Current Multiplexer for values between 100nA/10nA

Hi,


I've got a digital input, and according to that digital input I would like to generate a defined current (have a node where the current is set according to a predifined value chosen by my digital input). The order of magnitude of the desired current is 100nA/10nA (exactly 0->0.13uA, 1->0.17uA, 2->0.059uA, 3->0.06uA).

At first, I thought of just using different current sources and connecting them to P MOSFETs, connecting the drain of the mosfets together, and activating the mosfets according to my digital input. Put obviously after some thoughts it didn't work and I don't really know how to realize that.

Could someone point me in the right direction please? Please note that I'm using HSPICE, so maybe there is a way to script that behaviour, but I don't know.

Thank you,

I'd use 3 constant current (current mirror) sources with current sizes 60nA, 10nA, 1nA and digitally add or subtract them:

#0: 2*60+10 nA
#1: 3*60-10 nA
#2: 1*60-1 nA
#3: 1*60 nA

Suppose you had a means to switch in different resistors? Then you might get by with a single current source.

Perhaps using this programmable gain op amp (from the Forrest Mims Engineer's Notebook).



Resistor values can be chosen to amplify or attenuate. They can be switched one at a time, or in parallel.

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You may not need the op amp. Perhaps it will work if you just switch in different resistors.