LMP7721 fA bias current used as electrometer

LMP7721 fA bias current used as electrometer for capacitor decharge measurement

Hi All,

I'd like to know if I am making a big mistake with this circuit design or there is something good. The scope is to charge the cap and than leave it decharging. The key point is having a capacitor with high charge holding, so the discharge process will be very slow. I was thinking a descharge of about weeks with a polypropylene cap.

I am doing some sort of mistake thinking about this circuit?

The switch just recarge the cap and then leave it attached to the input of the Amplifier. The choise of the amplifier is critical and that's why I choose an electrometer grade amplifier such as the LMP7721. I was thinking also to use maybe a jfet. What do you think?


Do this circuit convince you? You see some problem with this idea?

Thank you in advance
Salva

The circuit can only work if the switch has similar low leakage as the amplifier. Did you identify a suitable transistor?

I was thinking at two options: 1) power mosfter with very high off resistance 2) An electro-mechanical switch (reed relay??)

What do you think?

For the first proto I can even think to use a mechanical switch (by hand).

No power-MOSFET has sufficient low leakage, some "high insulaton" reed relays are specified with 10e14 minimum open contact resistance, e.g. **broken link removed**

Needless to say that the mounting technology and enviroment matters, you'll need "flying" wiring with teflon standoffs and dry air/nitrogen atmosphere.

FvM said:

No power-MOSFET has sufficient low leakage, some "high insulaton" reed relays are specified with 10e14 minimum open contact resistance, e.g. **broken link removed**

Needless to say that the mounting technology and enviroment matters, you'll need "flying" wiring with teflon standoffs and dry air/nitrogen atmosphere.

Click to expand...

Thank you for the quick reply.

Do you think that even if I lower the specification regarding the discharging time (2-3 days) I should deal with teflon standoff? Naturally increasing the capacitance...

Thank you again
Salva

What is the purpose of holding this voltage from such a long period? Why not just record the voltage with an A/D converter and you can store the value forever?

The point is that this voltage is needed.. since the capacitor will feed a high impedence sensor which need a constant voltage... the sensor can be seen as a Ionization Chamber...