6733 AO slow rise time

[Steve Ray]

I am using a PXI 6733 AO board as output on a closed-loop PID control. There appears to be excessive lag on the output when compared to the error signal, and as an experiment, I gave an unbuffered 0-10V step output command to the card. I measured the rise time for 0-10V at the output (no load, directly connected to SCXI 1121 for acquisition) at approximately 200mS. This is nowhere near the rated 2.8uS...am I doing something wrong?

[Sean_C.]

Hello Steve,

How exactly are you measuring the rise time?  You would need to be acquiring the voltage channel at at least 350 kS/s to register a rise time of 2.8 us.  How fast are you acquiring from the 1121?

Regards,
Sean C.

[Steve Ray]

Sean,

I'm only acquiring at 1kHz, but I'm not really interested in the absolute value of the time, just need it faster than it is. My application is a real-time control loop operating at 500Hz, so 1k should be fast enough to show me the step response. In continuing my investigation, I have found that the 200ms time is constant regardless of the magnitude of step change (-10 to 10, 0-10 or even 0-1V). Makes me think that something somewhere is configured to generate a ramp, even though my code has set the buffer to zero. I also get the same rise time when I send a command from the test panel in MAX.

Steve

[Sean_C.]

Hi Steve,

Can you confirm that the rise time is 200 ms with an oscilloscope?

When acquiring at 1 kHz, does your input buffer show multiple points (around 200) transitioning from 0 to 10 Volts?

Regards,
Sean C.