Different result when measuring signal by 6133 & 6250

[Ryan_HK]

Hi there,

I am using PXI-6133 and PXI-6251 to measure a 1.3kH FSK modem signal with same SCB and cable, but the outcome is different.

The signal on PXI-6250 is shifted upward by a 50Hz signal, could any one explain a little how is this happening? Or is there any anything inside 6251 that could probably cause this?

Thanks,

Ryan

[Raajit_Lall]

Ryan,

The main difference between the 6250 and the 6133 is that the 6133 has simultaneous sampling.  This means that every analog input channel on the 6133 has a dedicated ADC to it.  On the other hand, the 6250 multiplexes through all the channels on the analog input and has only one ADC to which all the channels are fed.  What this might cause is some kind of ghosting, because the amplifier might not get enough time to settle into its measurements and readings from adjacent channels might leak onto the current channel.

How many channels are you using on the 6250?  It might help to ground adjacent channels to eliminate ghosting.  Here is a link that has other options to eliminate ghosting.

How Do I Eliminate Ghosting From My Measurements?

Another question is, how have you configured all of these readings?  Are you using RSE (Reference Single Ended) or a Differential configuration.  A differential configuration will get rid of common mode voltage and might be a better option.

Here is a tutorial that has a section that talks about the different methods of connecting your signals.

Field Wiring and Noise Considerations for Analog Signals

Hope some of this helps.  Please post back if you have any further questions regarding this.

Best Regards,