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OPAMP Configuration

parviz kalkhoran
Prodigy 50 points
Other Parts Discussed in Thread: OPA320, OPA197, TINA-TI, LOG114

I have a requirement for  opamp that the input is from 60 nA to 120 uA and ,I need 0 to 3 volt output .

 Any circuit recommendation ?

  • 0
    TI__Guru 62745 points

    Parviz,

    60nA to 120uA is right on the edge of ~3.5 decades of input range (60-600nA, 600nA-6uA, 6-60uA, 60-120uA) which is usually right at the far end of what can be accomplished with a linear solution. 

    We suggest trying the circuit configuration shown the TI Circuit linked below.  The circuit is described and design steps are provided on how to set the input to output transfer function as well how to select which bandwidth op amp you'll need.

  • 0 in reply to Collin Wells
    Prodigy 50 points

    OPAMP_SCH.pdfHi;

    Attached is the schematic , my result is not as linear as I want , can you make some comments please

  • 0 in reply to parviz kalkhoran
    TI__Guru* 93105 points
    Hi Parviz,

    The OPA320 when applied in a transimpedance application will produce an output voltage that is the product of the current produced by the input current source, times the feedback resistor value. If the input source exhibits a linear response across its current range, then the OPA320 will produce nearly identical linearity behavior over the specified output voltage swing range.

    The exception will be when the output voltage level approaches the swing limits of the operational amplifier. If the OPA320 output is minimally loaded it swing within a few tens of millivolts of the supply limits. In your circuit the supply levels are +5 V and 0 V so the output will exhibit that linear response from a few tens of millivolts above ground (0 V), up to a voltage a few tens of millivolts below +5 V.

    If the output voltage truly needs to swing from 0 V to 5 V, then it would be best to use a higher voltage op amp and power it with dual (+/-) supplies. The OPA197 precision op amp powered by +/-6 V supplies or more should work well in the application.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 50 points
    Hi;
    I agree with what you said,can you please simulate the schematic that I sent you and send me the result so I can compare with mine?
    May be in order to achieve (attached schematic)what I need is may be use a Logarithmic amp ?
    Please advise
    BR
  • 0 in reply to parviz kalkhoran
    TI__Guru* 93105 points

    Hello Parviz,

    If we assume that the OPA320 output is lightly loaded, then the output swing range will be from about +10 mV to +4.990 V, for Vs = +5 V.Using a 100 k feedback resistor with the OPA320 the input current range would be limited to 100 nA on the low end, and 50 uA on the high end.Certainly that is a narrower range than the 6 nA to 120 uA you are attempting to accommodate with your design.

    Maintaining a linear output voltage in relation to a linear input current will require keeping the output from swinging into the rails as previoulsy discussed. Applying a small common-mode voltage (VCM) to the non-inverting input, and then setting the transimpedance gain so that the output stays within the linear range will accomplish this task. Shown below is a TINA-TI simulation circuit with the OPA320 configured for +0.25 V VCM, and the transimpedance gain set to 37.5 kV/A. The output will then have a range from about +0.25 V to +4.75 V. Both linear and log plots for output voltage vs input current are shown. 

    If you think you would like to consider a log amplifier for the application take a look at the LOG114. It is very versatile and easy to apply. Here is the link to the LOG114 datasheet:

    Regards, Thomas

    Precision Amplifiers Applications Engineering