Rif.: max frequency control of NI-9476

[mikisi]

Hi to everyone,

i would like use NI-9476 to generate PWM signals (max 100 Hz). I would command 10 devices, so use 10 channels. Can I use this module? What are the maximum frequency that I can use ? I suppose 1/500us = 2 kHz, but I do not understand if this value is for each channel or for all 32 channels (therefore, 2000/32 = 62.5 Hz/ch ... in this case this module is not good for me... i need 100 Hz).
Thank you

P.S: I'm sorry for my terrible english.. I will improve it (i hope)

[alamanna]

Hi Michele,
the NI-9476 module is good for your purposes: the update rate is not divided among all 32 channels, but only among the 10 that you use, so you can easily meet the 100 Hz requirement.
You can find a lot of information about C Series module specifications on this page.
Hope this helps.
Regards,
Alessia
P.S .: Your English is not so bad at all, believe me! 🙂

[Kevin_Price]

That device can generate digital samples at 2 kHz.  Each sample can include anywhere from 1 to 32 digital lines, and the sample rate doesn't depend on the quantity.

However, if you want to generate PWM, you need differentiation between the on and off times.  100 Hz PWM with a 2 kHz sample rate means you have 20 time intervals for defining a duty cycle.  This means your PWM duty cycle will be quantized at multiples of 5%.

Also keep in mind that there will be some latency between the time you write new digital samples to the task buffer and when they actually get generated as real-world signals.  The PWM changes won't show up instantly.  The data has to first move through the task buffer and then through the device's internal FIFO before it becomes an output signal.

-Kevin P

[jozisoftware]

No, it only defines the gap between

1) 0% and min pwm

2) max pwm and 100% pwm.

500 us is only the switching time. When you initiate that switch is not limited to 20 time intervals. It limits a short OFF/ON/OFF peak  or  ON/OFF/ON peak.

Some valves etc you can set to react on 10%-90% (=0 - 100% switch) and have a dead zone below 10% and above 90%. Tried it out myself.

[jozisoftware]

EDIT: 5 - 95% is quite ok to use as the PWM area. The update rate of 500us contains rise and drop time both.

[jozisoftware]

It all depends which frequencies you want. If you check https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YIUWCA4&l=nl-NL you just pick the one you need.

Personally if I am at the edge of the specifications I grab a scope and check if it matches my needs. Because specs are often a soft limit. The truth is a bit different mostly.

So get a scope and benchmark 😉