I have a DAQ task with 80 channels. A few of these channels are high-impedance voltage divider sources (~1MOhm or so). When sampling, I see significant ghosting to the previous channel in the scan. If I hook up a scope to the test point, I can see a blip and decay that coincides with the overall scan clock.
We have tried to add a 0.1uF cap across the lower leg of the voltage divider:
Source------1M--------<Test Point>--------DAQ
|
------
| |
1M 0.1uF
| |
------
|
GND
(Note: I cannot change the 1M resistors in this voltage divider)
This helps, but doesn't alleviate the issue.
I have also tried slowing down the rate (just to see what I can do, I still need faster sampling):
Originally: 1KHz Scan Clock, ~90909 Conversion Clock
Test: 5Hz Scan Clock, 400Hz Conversion
This also helps, but it's not entirely stable yet.
I am not 100% familiar with the timing; does the MUX switch immediately after the previous measurement? What is my dwell time for the selected channel? Can I set that on a per-channel basis, or is it determined solely by the Conversion Clock?
I am considering adding dummy samples on these channels in a group, to increase the dwell. But if it isn't stable at 2500us of dwell (400Hz conversion clock), then would this help?
