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Having problem in NI 5733 readings

Hi,

I have a problem with ni5733 (Digitizer Adapter Module for NI FlexRIO). I am generating a 100KHz sinewave signal using a signal generator and trying to read that using ni5733 (mounted on my  FPGA NI 7965 flexRIO)  .
To make sure that my readings are OK , I have also connected the output of signal generator to a digital oscilloscope using a BNC.

 

Here is the problem. When the output of signal generator is only connected to oscilloscope , it shows correct amplitude of the signal ( for example 1.4 Vpp) .  But when the signal generator output is  connected to both oscilloscope and the NI5733, the amplitude reading shown in the oscilloscope drops Smiley Surprised!!! ( for example from 1.4Vpp to 1Vpp). Also I found that this is some how frequency dependent  and if I lower the frequency to 10kHz  it drops less ( for  example from 1.4Vpp to 1.3Vpp).

I was wondering why this happens. Is this a problem with my NI 5733?

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Hi alireza,

 

Depending on the impedance of your devices, you'll get a voltage divider effect to some extent when splitting your output signal.  The dependence on frequency output may be a result of some parasitic capacitance or similar non-linear characteristics of your connections/setup.

 

For example, the 5733 has a 50 Ohm impedance.  What kind of probes are you using?  How are you splitting the signal?  Is your oscilloscope set to high impedance?

 

Regards,

Regards,
Chris Elliott
x36772
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