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Small current to volts amplifier, choosing basic schematic + OpAmp

Thorsten Simon
Prodigy 20 points

Support Path: /Product/Help with Device Selection/Amplifier/filter solution/

Hi there,

I need some help choosing the right schematic / OpAmp.

I'm getting 2 single ended current sensor signals:
a) 15...2000nA
b) 0...-120nA (negative!)

That's all the information about it.
Power Supply is 24V

The job is to form the sum of the 2 signals and scale it to 10V (2000nA -> 10V)

First approach:
TIA (50V/mA too much?), Inv. Amplifier (x10), Diff.-Amplifier


Second approach:
2 shunts of 100 Ohm, Instrumentation Amplifier, amplifying the (2000nA * 100 Ohm + (-120nA) * 100 Ohm)

The Output is digitized by an given DAQ System (<= 1kHz)

Which one is the better approach?
Which parts could be used?


Thank you in advance

  • 0
    TI__Mastermind 23050 points

    Hi Thorsten,

    To confidently recommend a solution I would need to know more about the sensors you are trying to interface with and what your bandwidth requirements are. However, if we can assume that the sensors look like true high impedance current sources then you may be able to use a single TIA as a summing amplifier as shown below.

    Summing TIA.TSC

    I added a 1V offset generate from the 24V supply to keep the input and output within the linear range of the amplifier. The voltage at the non-inverting pin will also be impressed across the sensors so you would need to verify that this is acceptable. Otherwise, a negative supply would be needed.

  • 0 in reply to Zak Kaye
    Prodigy 20 points

    Hi Zak,

    thank you for your prompt response.

    Looks very good.

    These sensors aren't free available and there is no datasheet. But I was told they behave like "ideal current sources".

    It'll take some days to get an answer, if it's possible to connect the sensors that way.

    The Offset is no problem, because the data acquistition system has no GND connection (transformer coupled).

    So I can connect it to the pos. OpAmp Input and the OpAmp Output without loosing resolution.

    I'll be back when I got an answer.

    By the way:

    Do you have any proposals for small connectors?

    I'm planning a multi channel rack system with 100mm x 160mm 4-channel amplifier boards. Input connectors on the backside.

    The ordered Current Source (Keithley 6220) uses 3-lug female triax connectors, but they are to big for my system.