I have a floating stainless steel tube enclosed in an apparatus used for atomic physics experiments. A feedthrough + 10ft coax cable allow us to apply a voltage to this tube. Until now, we have connected it to ground.
Now I need to apply a 10us negative square voltage pulse to the tube. The amplitude must be adjustable between 10V-80V. The rise and fall time must be less than 300ns for 50V (so say 200V/us). Most importantly (!) the top of the pulse must be very very flat, with minimal overshoot and ripple.
My problem is this: applying an 80V square pulse to the tube whose over/undershoot and ripple are <0.5V over the entire top of the pulse.
I've attached a quick drawing of my current setup. I need to throw this together quickly, so I'm using whatever I've got. The idea is to use an Agilent 33220a 20MHz Arbitrary Waveform Generator (which I can control with a computer) to produce a 0-5V pulse, and then amplify it to 0-80V.
For the amplification step, I'm using an Apex PA78 power operational amplifier (datasheet). I've taken a circuit from their applications note.
I've attached some scope traces with the probe attached right after the output of the PA78. In one trace, I have disconnected the stainless steel tube, while in the other two it is connected. (Note that I've zoomed in on the top of the pulse in two of the traces!) The circuit itself is wired up on a breadboard. 0.1uF caps are used for bypassing power to the op amps.
Question: How can I get a very flat top to my pulse? Are there any tricks I can try? Could this be from reflections due to the way things are terminated (or unterminated..). This sort of thing is not my specialty, so please let me know if I'm doing something dumb.
Thanks in advance!
Now I need to apply a 10us negative square voltage pulse to the tube. The amplitude must be adjustable between 10V-80V. The rise and fall time must be less than 300ns for 50V (so say 200V/us). Most importantly (!) the top of the pulse must be very very flat, with minimal overshoot and ripple.
My problem is this: applying an 80V square pulse to the tube whose over/undershoot and ripple are <0.5V over the entire top of the pulse.
I've attached a quick drawing of my current setup. I need to throw this together quickly, so I'm using whatever I've got. The idea is to use an Agilent 33220a 20MHz Arbitrary Waveform Generator (which I can control with a computer) to produce a 0-5V pulse, and then amplify it to 0-80V.
For the amplification step, I'm using an Apex PA78 power operational amplifier (datasheet). I've taken a circuit from their applications note.
I've attached some scope traces with the probe attached right after the output of the PA78. In one trace, I have disconnected the stainless steel tube, while in the other two it is connected. (Note that I've zoomed in on the top of the pulse in two of the traces!) The circuit itself is wired up on a breadboard. 0.1uF caps are used for bypassing power to the op amps.
Question: How can I get a very flat top to my pulse? Are there any tricks I can try? Could this be from reflections due to the way things are terminated (or unterminated..). This sort of thing is not my specialty, so please let me know if I'm doing something dumb.
Thanks in advance!
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