sample large voltage ranges on 6071E

[marc lawrence]

Im using the 6071E to record voltages on 6 channels which are then fed into a FFT operation for frequency analysis. (Like Frequency Response Function).

I always need to sample CH1 - which is my source signal - and I then iterate through a list of CH2->6; CH7-11; etc. Therefore my sample list looks like
CH1, CH2, CH3, CH4, CH5, Ch6
CH1, CH7, CH8, CH9, CH10, CH11,
.....

CH1, CH49, CH50, CH51, CH52, CH53

Under some sets of circumstances; CH1 will be measuring in the +-10V range; and the remaining channels in +-1V or +-1mV range. These circumstances depend upon our system under test.

I understand that with the 6071E these channels are sampled asynchronously; and when the inputs have a high impedance there can be a problem with the ADC not being cleared (i.e. some the second sampled channel sees the wrong voltage). Currently some of the channels (e.g. 2,3) will be corrupted.

What is the best way to ensure that the channels sample the correct voltages and see no influence from the different ranges / charge? Would it make sense to record CH1 on a seperate DAQ card; and in which case how can I ensure synchronisation?

Thanks,

[AlexxxR]

Hi,

the Problem is the preamplifier. With high sampling rates, it could be possible that he is to slow to switch between the different gains. If you try this example:

http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=F4BF0F90E85E21B6E0340003BA7CCD71&p_...

do you see the same behaviour? If yes, best would really be to use a second device. Synchronism is not that diffcult. With PCI-devices you can use a small cable, called RTSI. With this cable you can exchange the sample clock easily. Best would be to call National Instruments for product consulting.

Best regards

Alexander Rudolph

NI Switzerland  

[marc lawrence]

Hi Alex

Thanks - this is basically the same conclusion as I came to. We have a spare 6023E card which I can use for the sampling of CH1 - and I hope the RTSI cable option works. I will need to test to see if I can then do sample rates from the boards fast enough - but it should be ok. I can also try to drop down the voltage range of CH1 so they are always in the same range.

Marc