Issue with Continuous Scan Using Python in MCC 128

[Samuel Alpha Gian]

Hi everyone,

I'm currently facing an issue with my MCC 128 device when trying to read voltages in differential mode from a function generator with an amplitude of 4 Vpp. When using the DAQHats app, everything works as expected, and the voltage readings are accurate.

However, when I switch to using the continuous_scan.py script for continuous scanning in Python, the readings are incorrect.
.

Here are the specifics:

Mode Used: Differential

Function Generator Output: 4 Vpp

Expected Behavior: Voltage readings should match those shown in the DAQHats app.

Observed Issue: Voltage readings are inconsistent and incorrect when using the Python script.

Has anyone else encountered this issue? Any advice on how I might be able to fix this? It seems like a configuration or software-related problem.

[Samuel Alpha Gian]

Hello everyone,

I have an important update regarding the issue I shared previously about inconsistent voltage readings from my MCC 128 device when using the continuous_scan.py Python script. After more tests, I decided to connect four channels to four different voltage sources. Surprisingly, both the DAQHats application and the Python script are now providing accurate and consistent readings across all channels.

Here are the details:

Test Configuration: Connected four channels to separate voltage sources.

Results: Accurate readings from both the DAQHats app and the Python script across all channels.

Attached Screenshots: Showing the results from both the app and the Python script.

Could anyone help explain why there were discrepancies in the readings with fewer connected channels? It seems that the device or software behaves differently based on the number of connected channels. Any insights or similar experiences would be really helpful!

Thank you all for your continued support and looking forward to your responses!

[DAQman]

The MCC 128 uses a multiplex scheme to switch each channel to the A/D converter for the measurement. The multiplexer IC chip is nothing more than a bunch of analog switches firing in rapid succession. However, between each switch is some capacitance that allows for a stray charge to transfer to the next channel. The stray charge is not a problem when the input is connected to a low-impedance source, like a function generator. Open or unconnected channels have high impedance, so the charge hangs around and appears as stray voltage. In short, turn off unconnected channels or connect them to the ground pin. 

The MCC DAQ HAT Manager reads each channel using a_in_read(), whereas continuous_scan.py uses a_in_scan_start(), which results in faster channel switching and different amounts of stray voltage.