current sink question

[ccesaretto]

Hello.
I do electronics as a hobby and I don't know a lot. I am going to explain what I need to do and hope somebody has suggestions on how to do it better.

In one of my projects I have a costant current sink, pretty much like the one in the picture below:

http://upload.wikimedia.org/wikipedia/commons/thumb/d/d1/Current_sink.PNG/250px-Current_sink.PNG

I am able to sink from 0 to 220mA.
The reference voltage Vin in the picture is created with a 12 bit DAC, so I am able to change the current in steps of around 53uA. (220mA/2^12)


Rc in the picture doesn't have an equivalent in my circuit. I just have a connection between the transistor's collector and another point of the circuit from which current is sinked.

Now I need to reduce the current range becuase 53uA current increment is too big. I would like change the current range from 220mA to around 100uA (even 200uA is fine) so that the current increments are much smaller, in the order of tens of nA (100uA/2^12).

I can't change the original current sink circuit but I can put something between the  transistor collector (where the load usually is connected) and the load.

I was thinking of putting a sense resistor (Rs) between the transistor collector and the 12V supply, so that the voltage (Vs) at the transistor collector will give me an indication of the current being sinked. then I would use Vs as a voltage reference for a second current sink module, that sinks 0-100uA over the Vs range.

I hope this this clear enough.

Do you think there is an easier way to do it? Basically I need to reduce the current increment steps.

Thank you
Cristiano

[P_Doped]

When you say you can't change the original circuit, does that mean you can't make any changes to the topology of the circuit or to the elements?

I think by far the easiest way to accomplish what you are looking for is to swap out the Rr resistor for something scaled larger.  That would be a pretty easy modification.  Next in difficulty, breaking the DAC connection and scaling it with some sort of divider.

Your suggestion, of doing some sort of sensing and then replicating the circuit, but scaled down, only seems to save you the DAC, but at the expense of adding some type of differential (or difference) amplifier (and it's error contribution).  At that point, I'd go the whole way and just replicate the complete circuit so you can have complete control.

[ccesaretto]

Hello.
Thank you for the reply.
I will add more information.

I have a board that contains the current sink module and other stuff.

to this board I connect an external device that is current driven (device connected to transistor's collector). So changing the current sinked I can change the way the external device works.

The external devices come in different models and expect different current ranges.

So instead of redesigning the board, I was thinking of putting something together using some resistors/opamp/transistors and place it between the board and the device.

I agree that swapping the Rr resistor would be easier but I was trying to avoid that, for a matter of time and cost.

Thank you very much for your help.

Cristiano

When you say you can't change the original circuit, does that mean you can't make any changes to the topology of the circuit or to the elements?

I think by far the easiest way to accomplish what you are looking for is to swap out the Rr resistor for something scaled larger.  That would be a pretty easy modification.  Next in difficulty, breaking the DAC connection and scaling it with some sort of divider.

Your suggestion, of doing some sort of sensing and then replicating the circuit, but scaled down, only seems to save you the DAC, but at the expense of adding some type of differential (or difference) amplifier (and it's error contribution).  At that point, I'd go the whole way and just replicate the complete circuit so you can have complete control.