hey everyone
i really really need your help , so please don't hesitate on answering or posting anything that make sens to you.

I am not an expert in analogical electronics and am supposed to build a AC current source that is very stable and can deliver an current with Imax=0.355µA
the frequency is about 0 - 20KHz

am really desperate
i will be gratefull for any help and thank u in advance

 

What is the voltage limit of said current source? e.g. 10V, 24V, etc.
Also, how stable?
I assume that it is also variable in that the Imax is stated.

 

the voltage limit is 15 V

 

This is an extremely low level, apparently specified to a precision of nano-amps.

How perfectly constant-current does it need to be? It occurs to me that you might get away with a voltage source of some tens of volts followed by a large resistance.

 

well this source will feed a sensor so it has to be very stable

 

Check out page 7 of this old National Semiconductor app note--it shows the classic Howland current source--it is applicable for both DC and AC--will work better with modern op amp.
http://www.ece.ucsb.edu/Faculty/rodwell/Classes/ece2c/resources/an-31.pdf

 

Numbers are needed here - "very stable" is too vague. What are the upper and lower limits for the drive level? Do you know what the sensor impedance is, and if relevant how it varies over the frequency range?

 

Depending on the sensor impedance, careful guarding of cable runs or even PCB tracks to the device may be required to get 20kHz bandwidth with nA precision.

 

ok ,
i really don't have much of informations all i know is that the current will feed a capacitive sensor , and that the current source will be connected to a quartz oscillator
i also have this equation
v=I/(jwc)=I*d/(jwES)

f = 32 kHz (oscillator frequency), V = 0 to 10 V. So if we take V = 10 V and d=0.2 mm, S= 4 mm² you can calculate the rms value of I to be constant. E = 8.854 10^12F/m the primitive of the space

hope i made a little bet clearer