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Perry
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ADC Channel crosstalk in xPC

Asked by Perry
on 16 Jul 2014
Problem Description:
xPC and Matlab (2013b) Board - Measurement Computing PCI-DAS-1002
The data acquisition card, in single ended mode, is connected only as follows
Ch2 (Pin 5) = 5V from card (Pin 48) Ch 9 (Pin 6) = connected via a 25k resistor to the 5V (Pin 48)
Everything else is completely disconnected.
With this setup, the measurement from xPC software for Ch2 and 9 are similar (near 5V) although Ch 9 is a bit lower (by ~0.03V) and bounces around more than . DMM measurement shows that CH9 pin is slightly lower than Ch2 pin (by around 0.01V) when measured with respect to ground.
However, when another channel, e.g. Ch1 on Pin 4 is grounded, then Ch9 reading, as read from software will drop to 3.88V. Ch 2 reading remains near 5V. DMM measurement of the voltage of Ch9 pin and ground is still 4.98V ish and measurement of Ch2 pin is steady at 5V (or 4.99V). Ch1 is read correctly as 0V.
Conclusion: The channel with the 25kOhm output resistance is significantly affected by the other channels' voltages (e.g. Ch 1).
Note that Measurement Computing's own software (InstaCAL) does not have this problem so it seems to be a driver issue. Does Mathworks have a solution (updated driver to use)?
Thanks Perry

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