# Open circuit ground multiplexing?

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?

• Possiby, the pico ampermeter is cheaper than multiplexer that could switch those pico amperes. A pico is very very small unit , that is easily "eaten" . – Marko Buršič Jun 26 at 16:28
• The pico-ammeter is extremely expensive. This is not a possibility. – keystoneclimber Jun 26 at 16:57
• You could look what's inside, then if there is a shunt resistor you add shunts to all positions and then multiplex shunt signals. I guess the input impedance to the amplifier is very high - gigaohms, so an additional relay or analog switch won't change the accuracy. – Marko Buršič Jun 26 at 17:07
• So you have some required switching speed? You need to mention ALL of your requirements. – analogsystemsrf Jun 26 at 17:14
• What's the current in case of a 4k spark gap? When not know: what's the current rating of the fuse? – Huisman Jun 26 at 17:46

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.

• "Rather than leaving the unconnected ground electrodes open you can short them to ground - there will not be any potential on them then." Why, what is the point? Mechanical relays are not feasible. There will be several hundred of these electrodes. Expense and switching (and debounce) time are prohibitive. – keystoneclimber Jun 26 at 16:59

A good mux to use would be the ADG798 which has a leakage of 10pA to 40pA of leakage in typical conditions and has 8 ports. If that doesn't work then use read relays. If that doesn't work Teledyne makes some good can relays also.

Marko has what I was thinking of but my suggestion is to use a 4kV voltage source and current limit each electrode in parallel with 100M and sense current with 50M with OVP & OCP.

• OVP using a low capacitance (pF) low voltage TVS to clamp ~5V
• OCP using a string rated for 4kV/100M= 40uA
• preventing any significant energy in the gap
• with minimal power in the HiV resistor string at 160mW
• the RC time constant from spray and stray pF “may” create a Partial Discharge (PD) Relaxation Oscillator at a slow rate when beginning to clog. Z

The current sense needs to be carefully located in close proximity to spray gap to reduce capacitance yet avoid crosstalk. And then impedance reduction to 50 Ohm coax

If a higher current loop is needed, PD arc current pulses or pC discharge levels can be detected with a current loop or a VHF loop antenna to 50 Ohm coax terminated by same into a high speed DSO.

You need a shunt and a very low bias input current opamp, like elctrometer ADA4350 with femto ampere bias. Then you amplify this small signal to your needs. Of course you would need to calibrate for yourself and it won't be such precise as already made pico ampermeter from reputable manufacturer, but it can be cheap enough to build it.

simulate this circuit – Schematic created using CircuitLab

You can also buy from china a module to play with it, just to try. Unfortunatelly it is configured as transimpedance amplifier, while in your case a buffer is needed:

To protect the current measuring system (when it needed to be grounded), I first thought of a clamping circuit as shown below to the left.
But then, I realised there is a way easier circuit, as shown to the right.

The right circuit consist of a few resistors (I only drew R15 ... R18) that are rated sufficiently to prevent break down and limit the current in case the air gap is short circuited (worst condition). With 400kohm, the current is limited to 10 mA, protecting the multiplexer solution and the current measuring system.
Because the resistors are in series with the multiplexer solution and the current measuring system, there is no leakage current.

simulate this circuit – Schematic created using CircuitLab

I kept the left circuit in case it might hint towards other solutions.
In the left circuit the cathode of the ultra low leakage diode D1 is biased at a voltage of 104 mV. When the current is "somewhere on the order of 20-100 nA", due to R3(1) the voltage at the anode of D1 will be max 10 mV. So, D1 is reversed biased and will leak only a few pA.
When the current increases past the about 6 uA, D1 will start conduction and it will clamp the voltage at its anode. The clamped voltage depends on R4...R10, D1 and R2. The max current is limited by R4...R10(2). For the FJH1100, $$\V_{F,max}\$$ = 1.07 V at $$\I_{F}\$$ = 50 mA, so, the clamped voltage with be about 2 V + 1.07 V = 3.07 V.
Of course, R4...R10 should be rated sufficiently to prevent break down of each resistor.

(1) The resistance of the multiplexer and current measuring system are assumed negligible compared to R3.
(2) This is where I realised the circuit could be made way easier.