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Suggestion on transconductance OPA app , range: Vin=1-10V vs. Io = 1m-10mA

Oreffocaet Iwan
Prodigy 225 points
Other Parts Discussed in Thread: OPA860

Hi, :

I please ask a question, I hope to design a circuit to convert a voltage range in 1-10V to output a current in 1m~10mA,

Can you please suggest some on this ? Can I use opa to do it ? Is it enough ?

Thank you very much

  • 0
    Guru 140200 points

    Hi,

    what load has to be driven by the 1...10mA output current?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 225 points

    Hi, Kai:

    I want to use it to drive a resistor, maybe not 'it', them, 3~4 parts, or more, it depends on different applications.

    The original thinking is sum up the current from every transconductances, sorry, I don't know the right name, some said 'OTA' operational transconductance amplifier.

    But from the datasheet, the OTA looks like different than regular amplifiers. I looked at OPA860, but the transconductance is not very linear.

    I don't need very accurate, but too distort still very bad, I can't evaluate the distorted result easily.

    Thank you very much

  • 0 in reply to Oreffocaet Iwan
    TI__Expert 8540 points
    Hi Iwan,

    You can use an OTA for your current source, or you could use a basic op-amp voltage controlled current source where you can drive MOSFET. The below application note goes into great detail on many different current source options that you could use.

    www.ti.com/.../sboa046.pdf

    Regards,
  • 0 in reply to Oreffocaet Iwan
    Guru 140200 points
    Hi,

    have I understood you correctly? You want to parallel several current sources? To drive only one resistor? Or more than one? Can you explain a bit your application?

    Kai
  • 0 in reply to Jacob Freet
    Prodigy 225 points
    Hi, Jacob:

    Thank you for answering me.
    I need some time to study the documentation,
    www.ti.com/.../sboa046.pdf

    I will try to find a solution in it.

    Regards,
  • 0 in reply to kai klaas69
    Prodigy 225 points
    Hi, Kai:

    The resistor is not only a resistor, some where other applications use resistor to sense current.
    In the idea, I thought to add other compensations (interferences) to the original applications. But some time use voltage compensation is very complicated, there are different voltage potential between every circuit portions, so I try to find and ask help ideas, is there any small current source can be controlled to feed compensation to interfere the original applications.

    I don't know if this is general, I am not familiar in amplifier, and OTA, or something about current source devices.

    Thank you very much

    Regards.
  • 0 in reply to Oreffocaet Iwan
    Guru 140200 points
    Hi,

    I have great difficulties to understand you... :-)

    Do you have any schematic of your application you could show us? This would make things clearer.

    Kai
  • 0 in reply to Oreffocaet Iwan
    Guru 48395 points
    Yes, it is difficult to understand your question, but aside from OTA's which are typically open loop and hence not very linear (I worked on the OPA860 release), there are a number of op amp circuits - the first is a Howland current source

    www.ti.com/.../snoa474a.pdf

    There are some more recent discussion in EDN & Planet analog as well,

    www.edn.com/.../Simple--precise--bi-directional-current-source

    www.planetanalog.com/author.asp
  • 0 in reply to Michael Steffes
    Prodigy 225 points

    Hi, Michael:

    First, thank you for answering me. In the application note

    says 'The Howland current pump is usually excellent for that. Current sources are often used for testing other devices. They can be used to force currents into sensors or other materials. They can be used in experiments, or in production test. They can bias up diodes or transistors, or set test conditions. When you need them, they are useful — even if you only need them once or twice a year. '

    I think this is what I need. Originally I don't know the 'But the practical problems involved are not so simple or obvious', How severe is it. <- I don't know this...so, I don't know if the choosing strategy is proper or not.

    I don't know the keywords 'howland' , 'current pump' before you told me.

    I need some time to study.

    Thank you for the guide.

    Regards.

  • 0 in reply to kai klaas69
    Prodigy 225 points
    Hi, Kai:

    I don't have schematic already finished. It's idea only in mind. The original needs is a power supply to supply several units, every units can have their reference ground in different, but total current must be controlled, and the number of units plugged can be changed. The connection can't be drawn in same schematic. It's diagram.

    The other idea is for verification, to interfere a design, and see the result reliable or not. I didn't do that before, only heard.

    FYI

    Regards.
  • 0 in reply to Oreffocaet Iwan
    Guru 140200 points
    Hi,

    every unit having its own reference ground can make it very difficult to find a solution. How much can the individual reference grounds differ from each other?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 225 points

    Hi, Kai:

    It's hard to answer your question, it should be concerned. But at the first, it's floating, then it's possible to ask to meet some criteria. But which criteria, it's not clear now.

    But thinking the worst case, if apply the isolating voltage in safety, 2KV or 4KV...

    Or...do you think except Howland (amplifiers), OTA , it's better to use other strategies/methods ? I ever though using isolators , but it needs more auxiliary power to supply the isolators.

    Regards.

  • 0 in reply to Oreffocaet Iwan
    Guru 140200 points
    Hi,

    unfortunately, without seeing a scheme or a schematic I cannot help you further. I can only say, that it will be very difficult to connect two circuits with another which have their reference grounds differing by 2...4kV.

    Kai