external clock

[yyan3]

I have a DAQPad-6070E and I am using MATLAB7.0 as software. we use it to digitize a 500khz bandwidth analog signal. at first we use the internal clock and everything works perfect, I can get data as long as 10 sencond or even longer (I didn't try). but when we switch to external clock, the device can only work for 0.02616 second, the MATLAB give us a "device buffer overflow" error. everything is the same except the clock source. we tried several machines and the results are exactly the same. the maximum sample rate for DAQPad-6070E is 1.25M and our external clock is rated at 1.25M. is it possible that our clock is slightly greater than 1.25M and cause this problem? or is there any other causes?

[Tom_W_[DE]]

Hi yyan3-

It is possible that your external clock is slightly faster than the maximum rate for your card. I would suggest that you decrease this slightly below the 1.25MS/s threshold.

This is probably not the cause of your problem though because it is possible to see the buffer overflow error even at very low frequencies. This error usually results from samples being placed into the buffer by the card faster than you are reading them out in software. Two possible solutions are to increase the buffer size and to increase the number of samples to read with each read operation you perform.

You will need to verify that you are reading samples from the buffer at a rate between the actual rate they are being placed in the buffer (to avoid a buffer underrun error) and the rate at which the acquisition speed builds up more significantly more samples than you are reading out(to avoid a buffer overrun error). This will require some testing based on system performance, but increasing the samples to read is usually sufficient in short-term acquisitions to avoid the error you're seeing.

Thanks-