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Current Sense Range of 10 nA

SANG YUN BANG
Prodigy 60 points
Other Parts Discussed in Thread: LMC6041, OPA381, LMC6061, LMC6081, LMC7101, LMC6482

Hi, 

I am trying to sense the current from our chip by using transimpedance amplifier. And we have the -5V to +5V power supply option on our PCB board.

The current range will be around 10nA. I was going to use OPA129, however, there was some issue with a power supply of 15V. 

Is there any recommendation for other amplifier or circuit design to sense the nanoamp current? 

I am attaching basic block diagram for the question. 

Thanks!

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While I was searching some components, I designed the circuit. However, I am not sure to verify to sense 10nA range.

Any suggestion for this circuit design?

Thanks,

  • 0
    TI__Mastermind 25805 points
    Hello Sang,

    Please refer to the following article series authored by my colleague Paul Grohe.

    www.edn.com/.../Design-femtoampere-circuits-with-low-leakage-part-one

    Given your supply voltages you may want to consider the LMC6041.
  • 0 in reply to Pete%20Semig
    TI__Guru 64290 points

    Sang,

    OPA129 is NOT rail-to-rail input nor output voltage (far from  it) and this is most likely what caused the problem using +/-5V suppies - see below.

    The minimum input common-mode voltage is specified 5V from either rail so it would only work properly on your supplies if the input voltage was around 0V.

  • 0 in reply to Pete%20Semig
    Prodigy 60 points
    I truly appreciate your reference. While let me review the datasheet, would you verify the design that I updated?
  • 0 in reply to Marek Lis
    Prodigy 60 points
    Yes, I realized there was misunderstanding on supplies. Meanwhile, I updated one design by using OPA381, do you have any suggestion on this design?
  • 0
    TI__Guru 53846 points
    Hi Sang,

    The LMC6041 is the low power device of the LMC6xxx family. The LMC6061 is the precision low power version. Use these if you need to save power.

    The LMC6081 is the "normal" power precision version with lower noise and lower offset.

    If you need R-R input, the LMC7101 (single) and LMC6482 (dual) are the Rail to Rail input and output devices of the LMC family.

    Since the LMC CMOS series uses an a older, larger geometry and less aggressive ESD devices, the leakages are typically very low, typically in the teens of femto amps right out of the sealed rail/reel.

    All the LMC6xxx's are 15V max supply with R-R output - so the supplies can be ±7.5V max (or bend the supplies to favor your measurement range). They should all run happily at ±5V.

    Just remember to remove any bypass caps from the ASIC, or add a resistor in series before the cap, otherwise the TIA may go unstable due to large capacitance directly on the summing node.

    Thanks for the plug, Pete!

    Regards,