Application: I have a copper mesh (10cm x 10cm square) in a vacuum chamber connected to a BNC connector by a 24-cm-long copper wire. The goal is to switch the mesh voltage (referenced to ground) from 8 V to ~0 V quickly. (This will switch the electric field in the chamber, which is a control mechanism for our atomic physics experiments.) It is essential that roughly 500 ns after the switching starts, the signal settles to <10 mV (~<0.1%). The mesh is floating; it is not terminated in the chamber.
Problem: There is a "hump" at the bottom of my inverted square pulse. I need to flatten it.
Circuit: I have settled on a simple MOSFET switching circuit:
Description: The MOSFET (ZVN2110A-ND, N-Channel Enhancement Mode) is driven by a IRS2117PBF-ND driver, which outputs a 15 V positive pulse. The baseline of this trigger pulse floats on V_S, which is tied to V_LO by a small resistor. The mesh is connected to Point B. The output low-pass filter was an attempt at fixing the problem. All resistor values were determined experimentally (i.e. by initially using potentiometers). The result was hard-wired using a "dead-bug" style on a copper-clad board.
Probe Details: To simulate the mesh, I soldered a 24 cm wire to a piece of copper-clad perf board and connected it to the circuit output (Point B). I probed the signal on the perf board with a Tektronix probe (500 MHz, 8.0 pF, 10MOhm, 10x) into a Tektronix scope (TDS3012 100 MHz digital scope).
Observations: It switches quickly enough (although I could speed it up by removing the filter), the ringing amplitude and duration is tolerable, but on the (essential) microsecond time-scale, there is a large "hump" and droop/sag of 20 mV (labeled in image by red line). This is unacceptably large and makes it impossible to do our experiments, which take place from the moment of switching until about 10 microseconds after switching.
Details of Application: We use electric fields to tune atomic resonances in our experiments. Scanning the electric field applied to the atoms lets us record a "spectrum" of these resonances showing their location and shape. The widths and separations of these resonances are on order of 1-10 mV/cm (very small!). To apply the electric field, we place the atoms between two flat copper pieces of mesh, separated by 1 cm. The E-field between the copper mesh pieces is just the potential difference between the mesh pieces (1 V difference equals 1 V/cm E-field, a 1-to-1 conversion). In collecting a spectrum, we sample an E-field value by switching to the corresponding voltage and waiting a few microseconds before detection. If the voltage (and thus the E-field) drifts during the sampling period more than the size of the resonances (<10 mV) the resolution is degraded to the point where our spectrum picture becomes blurred beyond recognition.
Additional Thoughts: I have considered the possibility that the MOSFET is heating up, thereby changing its on-resistance (normally ~4 Ohms). To test for this, I tried two things: (1) placing two MOSFETs in parallel, and (2) replacing the ZVN2110A with a IRF1010EZ MOSFET that has a much lower on-resistance (100 mOhm). Neither things helped, the "hump" is still 20 mV and still lasts a few microseconds. It seems to me that increasing the pull-up resistor (as suggested in the comments) could also help, so I will try this.
Update 1: I have tried increasing the pull-up resistor from 470 Ohms to 10 kOhms. There was no effect on the output; it still has the 20 mV "hump" after the initial ringing.
Update 2: Disconnecting the "mock-up" wire + mesh from the circuit and probing Point B directly has no effect on the measured signal.
Update 3: Below are traces for the corresponding points in the schematic above:
It appears as though the "hump" appears on the gate pulse, too. Point "D" right near the FET does not look any different than probing the mesh.
Update 4: I have (1) increased the pull-up resistor to 1kOhm, (2) removed the filtering 1000pF resistor, (3) disconnected the mesh, (4) added two "jam can" 470uF electrolytic capacitors to the rails, and (5) replaced the pulse generator with a faster one (Agilent 33250A). New schematic and traces:
Even with a faster trigger pulse for the FET driver, the problem remains. The "jam can" caps do seem to filter out some high frequency oscillations, but the "hump" remains.