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TINA/Spice/LMP7721: lmp7721 nano ampere current biasing

MAHENDRA JADHAV
Prodigy 50 points
Part Number: LMP7721
Other Parts Discussed in Thread: TINA-TI, , LM7705

Tool/software: TINA-TI or Spice Models

Hi, 

I'm using LMP7721 op-amp for ultra low biasing, so I used lmp7721 as  a transimdence amplifier with appropriate gain. I want to bias nano ampere current but cant done yet. can you suggest what kind of schematic and layout of PCB required to complete task?

  • 0 in reply to MAHENDRA JADHAV
    Guru 140200 points
    Hi Mahendra,

    can you explain in detail what the circuit is intended to do?

    One improvement could be to route the signals in the connector P1 in a different way. The input signal and the output signal should not be routed next to each other. This can add unwanted stray capacitance between the input and output and can destabilize the circuit. So, I would route the signal ground between the input signal and the output signal.

    Also, you should add an isolation resistor to the output of LMP7721 to isolate the output of LMP7721 from cable capacitance and other load capacitances, like stray capacitances. e.g. Without isolation resistor the LMP7721 can become unstable.

    More, don't omit the supply decoupling cap! Connect a 100n cap, or similar, directly across the supply terminals of LMP7721.

    Kai
  • 0 in reply to MAHENDRA JADHAV
    TI__Guru 67601 points

    Adding to Kai's comments, since you attempt to measure I1 input current, there is no need for R1 resistor unless there is a possibility of high input voltage glitches (e.g. during power up) beyond the supply voltage, +V, and the input resistor is there to limit the input current during such fault condition.

  • 0 in reply to kai klaas69
    Prodigy 50 points

    Thanks for reply,

    As per your question, I design a circuit for nano ampere current biasing. The above shown circuit is not work for nano ampere i.e. unstable output.

    as per suggested improvements, I'll do that and verify earliest.

     

  • 0 in reply to MAHENDRA JADHAV
    Prodigy 50 points

    As per previous discussion, I test the circuit with improvements which shown below, but I cant bias nano ampere current. Please suggest...

  • 0 in reply to MAHENDRA JADHAV
    Guru 140200 points
    Hi Mahendra,

    if the current to be measured flows into the terminal "I1", then you will get a problem because this would cause the output voltage to go negative, which is not possible because the LMP7721 is powered by a single supply voltage. In this case you would need a bipolar supply voltage.

    10nA x 500k = 5mV. So, if your input current isn't much bigger, you could use the LM7705 to provide the negative supply voltage.

    What is your maximum measuring current? And which direction does it have?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points

    Hi sir,
    Actually, I1 is source current fed from Source and Measurement Unit and I want to bias current from 1nA to 50nA using LMP7721 and get the output interms of voltage which is in range of 0.5v to 3.3v. Now, can I done this task with LMP7721? if yes, How?

  • 0 in reply to MAHENDRA JADHAV
    TI__Guru* 93105 points

    Hello Mahendra,

    Be sure that you observe Kai's point about the direction of the input current. If you try to source current into the LMP7721, the output will try to move below 0 V. However, because the op amp is powered from a single positive supply the output won't be able to move below 0 V. You would have to use a dual polarity supply such as +/-2.5 V as Kai pointed out.

    Additionally, with the transimpedance gain set by the feedback resistor value, it could be increased considerably to expand the 1 nA to 50 nA input current range to a much wider output voltage. The output voltage is simply the input current multiplied by the feedback resistance. That 1 uF feedback capacitance is huge and will very much limit the bandwidth. If you don't need wide bandwidth, then it is okay.

    Regards, Thomas

    Precision Amplifiers Applications Engineering