Using USB-6259 BNC vs SCXI-1000 and SCXI-1600

[jeongho20000]

Hi,

I'm trying to collect electrical stimulation data through SCXI-1000 and 1600, but the data behaves in a strange manner.

The stimulation trains which are generated from the electrical stimulator should be perfectly square. When I collected the data by USB-6259 BNC, the data looked OK.

If I collect them by SCXI-1000 and SCXI-1600, however, the data look strange (having some negative values). Please see the graphs below.

I supposed that the excitation from SCXI-1000 would have affected the data, so I turned off the excitation from "Measurement & Automation Explore".

However, the problem has not been solved.

Any suggestions or thoughts?

By the way, I'm using LabView 2009.

Thanks,

Jay

[RT4CY]

Hello Jay,

The first thing I would do is to ensure that the hardware is working properly.  Measure a known voltage source (from a battery or power supply) with both the SCXI-1600 and USB-6259 to ensure that you are getting the same readings.  I would suggest moving away from LabVIEW when troubleshooting hardware in order to eliminate the possibility of a software or programming problem.  Take your measurements from the Test Panels in Measurement & Automation Explorer (right-click on the device and choose Test Panels).  Try some test signals from a function generator or your USB-6259 analog output such as a sine wave at varying amplitudes and frequencies, and ensure that you are getting proper measurements. 

If you confirm that the hardware is working as expected, I would investigate the differences between the SCXI-1600 and USB-6259 specifications and ensure that the SCXI-1600 will meet the requirements of your test system.  For example, the USB-6259 has a much better settling time than the SCXI-1600 (1 µs versus 5 µs for ±4 LSB).  This may be significant considering the source impedance of your "electrical simulator".  For a good tutorial on settling time, see the following link.  Additionally, I am not sure that either of these devices is appropriate for your application.  You seem to be worried about the shape characteristics of a signal with very fast transient response.  Note that the max sample rate of the USB-6259 is 1.25 MS/s.  The max sample rate of the SCXI-1600 is 200 kS/s.  If your application requires shape characterization of the signal, I would identify the highest frequency component of your signal.  Then multiply that number by 10, that should be the sampling rate you use on your measurement device.  That being said, you should not put a signal with frequency components greater than 20 kHz into the SCXI-1600 (to characterize the shape).  At the very least, you should follow Nyquist and avoid putting signal frequencies above half the sample rate.  We usually put an analog filter on the front end of the device with a cut-off frequency of half the max sample rate, and this may also explain the measurement error you are seeing at the transients.  Have you considered a high speed digitizer?

Since you seem to be getting the results you want from the USB-6259, do you have the option of using this device for your application?  SCXI was not really meant for high-speed applications and you might consider a different platform (such as PXI with a high speed digitizer or multifunction DAQ module) if this is what you require.  

[jeongho20000]

Hi, Rod.

Thanks for your suggestions.

I've already checked all the hardwares work fine and all the devices are proper for our experiment. Since we know that 6259 is better, we bought 6259 for our experiment, but our accelerometer needs 4mA of excitation which 6259 doesn't provide. So, we decided to use 1531 (which provides excitation) to collect data and degitize the signal with 1600 in order for us to use USB. Due to the reason, we can't use 6259 to collect  data from the accelerometer.

Do you see any possibility that the excitation originated from SCXI-1532 might have influence on the signals?

Best,

Jay 

[RT4CY]

Hi Jay,

My feeling is that the signal looks the way it does due to settle time and bandwidth limitations, not the excitation from the SCXI-1531.  But if you want to narrow down the issue to remove the excitation as being a possible cause of the error, go ahead and remove the SCXI-1531 module from the system as a troubleshooting step.  Let me know how that goes.