Need Help Design Current Source - Electronic Circuits Projects Diagrams Free

leofur · 6th March 2008, 11:25 AM

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 · 6th March 2008, 11:28 AM

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** · 6th March 2008, 11:48 AM

How about a resistor fed from a voltage?.

leofur · 6th March 2008, 11:53 AM

Thanks for your answer, but I need constant current.

**Nigel Goodwin** · 6th March 2008, 11:59 AM

Quote:

Originally Posted by leofur

Thanks for your answer, but I need constant current.

Use a higher voltage!

It's a VERY old technique.

leofur · 6th March 2008, 12:02 PM

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 · 6th March 2008, 02:07 PM

Well, you'll find that 10G

hm: 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 · 6th March 2008, 04:50 PM

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 · 7th March 2008, 10:05 AM

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.

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6th March 2008, 11:25 AM	(permalink)
leofur	Need Help Design Current Source 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.

6th March 2008, 11:28 AM	(permalink)
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.

6th March 2008, 11:48 AM	(permalink)
Nigel Goodwin	How about a resistor fed from a voltage?. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

6th March 2008, 11:53 AM	(permalink)
leofur	Thanks for your answer, but I need constant current.

6th March 2008, 12:02 PM	(permalink)
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.

6th March 2008, 02:07 PM	(permalink)
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. Last edited by Bob Scott; 6th March 2008 at 03:10 PM.

6th March 2008, 04:50 PM	(permalink)
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. __________________ Ron

7th March 2008, 10:05 AM	(permalink)
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. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.