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Single chip digitally programmable current source: 0-2mA

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orient

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Hello there,

I am designing a PCB for testing an ASIC taped out a few months back. I wonder if anyone knows a single component that can output a digitally current from 0 to 2mA, a DAC basically.

Of course, it is always possible to add an opamp to a voltage DAC to make it work as a current source, but perhaps someone knows a single component that does that. If so, please...

The resolution is not so important, it will be used to set some bias current for the ASIC.

Thank you.
 

orient

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Yes, this DAC seems very promising indeed. Thanks very much.

However, I couldn't figure out a way to use it as a current source without a companion Opamp. If you have used it before in this mode, would you mind elaborating a bit more, please?

Cheers.
 

skogsjanne

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Many years ago we used a multiplying DAC to add a programmable current to a summing node.
Figure 43 show how it works. It may be a problem that you need a virtual ground (or at least a constant voltage) for the current to flow into.
I also didn't realise that the values of the resistors could be between 7k and 11k. Only the ratio between the R and 2R is accurate.

Maybe a digital potentiometer is a better choice.

What impedance do you want to inject 0-2mA into? Is the voltage at the input pin constant?
 
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orient

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Indeed, I think a virtual ground is necessary, thus requiring an opamp. The current will be injected into diode-connected NMOS transistor inside the ASIC, which is then used as a reference for mirroring the current for other devices. Another issue is that I need to source a current into the NMOS transistor (not sink current from it).

Anyway, thanks for the help. I am kind of settling to solution based on a simple DAC + an opamp + a reference resistor + a discrete PMOS transistor. I wished I could find a single component solution. I am sure such a component would sell quite a lot... so convenient!
 

skogsjanne

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A diode is of course not a perfect virtual ground but a fair (?) approximation.
If you just need to vary the current into the pin between approximately 0 and 2 mA I think the DAC or even a digital potentiometer would do.
If, on the other hand, you need to know with any precision how much current you source it's not a solution.
 

orient

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I agree, but about 5% of accuracy is good enough for me... This is only to set a bias point for my on-chip circuitry. Thanks.
 

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