I've had a nice development session with NISCOPE C/C++ API, with Visual Studio 2012, last weeks.

My NI device is a scope NI-5132, nice top 5Ms/s, 8bits, etc.

http://sine.ni.com/nips/cds/view/p/lang/fr/nid/203719

My software can now launch acquisitions, for A or A&B, I feed waveforms from a synthesizer output (50ohm, 0-1500kHz)

to generate signals.

For now, I plots signals, and FFT diagrams.

All works nicely, thanks for the API !

The issue happens when I switch impedance (Configure Chan Characteristics) : 

• with 1Mohm : things works

• with 50ohm : my application crashes.

Why switching ? 

Cause my aim signal generator output is 50ohm impedance,

before switching, I ve used :

• 1 Mohm NI5132 impedance

• on its input : a T BNC with a 50ohm caps to align impedance with my generator, and protect it from mirrored signal by the 1Mohm impedance.

Please help :

Why 50ohms make my app crashes ?

Do I need to re-calibrate ?

Much thanks

NI DOC :

Configure Chan Characteristics allows configuration of the less
common vertical parameters. The input impedance parameter allows you
to switch between 50 Ω and 1 MΩ input impedance, depending on your
digitizer. See Appendix B, Features Supported by Device, to find out the
input impedance your digitizer supports. The allowed vertical ranges may
be fewer for low input impedance settings, since the amount of power
dissipated through the 50Ωresistor quickly increases. Generally, you want
to match the input impedance of your digitizer and your source, so if you
measure a signal from a 50Ωoutput function generator, you should set the
digitizer for 50 Ω input. Impedance matching becomes much more
important with higher frequency signals to avoid reflections of the signal
that may distort your measurements. Alternatively, if you probe a circuit,
it is best to use the high impedance setting on the digitizer to avoid
changing the characteristics of the circuit.
To protect the 50Ω resistor, a thermal sensing circuit will open the input if
the power dissipation is too high. If this occurs, a warning will be returned
from all Read, Fetch, and Status functions. The circuit will try to reset itself
when Read, Fetch, or Status functions are called. Some digitizers also
support the detection of an ADC overload. This works the same as the 50 Ω
overload detection circuit described previously