Digitizing and impedance matching of inputs to signal source

[WilliamdorHwSw]

I'm trying to measure the voltage and time difference between the two inputs to a USB-5132 Digitizer. The switching interval that I'm trying to capture is ~ 150uS and I plan to acquire ~ 25K samples (at 50MS/s for 500uS, although that's an overkill for sampling rate). The digitizer has 1M ohm inputs and I wanted to use the SMA 100 50ohm cables connecting to source signals whose impedance in greater than several K ohms. My RF knowledge is weak and I am concerned about impedance matching between digitizer inputs, cabling and source impedance. What sort of considerations to I need to make to avoid degrading the signal? The switching signal is pulse-like and so it's rich in harmonics and on a scope sampling 500K samples/sec it is accurately represented. What sort of considerations do I need to make regarding impedances?

[Chris_W. · Accepted solution]

Hi William,

There will be some issues if you try to use a source that has an impedence of several kohms. You will likely get signal reflections. What is the source of your signal that has the impedence of multiple Kohms?  I would take a look through the following Developer Zone article which describes some considerations when it comes to impedence matching. I would say specifically check the last section labeled Resistive Matching. This should give you a better idea of what you need to be concerned about for your application. 

http://zone.ni.com/devzone/cda/tut/p/id/3475#toc4

Here are also a couple more developer zone articles that you might be interested in with regards to your situation.

http://zone.ni.com/devzone/cda/tut/p/id/5779

http://zone.ni.com/devzone/cda/tut/p/id/2892

Chris W

[WilliamdorHwSw]

Chris,

Thanks for the prompt reply. I'll check out the information at the links you included. In answer to your question re: the impedance of the signals being sampled, both a switching transitions of DC signals. One channel is at the anode of a diode and the other channel is at the cathode of a Schottky diode, so their impedance is high.

I'll reply with an update based on my findings.

Thanks,

Bill