Hi everyone. I am looking to measure 2 capacitors in parallel, one is 50pf and the other is 10pf. My circuit is attached, it uses a FET to switch between measuring the 50pf cap and the 50+10pf cap at the output. The capacitance meter measures the the caps in parallel (60pf) when the FET is on as well as off (measuring between the source (ground) of the FET and the output). The capacitance drops only about .4pf when the FET is off. The off resistance of the FET I measured to be about 1MOhm. Could it be that the off resistance is not high enough to cut the 10pf cap out of the circuit? Thank you.

 

The 1.25v pulses on the gate of the 2n7002 won't do much; that isn't even up to threshold level.

Did you look at your input and output capacitance? That's going to interfere with your readings; as the parasitic capacitance is anywhere from 4pF to 50pF.

You'd be better off using a relay.

 

I don't quite see how it's supposed to work, but turning on the FET might help. I am guessing that the device is really a 2N7002. The gate has to be raised to at least 10 volts above the source potential to turn fully on.

One thing you may notice is that measuring picofarad value capacitors is affected by many things, and in sometimes unpredictable ways. If your meter will handle larger values of capacitance, I might suggest you try it with much larger capacitors, like a pair of .1 uF.

What is the duration of the pulses?

 

Yes, my mistake. It is a Supertex 2N7002. The device is turning on an off, as the threshold voltage is 1V minimum. I know it is turning on and off as I measure the conductivity between drain and source and the pulse hit the gate. The meter goes between conducting and about 1-2MOhm when on and off, respectively. Even if the FET is not conducting, I should only be reading the 50pf cap, correct? Here my cap meter shows 60.

How can I configure a relay to do this? I haven't worked much with relays.

 

That threshold is a point where applied voltage on the gate just starts having an effect on the conduction channel. I have used these things more than once.

Even if the FET is not conducting, I should only be reading the 50pf cap, correct?

Not at all. The sergeant mentioned stray capacitance, which you have not taken into account. Remove the caps and check with just the 2N7002 in circuit. Bet you read something.

 

Thanks. I increased the gate voltage to 5V and I am still getting the same readings, .4pf difference when on and off.

I also measured the capacitances. When VGS=0, C(GD) = 276pf and C(DS) = 271pf. When VGS=5V, C(GD)=446 and C(DS) = >2000pf.

Also I grounded the source, shorted the drain and source, and measured the capacitance of GS and GD. They were both 36pf.

 

OK, so...
What capacitance would you expect to measure with 10pF and 271pF in series?
What capacitance would you expect to measure with 10pF and >2000pF in series?

Here is how you could use a uC to control a relay to switch your 10pF cap:

Note that you don't want to connect the cap to the common terminal, as it will likely have more stray capacitance than a NO or NC terminal.

Don't try to operate the coil of the relay directly from your uC; it probably won't be able to sink/source enough current, and it will likely be damaged.