# Stable Ac current source

#### sanoua20

Joined Dec 6, 2011
4
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

#### jimkeith

Joined Oct 26, 2011
540
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.

#### sanoua20

Joined Dec 6, 2011
4
the voltage limit is 15 V

Joined Dec 26, 2010
2,148
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.

#### sanoua20

Joined Dec 6, 2011
4
well this source will feed a sensor so it has to be very stable

Joined Dec 26, 2010
2,148
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?

Joined Dec 26, 2010
2,148
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.

#### sanoua20

Joined Dec 6, 2011
4
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