multiplexer delay

[orsogna]

Hello,

I have a PCI 6221, I am acquiring 3 analog signals at a sample rate of 70 k. Which is the delay introduced by the multiplexer?I don't have any problem with the measurements but I don't know how much the signal are shifted. Thank u in advance.

Antonio

[Logan_081]

Hello Orsogna,

The parameter you're looking for is known as Interchannel Delay. Here below you can refr to multiple Knowledgebese documents in order to calculate you multiplexer delay during switching from one channel to another:

Data Acquisition Sampling Terminology

How is the Convert (Channel) Clock Rate Determined in NI-DAQmx and Traditional NI-DAQ?

Minimum and Maximum Values for the Interchannel Delay Setting

I hope this could help. If not, don't hesitate to write me back your questions.

Best regards

[orsogna]

hy,

thank u for answering. In one of the post there's:

Maximum Interchannel Delay Setting = 1/(scan clock rate) divided by the number of channels in your scan list. For example, if you scan all 16 channels at a scan rate of 10,000 scans per second, your Maximum Interchannel Delay setting would be (1/10,000)/16 = 6.25E-6 seconds.

so in my case I use an encoder to generate an external sample clock for sampling 3 analog signals and the max freq I have is 17 Khz. I will have

(1/17000)/3=1,96*10^-5

it means that as my shaft runs at 16.6 rev/s I will have a delay of 1,96*10^-5 * 16,6*360=0,11° ? that's right

thank u again

[John_P1]

Hi Antonio,

If you are using an external sample clock, the convert clock is still being generated internally based off of the specified sample rate (unless you tell the driver to do otherwise).

If you specify a sample rate of 70k, and your task contains 3 channels, by default the convert clock rate will be 210k regardless of the actual rate of your external clock.  The inverse of this is about 4.76 us, which would be your interchannel delay.  You can also directly set the convert clock rate or use an external convert clock if desired by setting a DAQmx Timing property node.  \

For slower rates, DAQmx actually uses a default minimum convert clock rate.  The minimum convert clock rate is computed by adding 10 us to the minimum convert clock period (4 us on the 6221, or 1/250kHz).  On your 6221 the minimum convert clock rate is about 71.43 kHz (1/14 us).  You can specify a convert clock rate lower than this, but DAQmx will not set the rate lower than this by default regardless of how slow the sample rate.

Best Regards,