maximum input frequency of the PCI-6014?

[St Augustine]

I am using the PCI-6014 configured for digital inputs. At low frequencies my quantized waveform is very smooth however wheni increase my input frequency, teh waveform becomes highly distorted. Im sure its not my A/D converter becomes i am sampling more that eight times the input frequency. Can you please help me!

[rgharrison]

Hi St Augustine,

This issue could occur if you are sampling on 2 or more channels. If this is the case, you are probably seeing cross-talk (channel ghosting) where voltages on previous channels are being seen on subsequent channels in your sample list. For example, if you have a sinewave on the first channel and 5V on the second channel, you might see remnants of each channel on the other, distorting your signal slightly. The best way to correct this is to increase the interchannel delay.

Data Acquisition: Troubleshooting Unexpected Voltages or Cross-talk in Analog Input Channels
http://digital.ni.com/public.nsf/websearch/B9BCDFD960C06B9186256A37007490CD?OpenDocument

If you are only sampling on one channel, you could check in Measurement & Automa
tion Explorer (MAX) to make sure that you are seeing the same phenomenon being exhibited by the waveform. In MAX, you can right-click your PCI-6014 device and select Test Panels. Then you can adjust the sampling rate and watch the waveform directly from the front end of the driver.

Another point is that it could still be that you are seeing granularity because of the high frequency of your signal relative to your sampling rate. A sampling rate of 8x higher will still look much more "squarish" then a sampling rate 100x faster.

Anyway, hope that helps. Have a good day.

Ron

[St Augustine]

Ron that was extemely useful information, however for this application i am using digital inputs and reconstructing the waveform in labview. i guess this would make the problem i am having more clear. can i vary the sampling rate for digital inputs in measurements and explorer?

[waldemar.hersacher]

Dear St Augustine,

a 6014 has not the abability to do HW timed digital input sampling. The timing is always a SW timing and therefore its accuracy decreses with increasing frequncy of the sampling rate. If you need exact timing of the digital sampling you must use a 653x card.

You cannot set the sampling rate for digital inputs for a 6014 in MAX.

Waldemar

[Anonymous]

I see your situation. I am assuming you have an A/D converter and the
digital lines are connected to the digital lines of the 6014. Wald is
correct in that the 6014 samples the digital lines through software.
The problems with this is that the sampling is not predictable in
period (sampling interval). The timing can usually be off by a couple
ms (due to the OS, the application etc.). When you are sampling the
digital lines every 1s, then being off by a couple of ms is such a low
percentage of our sampling rate that it will seem like stable
acquisition. However, as you speed up the sampling of your digital
lines to 10ms for example, a couple ms of error due to the OS or the
application software is like 20% of the sampling rate and this could

greatly affect if your digital lines are acquiring the last value or
the next value. Therefore, your digital input will give you incorrect
values in a greater percentage of the samples. This will lead to a
very choppy waveform.

I believe that the above is what is actually occuring in this case.
The only way around this is to use the onboard A/D of the 6014 or to
use a 653x board as Wald has suggested or using a S Series board that
supports "timed" or correlated digital input.

Anyway, hope that helps. Have a good day.

Ron

[Anonymous]

thank alot guys, i think that the support at your company is
excellent. You all have saved me alot of time.thanks again
vijay