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Input impedance changes while streaming


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3 replies to this topic

#1 DanT

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Posted 27 June 2013 - 07:28 AM

I have a setup with streaming capture of two signals using a U6-PRO that is having a problem with loading changing when it goes into streaming. The signal I am capturing is a clock signal that has a high value of about 8V and a low of -8V and I am capturing bursts of data for one second every period that can be changed from 10 to 60 seconds. When it goes into streaming, the LabJack loads the signal so that it is shifted about 5 volts negative, although the negative clamps at about -10V. Is there some setup parameter I should look at? I haven't been able to find exactly what is causing the loading, only that it shifts when going into streaming mode and later gets reset. I would rather not have to add an external buffer to my test setup, especially since that would require adding two additional power supplies.

#2 LabJack Support

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Posted 27 June 2013 - 09:05 AM

I can think of a couple things that might make a difference in terms of streaming versus the U6 sitting idle. 1. When you change channels your signal must sink or source transient extra current to charge the analog signal change from the previous channel's voltage to the voltage of the current channel. Appendix A of the U6 User's Guide provides bias current and input impedance specs at steady-state, but when you consider the channel changing the more important spec is the max source impedance. 2. During stream mode the U6 could consume extra current or varying current, and depending on the USB host/hub this could affect VS, and if VS is powering your signal it might then be affected. Are you streaming multiple channels? Is so, try just streaming the 1 channel and see if it makes a difference. Does changing the scan rate substantially make a difference? If you do need to buffer, you can draw enough current from the Vm+/Vm- supplies on the DB37 to power an op-amp. See Section 2.11 of the U6 User's Guide.

#3 DanT

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Posted 27 June 2013 - 11:13 AM

I tried some more experiments. Slowing the sample rate reduces the loading effect. It gets better as I go slower so that by the time it is sampling at 50Hz, the effect is minimal. Unfortunately, I need to sample at least as 10Khz to get above the necessary Nyquist sampling rate so that doesn't help me much. I tried only running on channel and that didn't make any difference in the result. I also tried changing the settling time and that had minimal effect. I will look at the potential of adding a buffer which should work.

#4 LabJack Support

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Posted 28 June 2013 - 08:54 AM

With a single-channel stream, the multiplexer is not changing channels, so the input impedance should be the same as whether you are streaming that single channel at 50k or not acquiring data at all. If you look at the block diagram in Figure 2-2, you can see that there is an in-amp in front of the ADCs. That in-amp buffers the ADCs, so if the multiplexers in front of the in-amp are not changing the input impedance is steady state:

http://labjack.com/s...6/users-guide/2

So ... I can't quite explain why you still see the effect with a single-channel stream. We will try to replicate this by streaming a +/-8V square wave on AIN0. Sounds like your square wave is at 5 kHz? We will generate the square wave from a function generator and add a series resistor to create high source impedance. We can try streaming AIN15 & AIN0, AIN0 & AIN15, and just AIN0.


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