I am experimenting with electrospray ionization. The setup has a 4kV power supply connected to an electrode that is separated from a return lug (or ground plate) via an air gap, followed by an inline pico-ammeter and finally ground. The way this works is that a small drop of solvent is applied to the electrode, the 4kV power supply is energized, and the solvent forms an aerosol electrospray "Taylor cone" between the electrode and return lug. When this is occurring (it is not especially visible except via laser light) the inline pico-ammeter reads somewhere on the order of 20-100 nA resulting from the ion transfer.
I would like to test a very large number of electrodes by systematically switching the pico-ammeter & ground between the various return lugs. My first thought was to use a multiplexer, but I'm not sure if this is even possible, and I have a few concerns. Specifically, iIs there a mux that can switch a ground reference (through the pico-ammeter) to an open (high impedance) circuit, and has a low enough on resistance as to not substantially effect the 20nA ion transfer current?
Also, every so often the air gap gets a little too small and turns into a 4kV "spark gap" which results in having to change a fuse in the pico-ammeter. What would be the best practices with regards to circuit protection for preventing this scenario from frying the switching circuitry?
For clarification, I already have a current measuring system, so I would rather not design one from scratch if it isn't necessary. I am specifically asking about the identification of multiplexer transistor architectures (if any) that are capable of switching ground to a no load, high Z or floating condition. (like a mechanical switch or relay but much faster and more scalable) Normally, low side switching with a low Rds(ON) would be the job of an N-Channel Mosfet except they generally won't turn on with an unloaded drain pin (to my knowledge). Are there any solid state switching topologies that are capable of this?
Best Answer
Rather than leaving the unconnected ground electrodes open you can short them to ground - there will not be any potential on them then.
Since the current is so low I would not use a semiconductor multiplexer - the leakage currents will swamp the signal being measured. I would recommend mechanical relays (possibly reed relays). The voltage across the relay will never be more than the input to the pico ammeter (<1V).
Use a single-pole changeover relay per ground electrode with the common going the electrode, the NC terminal to ground and the NO terminal tied to all the others you can select between the multiple experiments with the unused ones being grounded.
Another way that avoids mechanical relays but with lower accuracy is to use diodes as the switching element to combine the electrode signals and short to ground all but the sensor being measured before the diode. The active element shorting the unwanted signals to ground needs to be selected to have very low leakage (<1nA) with a diode drop across it. The diodes also need to be low leakage parts with <1nA lead with a reverse bias of 1 diode drop.