Setting up an internal clock to take data acquisition at one minute intervals 
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 Setting up an internal clock to take data acquisition at one minute intervals

I ran the modified version of the code you sent. The loop does what I
needed as far as taking the measurements every minute but it seems to
corrupt the data received. If I hook up a signal to one channel, the
data looks fine. If I hook up a signal of different magnitude to
another channel, it follows the first signal but with a slight delay
in phase angle and a slight reduction in magnitude. This occurs
regardless to the magnitude or phase of the signals. I even removed
the second signal but left the VI set up so it would record the data
from both channels and even with the signal removed, I am still
recording a waveform. I will repost this question in the event no one
sees it here.

Mon, 06 Sep 2004 10:58:09 GMT  
 Setting up an internal clock to take data acquisition at one minute intervals
A couple things: First, unless you have a sample-and-hold board in
front of your analog inputs there will be phase delay between the
channels. The channels are being sampled very rapidly, but not
simultaneously. Second, if the amplitude of one channel appears to be
effecting the measured amplitude of a channel on either side of it (in
the scan order)there is probibly a settling time problem. NI's cards
internally only have a single A/D converter which is connected to each
of the input channels through a multiplexer. Simple enough, but things
start getting complicated when are doing on-board gain adjustments
too. Now you have to adjust switch the multiplexer, allow it to settle
and change the gain of the input amplifier. In addition, the amplifier
itself has what is called a slew time, or how fast its output can
change in response to a step change in the input. Put all together, if
there is not adequate interchannel delay in a scan the input will
still be showing the effects of a previous channel when you read the
next one.

If this is the problem, there are two fixes: First don't put signals
on dramatically different levels on the same card. This reduces the
amount of time the DAQ card needs to adjust to each input--at the
price of an additional DAQ card. Second, increase the amount of
interchannel delay for the scan to give the card the time it needs to
settle--of course this added time will also increase the apparent
phase shift between channels so you might need to add a
sample-and-hold card to get simultaneous sampling.

Personally, I would try the increased interchannel delay first to see
if what you are fighting is a settling time problem as this is only a
software change. If increasing the delay fixes the amplitude problem,
you then can analyze the hardware solutions to determine which is the

Of course, if you really need absolutely simultaneous sampling, the
decision is already made for you. Call NI and get an appropriate
sample-and-hold card.

Hope this helps, as in a lot of things in electronics, setting up an
acquisition system is a balancing act where you have to learn what
compromises you can, might, should or need to make technically and


Mon, 06 Sep 2004 21:57:03 GMT  
 Setting up an internal clock to take data acquisition at one minute intervals
Mike, I understand what you saying except I am able to run the
original program before the loop was added and do not have any
disturbance from other signals as well as another version that is
sampling continuosly at the same rate. I think there is something
going on since the automatic one minute sampling took place that is
causing the issue. That is the part I need help with.

Mon, 06 Sep 2004 23:46:06 GMT  
 Setting up an internal clock to take data acquisition at one minute intervals
I just looked at your original code again and now I am confused too...
Perhaps it's "call NI time". At least now you have a couple small code
samples you can send them--which is what they are always wanting to
see anyway.


PS: If you get an answer from them let me know what they said so I can
resolve my confusion as well...

Tue, 07 Sep 2004 00:12:03 GMT  
 [ 4 post ] 

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