Interference between channels in the Analogue Input block of Simulink Desktop Real-Time

I'm experiencing a problem with channel interference when using the Analog Input block of Simulink Desktop Real-Time (SDRT). Although it initially appears to be a case of ‘crosstalk’, the problem also occurs when measuring DC signals from very low impedance sources compared to the input impedance of the data acquisition boards used (National Instruments).

Test description

- I set up several simple voltage dividers (two 10 kΩ resistors in series) powered by an external 5 V supply.

- The voltage at the center of the dividers was measured using a single Analog Input block configured to access multiple channels in vector form [1:n].

- When I short-circuited the resistor connected to GND on the channel (1), its voltage dropped to zero as expected, but I observed a change of around 30 mV in the voltage measured on the channel (2).

- This behavior occurs consistently between any channel (n) and the subsequent channel (n+1).

Additional Tests

- I repeated the test using independent Analog Input blocks for each channel, and, in this case, the problem did not occur for any of the channels, demonstrating that the behavior is related to the vector configuration of the block.

- However, according to the SDRT user guide and the recommendation of MathWorks experts in the past, using a single Analog Input block for multiple signals, using vector notation for channel selection. This approach is necessary to maintain real-time operation, as using independent blocks introduces significant delays, even at low sampling frequencies.

Additional Information

- The same tests were carried out on two different computes (one slower and one faster and robut), with different cards of acquisition (PCI-6221 and PCIe-6323) and in two versions of MATLAB (2021b and 2024a).

- I also randomly changed the order of the input channels to rule out possible problems related to the sequence of the channels.

- The behavior persisted in all the configurations tested.

Requesting Assistance

Are there any known limitations or solutions to this behavior when using the Analog Input block in vector configuration? Considering that SDRT recommends this approach for multiple channels, would a technical review or guidance be possible to solve or mitigate the problem?

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Answer 1

[Jan Houska]

Hi Fabrizio,

first, please check if your signals are correctly grounded. Are you using single-ended, NRSE, or differential input mode? If single-ended, using one of the other two modes may help.

Then, please try to change the resistors you are using to lower values - 1 kΩ or even 100 Ω if your power supply allows that. The analog inputs have some capacitance and it might be that this, in combination with the input resistance, is causing them to settle more slowly than expected.

If this does not solve the issue, please try to short-circuit channels 2 and 3 and read 1:3 instead of 1:2. If the reading of channel 3 is more precise than channel 2, this could be due to additional time given to the signal to settle. This would also suggest a workaround: with the exception of channel 1, always use two connected channels for each subsequent signal, ignore the value from the first and use the value from the second.

Then, using two Analog Input blocks is not prohibited. It is just much slower. This extra time may help the signals to settle though. So if the reduced performance is not a problem for your application, you may just stay with this solution.

Good Luck, Jan