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Ref200 Floating Current Source <100uA

Kyle Green
Prodigy 30 points
Other Parts Discussed in Thread: REF200

Hi,

I am in the process of a designing a project and I need a floating adjustable current source.  The REF200 looks like a promising little chip and in both the datasheet and application notes (SBOA046) for it there are circuits that do almost exactly what I want to do.  The circuit I'm specifically talking about is Figure 10e in the datasheet and Figure 16 in the application notes.  The circuit is specified as a Floating Current Source for outputs greater than 100uA. 

For my project, I need a floating adjustable current source just like this one, except I need outputs as low as 10uA.

If anyone has insight or previous experience with this chip that could help me adjust this circuit to allow current outputs down to 10uA it would be greatly appreciated.

 

Thanks!

Kyle

 

  • 0
    Guru 21975 points

    Kyle,

    I don't have a good solution for your application. The circuit below is a not so good one. It requires a reference cell, Vb, to create a bias voltage. It would last a long time as it only supplies the input bias current of the op amp. The output current is Vb/R1. The op amp supply current is included in the two-terminal floating current so the op amp supply current must be less than the set current. The op amp must have common-mode range to the negative rail and the compliance voltage is equal to the max operating voltage of the op amp +1.35V. Stability may be an issue, requiring a cap from gate to source of the MOSFET. It may also require a bypass capacitor across the supply pins of the op amp which would reduce the output impedance at high frequency.

    This circuit matches the two-terminal functionality of the REF200. If you are willing to have separate power supply sources or make other compromises there might be some other, more attractive solutions. Or maybe some else has a better idea.

    Regards, Bruce.

     

     

     

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Thanks for the quick reply Bruce!

    I was also worried about the stability of the aforementioned circuit and that's a good point about the op amp supply current.

     

    I think i've been thinking about this design problem too long that I'm starting to think in circles...  The end game of what I'm trying to accomplish is to have a circuit that will limit the output current of a waveform to a wide range of loads.  The waveform has patterns of alternating +16V, -16V pulses in it and the load resistance will range from 100kohm to a few Megaohms.  I need a way to set the current output of the waveform to preset outputs (10uA, 25uA, 75uA, 150uA, 300uA).  If the load & current combination wind up needing more voltage than is available, whatever smaller current the voltage can produce at that load is sufficient.

    Right now I'm trying to come up with a circuit that will limit the current at the uA level like I need. 

    Maybe you (or someone else) has an idea that I haven't thought of.  Right now I feel I have hit a bit of a mental wall =P

     

    Thanks everyone!

    Kyle

  • 0 in reply to Kyle Green
    Guru 21975 points

    Kyle,

    I have not seen other ideas, so far. If you want to contact me off the forum, I have some other thoughts that might help.

    Regards, Bruce

    trump_bruce@ti.com

     

     

  • 0 in reply to Kyle Green
    Prodigy 30 points

    There are current regulating diodes that might help in this regard.

    Craig

  • 0 in reply to Craig Graham
    Genius 5430 points

    "Current regulating diodes" are simply JFETs selected for Idss