EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: nitish111 on October 08, 2018, 05:37:31 pm
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Hi,
I am designing a voltage controlled current sink(schematic attached). The minimum current on the output fro the simulation is around 1.05 uA but the circuit I built has a minimum output current of 25uA. I need minimum output current to be less than 10uA. How do I bring output current down to 10 uA or less? All resistors are 0.1% tolerance.
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It looks to me as if you are using a single supply of 40 V. The opamp will have a few hundred millivolt minimum output voltage that will cause an offset current to flow in the load. If you don't have a negative supply of a volt or two available, you could put a device with a threshold like back to back diodes in series with the output of the upper opamp to drop the offset voltage. You may also need to add a resistor after the diode to circuit common to absorb any leakage current. Note that this this could affect AC stability so there could be other changes needed such as a capacitor across the diodes.
Cheers,
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Definitely add a negative supply to your opamps. Doesn't need to be symmetrical. A small negative supply should be enough.
It will still be hard to guarantee a "zero-volt" current of 1 µA or less, but you can get closer to it.
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Thanks SiliconWizard and duak, I connected opamp to negative 3.3 V supply and biased input voltage to have minimum of around -0.1 V with resistor divider and it gives me around 2 uA which is perfect. Now the circuit has a small DAC offset which is not so bad.