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I have an older NI DAQ (PCI-6251) which comes equip with a bunch of ADC's, two DAC's and a hand full of DIO ports. I need to output a pulse of variable width, amplitude and DC offset which the DAQ can do but once you fall below about 5us pulses the rise and fall times become a considerable amount of the actual pulse. For example, I would like to idle the DAC at 5V and then have it pulse to 0.5V for 1us and return to 5V without having the pulse be too distorted.

Is there a simple circuit that could sharpen the edges of the pulse while still allowing me to keep a DC offset?

My first thought was to use a P-TYPE MOSFET in a pull-up configuration with the gate tied to a Digital Output signal but then I wouldn't be able to use just any DC offset or pulse amplitude. The rise/fall times of a Schmitt trigger would be perfect but I'm not sure how I could use the digital output in an analog signal.

Let me know if there is more information I can provide. Any help would be very much appreciated!

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  • \$\begingroup\$ Do you have any waveforms? Are you using a coax cable? If so, what is \$Z_0\$ and how long is it? What is the load you are trying to drive? Are you using \$10\times\$ probes for your measurements? \$\endgroup\$ – Zulu Apr 7 '15 at 3:59
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How about a fast SPDT CMOS switch that flips between the two DAC outputs and is controlled by one of the digital pins? This eliminates any issues involved with the DAC settling time as the DACs will provide the high and low levels and the digital pin will control which level is selected.

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It sounds like you may have too much capacitive load on the output? Try using a shorter cable or a higher-impedance buffer amplifier.

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