I am tasked with making a constant current SINK for a testing device. It must outputs 4 separate values, -10pA -100pA -1nA -10nA. I need the current to last for at least 10-20 seconds, preferably up to 100 seconds if possible. These current values are very small so I won't be able to use a simple current mirror with transistors.
The reason I need to make this device is it needs to be much smaller than a test bench instrument, think handheld, and only needs to work for those specific current values. I also do not know the load, this is a source, it shouldnt matter?
So far, all I have come up with is to use a voltage ramp to charge a capacitor (Ic= C dv/dt) so it can output the current. I would use a mechanical switch to change the capacitance value so the ramping time stays the same and the current can be changed between the 4 values. The waveform would need to be a sawtooth so as to ramp back up in ~1 second. I don't know how to make a sawtooth or really any voltage ramp myself and need it to be linear so as to get the proper current from the cap.
Please give me any suggestions and ask questions about whatever else I forgot to tell you, I'd like to figure this out soon.
EDIT: hopefully its a bit more clear
Best Answer
Linear have a Precision Nanoamp Bidirectional Current Source application note that may be of interest.
Figure 1. This circuit sources and sinks just nanoamps of current with precision due to the low input bias current of the CMOS op amps. A buffered difference amplifier and an integrator force the voltage across a 10 megaohm set resistor to be 1/1000 of the control input voltage in either polarity.
Whether this is suitable for your application is difficult to say due to the lack of information supplied. It has the advantage of not requiring any ramp generation or precision capacitors.