Need Help Design Current Source

[leofur]

Hi all,
I need to design a current source that will charge a capacitor, because the capacitor is very small( about 0.7 pF) I need a 1-10 nA current source, somebody can help me, and suggest how I can design such current source?
The circuit I'm designing measures changes in MEMS capacitor.
P.S
I need linear charge of the capacitor, so I need constant current source.


Leonid.

 

[leofur]

Forgot to add, I need to charge the capacitor to 5-10V, so I need a current that wil do it in about 1mS.

 

[Nigel Goodwin]

How about a resistor fed from a voltage?.

 

[leofur]

Thanks for your answer, but I need constant current.

 

[Nigel Goodwin]

Thanks for your answer, but I need constant current.

Use a higher voltage!

It's a VERY old technique.

 

[leofur]

I need to charge it to around 5V, so let say I use 100V source, and if I want 10 nA, I'll need 10Gohm resistor.

 

[Bob Scott]

Well, you'll find that 10Ghm: resistors are about as rare as 700 femtofarad capacitors.

Are you pulling our legs? linear charging a 700 fF cap?
Sensitive to non-linear charging is it?

It just needs a few electrons!

Seriously, I would suggest attaching a larger .01uF or so capacitor in parallel with the tiny cap. Charge them both with the manageable constant current, then disconnect the larger cap. Use a connection smaller than a gnat's eyelash.

 

[Roff]

Any current source that I can imagine will have at least a few picofarads of stray output capacitance. You will have to periodically discharge the capacitor, and that switch adds more stray capacitance.

 

[Hero999]

Why do you want to do this?

Normally capacitors this small are used in UHF circuits and are often etched onto ICs.

How are you planning on measuring the voltage?

Most op-amps will load the circuit far too much for them to be useful.