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PXI4065

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Can you use a PXI4065 to measure a pulse??  15 volts for 10 millisec.

Thats my requirement..."The voltage output SHALL be > 15 volts for at least 10 millisecs.."  There was something I found in DAQmx that allows you to get an array of values using the DMM but I can figure out where you set the sampling rate.  Plus I don't know what parameters to use to determine sample rate??..for a pulse??

Thanks

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Hi Clint100,

 

The PXI-4065 is a DMM meant specifically for analog measurements. You would have no issue with the amplitude of the voltage, but you could not sample a digital pulsetrain reliably unless the frequency of the pulsetrain is significantly less than the sampling rate (probably a maximum frequency of 600 Hz in this case). You will be able to measure an individual pulse's DC value, but you could not get any a precise result for the timestamp of the pulse's occurrence. If you were sampling at 3 kS/s, you could conceivably get 30 samples(of "high") during a 10 ms period of a pulse. If you would like to know any specific information about the DAQmx example you were referring to, could you please include the name of that example so that I can reference it?

 

Best,

Jason M.
Applications Engineer
National Instruments
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Hey Jason,

Thanks.. the vi I was referring to was "NI-DMM Read Multi Point.vi" in the shipping example "Acg & Graph Multiple Samples.vi".  There is also another one I thought of trying but it may only be for the 407x series and not my 4065.."NI-DMM Waveform Demo.vi".

So I understand this better:

The 4065 samples at 3k samples per sec or 1 sample every 0.33 milli secs.  That's 30 samples in 10 milli sec...my requirement time.  You don't think 30 samples is enough??  What do you usually like to see??  100 samples?  What do you use for Nyquist in this case??  Can you assume the pulse is continuous so the freq would be 100 Hz??  Then 600 Hz should be plenty for a sample rate.  I have a PXI-6230 which I would just as soon use.  I think you get more freedom when it comes to reading/manipulation of data w this DAQ Card than the DMM but I have to convince the powers to be.  Actually had initially written code to use the 6230 NEVER considering the DMM.  The only reason they want to use the DMM is the 15 volts.

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Hello Clint100, 

 

30 seconds should be enough, but keep in mind that you are using the DMM for things it was not designed to do. Nyquist determines the minimum sampling frequency to avoid aliasing. So, to retrieve all of the frequency information, you need to sample at twice your input signal frequency. To maintain the shape of your input signal, you need to sample about 10x faster than the input signal. 

 

Your PXI 6230 is more capable of digital input. However, the digital input range is only 0-5.25V. So, you could not use this card with your 15V range unless you attenuated the signal first. 

 

With DMMs, you cannot explicitly set the sampling rate. The sampling rate is dependent on the measurement cycle, which you can read more about here. The maximum sampling rate of the 4065 is 250 Ks/s, however, that will give you the worst resolution. 

Maggie
National Instruments
Applications Engineer
ni.com/support
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I'm a little confused why you refered to the digital input of the PXI6230.  Why couldn't I just use the analog in??  Then scale it down ( voltage divide)  so that its below the input max of 10 vdc??  I was going to treat the 4065 as if it were a "handheld" DMM to take ( if it were possible) the pulses measurement.  After all doesn't the pxi-4065 digitize the signal so if I used the array of "ni dmm read multi point.vi" I could get to each value (point) in the array if I knew the sample rate??  Thats why I asked.  I don't know how to treat a pulse as far as freq goes.  I want to sample enough so that I don't miss a possible peak voltage that may be out of my spec for that test.  The spec sheet says the 4065 samples at 3Ks/sec.  How do I get it to sample at 250 Ks/sec??  Why is that the worst resolution??  Isn't that a sample every 4 microseconds???

 I guess I should just use the PXI-6230 AI to measure the pulse after I voltage divide it.  Just wanted an explanation to provide to someone who didn't understand why we couldn't use the DMM.

Thanks..

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Solution
Accepted by topic author clint100

Hi clint100,

 

Maggie was only recommending that you use a digital input instead of an analog input if you are able to attenuate your signal within the digital input bounds. The reason we recommend this is because you will get a steady signal at "high" and a steady signal at "low" with accurate hysteresis built into the data acquisition. This way, you will not see any noise on the signal and it will appear to be a perfect square wave. You are absolutely able to measure a DC signal with an analog input, but it would be subject to noise and you would not see a perfect "rising edge" as you would with an analog signal. Your DMM will sample the signal as an analog signal, and you will see a decent square wave (with some noise and a slightly sloping rising and falling edge since it samples slowly). You can use the analog input where you will see more noise, but a less sloping rising and falling edge since it can sample much faster. You will have to do a voltage division, however. With a voltage division and a digital input signal, you will get a clean square pulse. It is up to you what you would like to do.

 

Best,

Jason M.
Applications Engineer
National Instruments
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Thanks Jason...never had to worry about the noise issue before.  Good point.

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