measured buffered period(DAQ-STC)

[yehya]

Hello,
I ve problem using Measured Buffered Period (DAQ-STC)for multiple period measurements.
by providing external switching device and encoder connected to the Gate (PFI9) and Source (PFI8),through Signal conditioning card 12/5VDC.
some times i get two consecutive values read in Data
Buffered Array with noumber read indicator of value"2".
This happened between rising and next falling edge
of one gate signal in random way.
i couldn't fined specific reason for this.
could it be noise? taking my measurements in a lab,
only encoder cable(without switching devise)is shilded.
the t(PLH) of the Gate signal is about 15 n sec.
and source signal doesn't exceed 10 KHz.
is there something related to Read Buffered.vi?
my Questions
are, in which conditions more then one
count could be returned in the data array? and
what are the effects of (counts to read) and (time
limit) controlleres?
thanks for help.
Yehya SAlman

[rgharrison]

Hi yehya,

It is quite likely that what you are observing is noise. The DAQ-STC chip is capable of detecting signals as narrow as 10ns. This means that you could potentially have noise which is causing a misreading on the line. Unfortunately, the DAQ-STC hardware doesn't have immunity to noise of this type. It is up to the user to shield and minimize the noise to the counter inputs.

The PCI-6601 boards on the other hand of built-in filtering which allows the programmer to specify the minimum pulse width that will be applied to the counter inputs. This way, it minimizes the effects of noise.

The following knowledge base discusses how to create this type of filter using the DAQ-STC but it requires a counter to be used as a filter for only one sign
al. Not sure how useful that will be in your case. Typically, it will help to twist a ground signal around your counter signal. This acts as shield to minimize the effects of noise.

Anyway, hope that helps. Have a good day.

Ron

[citizenDAK]

(which KB article?)

[rgharrison]

Sorry... forgot to link.

How Do I Remove Glitches or Add a Debounce Filter to My Digital Signal?
http://digital.ni.com/public.nsf/websearch/09A669242FC5645286256D890081EC12?OpenDocument

Ron

[yehya]

Hi Ron,

Thanks for your reply and advice.
The problem was due to noise coming from conditioning card.
100Pf capacitor was picking up external noise and transfer it to the gate terminal.

Reading now is stable after removing this capacitor.
Thanks, and best regards

Yehya Salman

[Anonymous]

Good catch Yehya, that would do it. Good Luck.

Ron