is the SCC-CO20 output current truly constant regardless of variable load?

[dygarth]

I am currently trying to put together a hardware system that (among other things) will provide a train of current-controlled pulses [required amplitude range = 0 - 10 mA (but fixed at a constant level during each pulse); possible pulse duration range = 1 - 100+ ms] into an external load whose impedance might vary in magnitude during the duration of each pulse.  I came across the SCC-CO20 module, whose User Guide states (page 1) "...the output current is proportional to the voltage applied to the module by an analog output channel."  Unfortunately, I can't seem to find any additional info within that Guide that explicitly speaks to whether that current output is truly independent of external load variations.  Does anyone know this info (and/or where I can find it)?  Furthermore, if it really is independent, then would some also know where I could find specs detailing the frequency response and/or stability of the output to variable loads (that is, specs regarding the ability of the module to maintain a constant current level in the face of varying loads)?  Thanks.

[Justin_M]

Hi dygarth,

The SCC-CO20 module will keep the current constant independant of the load, assuming it can source enough current for the load.  The following (from the SCC-CO20 Manual) shows the maximum load you can use at each specific current output amount:

5 mA ................................................2,500 Ω
10 mA ..............................................1,250 Ω
15 mA ..............................................830 Ω
20 mA ..............................................625 Ω

We do not provide any specs detailing the frequency response for variable loads.  However, converting voltage to current is in essence just a first order circiut (voltage across a resistor), which in theory doesn't add in any delay.  Generating pulses in the millisecond range with the SCC-CO20 should not be a problem.

Thanks,

Justin M.

National Instruments