Quick connection to NI 9215 BNC

[xl600]

I need to monitor a differential +30 to -10V signal using a 9215 BNC.  What would the recommended approach be (quick and dirty... )?  A simple potentiometer divider?

the specs for the 9215 BNC say there's 200k input impedance , but it caveats that by saying thus is between any two sides- terminals.  That doesn't really make much sense if I need to know what the signal load is on the signal under test.  Is it 1G or 200k or something else the specs don't really make clear?

Thanks,

XL600

[Henrik_Volkers]

As far as I interpret the spec you have 200k between any two (AI-) .... and I would assume ~1GOhm, 25pf for the (AI+).

I would go with a 4:1 divider ...  depending on the output impedance of your source, grap 4 equal resistors to build your voltage divider..... 

Do a quick check with a known voltage source 😉

With 100k resistors and no capacitive compensation you get about 1% error at 12kHz

( source impedance of 50 Ohm, shown is 1V AC simulation -> ~250mV)

If you add 22pF caps in parallel to R1..R3 it should work better but is not Q&D 😄

[xl600]

Thanks for the detailed input!  I had kinda guessed the A+ impedance was 1G (Even though it isn't really specified).  There was no attenuation when using a 10x o-scope probe (9Meg) so the input impedance must be really large in comparison.

It sounds like the simple divider is the easiest way to go with little error (I'm sampling at 10kHz and looking for transients that are around 1kHz and 1.5% error shouldn't make the distortions significant.

[Henrik_Volkers]

Just add a 1Meg between AI+ and AI-  and use your 10x probe... ..