NI USB-6356 Analog Sawtooth Output Overshoot

[LauSchw]

Hi all,

I'm using the NI-DAQmx USB-6356 via LabVIEW (2016) to continuously source an analog sawtooth output on Analog Output 0, frequency is 20kHz and amplitdue 6.8V. I'm using the onboard buffer to enable higher output sample rates (for the following examples 1MS/s is used). The signal is used for a modulation, the optimal receiver signal should then be a sine / cosine. Due to overshooting (i think thats why...) i receive a peak in the modulation signal at the switching point of the sawtooth:

Sawtooth generation with DAQmx & Receiver Signal

At first i made sure there is no cable reflexion and the used screw pinout doesn't cause any problems. The problem remains even if i connect only my receiver signal to the scope and the sawtooth is connected straight to the modulator:

Receiver signal with sawtooth connected directly to the modulator

I created the exact same sawtooth signal with a waveform generator and used the same cable connection (except the cable to the screw pinout) and the peak doesn't appear:

Blue signal by WFG, pink signal by DAQmx, receiver signal is here modulated by the WFGBlue signal by WFG, pink signal by DAQmx, receiver signal is here modulated by the DAQmx

The modulator impedance is a small capacity, the system was used in the same way already half a year ago without problems. Only things that changed is the LabVIEW program and new versions of DAQmx drivers. Is there any further options to the analog output except the terminal configuration one can use to supress the overshooting? Or is there other reasons to the peak in the modulation signal i'm not seeing?

I've tested higher min & max values for the analog output limit, output sample rates, output amplitudes and the signal frequency but the peak always appears.

I'm thankful for any suggestions and help!

greetings
Lauri

[TSE_Expert]

Hi Lauri,

it definitely looks like a dU/dt or dI/dt crosstalk in your system. When switching, the slope of the signal is very high in absolute terms,

enabling inductive coupling between the sawtooth signal and the sine signal. The bigger te slope, the bigger the induced parasitic voltage will be based on Faraday's law. Try to increase the distance between the two cables/wires or any other corresponding parts and try to decrease the signal slope, just to identify the source of the problem, for example by replacing the sawtooth with an (asymmetric) triangle wave.

Best Regards,

NI TSE

[LauSchw]

Hi,

tanks for your reply. It definitively looks like a crosstalk, but as i mentioned i've tried different cable connections to identify the source of it. Today i'll try some more things (more different cable & pins) but so far it looks like the crosstalk may origin from the NI-DAQmx screw pinout itself.

I'll post some updates here today or tomorrow. Thank you so far.

Greetings,
Lauri

[LauSchw]

Ok, so i've just connected another (better isolated cable) to the screw terminal and tested it with both analog outputs on the device. Also i've tried using the AO 0 signal terminal (pin 15) together with the AO GND on pin 32 (& vice versa with AO 1) to create some phsical space (~1cm) between the unisolated cable ending of signal and ground path screwed into the pinout.

The described problem remains the same. The lab next door is using a similiar (just faster) NI-DAQmx card in the same way & system to provide a 250kHz sawtooth signal, so even a steeper slope without any issues.

Is there known crosstalk on the NI-DAQmx with high frequencies/ high dU/dt for slower models?
My card is:
NI USB-6356; 8 AI, 1.25 MS/s/ch, 2AO, 24 DIO, 4CTR, 64 MS Onboard Memory

S/N: 01DC7075

Greetings,
Lauri

[TSE_Expert]

Hi Lauri,

try to make sure, that using the very same (so identical) signal sources as your colleague next door results also in error with your card.

Alternatively, try to use exactly your signal source on the neighborous DAQmx card.

If you proved that the NI USB-6356 card causes the crosstalk, we can initiate an RMA process either to repair or to replace your device.

Best Regards,

NI TSE