I'm attempting to build a custom pulse generator that I can trigger with a digital I/O pulse. So far I've used a TI SN74LS123N Retriggerable One-Shot Multivibrator in combination with a 22 pF cap and 10 k + 100k potentiometer to create nice looking square wave pulses from 200 ns to 2000 ns.
The voltage levels of the output pulses range from 110 mV (low) to 3.4 V (high), which is congruent with the datasheet. What I need though is an adjustable voltage range from 0 V to anywhere from 2.5 or above, the critical part of the range being the in the 10s of millivolts to about 1.5 V. The adjustment should come from a potentionmeter and the circuit should drive a 2 k to 100 k Ohm load.
What is the best next stage to complete my pulse generator?
Here are my ideas/attempts:
- Follow with an opamp configured as a comparator, followed by a potentiometer to adjust output between low and high, followed by a voltage follower to drive the load. The issue is that non-deal opamps (for the comparator) have non-zero low voltage (5mV in my case with a LM358A. I need zero volts low voltage.
- Use a +/- supply opamp for the comparator and voltage follower and clamp negative parts to ground with diodes (???).
- Find different opamps/comparators that have a true zero V low voltage level (???).
How can I best finish my pulse generator with the least amount of stages?
Edit: Besides minimizing components, the major issue I'm trying to solve is to get the "low" part of the pulse as close to zero volts as possible. Even rail-to-rail op amps have non-zero minimum output voltage. One idea I've stumbled upon can be found here: Single Supply Op Amp with True Drive to GND.