differential (as default if this is correct)? The pinout is totally different....

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When you have a floating input, who knows what is left on that channel.  It most likely would trail the positive input or even stay at or near ground.