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Channel coupling analog streaming


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#1 brian6091

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Posted 14 August 2012 - 05:02 AM

I am using LJStreamUD (1.10) to stream two analog channels. One is a voltage reading (mV range) from a photodiode (connected to AN0 and GND), and the other is a voltage output (<5V)from another computer (AN2 and GND). When I set the scan rate greater than about 1250 Hz, there is contamination from AN2 onto AN0 (ie, I see the voltage steps of AN2 on AN0). This disappears when the scan rate is 1000 Hz. How do I get rid of this coupling?

#2 LabJack Support

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Posted 14 August 2012 - 12:02 PM

Sounds to me like the photodiode signal might be very weak. That is, it has substantial source impedance. When you scan slower you might be getting a little extra settling time that makes the reading good. For comparison, put a known strong signal (such as a jumper to GND) on AIN0 and see if you get good readings at all speeds. If you want to sample the photodiode faster, the likely solution is a buffer of some sort. What is the max and min voltage you want to handle from the photodiode?

#3 brian6091

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Posted 14 August 2012 - 12:29 PM

Sounds to me like the photodiode signal might be very weak. That is, it has substantial source impedance. When you scan slower you might be getting a little extra settling time that makes the reading good.

For comparison, put a known strong signal (such as a jumper to GND) on AIN0 and see if you get good readings at all speeds.

If you want to sample the photodiode faster, the likely solution is a buffer of some sort. What is the max and min voltage you want to handle from the photodiode?


The max voltage is around 50 millivolts, and the min voltage is maybe 0-5 millivolts.

#4 LabJack Support

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Posted 14 August 2012 - 01:19 PM

The LJTick-InAmp should work good for that. I would go with Voffset=0.4 and Gain=x11. You could just use it as a buffer with Gain=x1, but might as well take advantage of the available gain. Start by connecting a couple jumpers from IN+ to IN- to GND on the LJTIA, then note the offset voltage (should be near 0.4 volts) to subtract off your readings. Then remove the jumpers and connect +signal to IN+, -signal to IN-, and a jumper from IN- to GND. You could make your own buffer or amplifier using a simple op-amp, but you would have to be careful to select an op-amp that works so close to the negative rail (assuming you power the op-amp from GND and VS).

#5 brian6091

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Posted 15 August 2012 - 02:29 AM

The LJTick-InAmp should work good for that. I would go with Voffset=0.4 and Gain=x11. You could just use it as a buffer with Gain=x1, but might as well take advantage of the available gain.

Start by connecting a couple jumpers from IN+ to IN- to GND on the LJTIA, then note the offset voltage (should be near 0.4 volts) to subtract off your readings. Then remove the jumpers and connect +signal to IN+, -signal to IN-, and a jumper from IN- to GND.

You could make your own buffer or amplifier using a simple op-amp, but you would have to be careful to select an op-amp that works so close to the negative rail (assuming you power the op-amp from GND and VS).


Jumpering AN0 to GND produces a clean signal at all speeds, so it looks like LJTick-InAmp is what I need. While I wait for delivery, I can easily filter out the artifacts for now. Is it safe for the U6 to do so?

#6 LabJack Support

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Posted 15 August 2012 - 09:29 AM

Are you wondering if it is safe to stream even though the signals are not settling properly? Yes it is safe, as doing so will not cause damage.


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