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LMP2234: Measuring input bias current

Gregory Duperon
Expert 6280 points
Part Number: LMP2234

I have a customer designing with the LMP2234.  They are interested in measuring the input bias current for the part.  Do we have any recommendations?

In addition, they are interested in additional ESD protection on the amplifier inputs.  How will this affect the input impedance?

Thanks.

  • 0
    TI__Guru* 93105 points
    Hello Gregory,

    The subject of measuring the input bias current of ultra-low input bias current op amps such as the LMP2234 has been discussed before on this e2e forum. Please access the following link for more information:

    e2e.ti.com/.../710223

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Expert 6280 points
    Thanks, Thomas.

    Customer wants to add additional ESD protection to the inputs. Can you comment on how this will input input impedance?
  • 0 in reply to Gregory Duperon
    Guru 140200 points
    Hi Gregory,

    this can become really tricky, if you want to keep the resulting input bias currents ultra low. You can use an ultra low leakage diode like the BAV199 e.g. and clamp the overvoltages to the supply rails. This works best when you slow down the slew rates of ESD by the help of a low leakage cap of 10...100nF. In any case a decoupling cap must be mounted directly across the supply terminals of OPAmp.

    Kai
  • 0 in reply to Gregory Duperon
    TI__Guru* 93105 points

    Hello Gregory,

    The LMP2234 has built-in ESD protection on all pins. A special leakage current canceling technique is used with the input pin ESD protection cells. This internal circuitry assures that the LMP2234 femptoampere level input bias current doesn't become overwhelmed by its ESD cell leakage currents. Therefore, adding external ESD circuitry will almost assuredly destroy the ultra-low input circuit current that the LMP2234 can attain in a good, clean, low-leakage circuit environment.

    If the customer does decide to add external ESD protection, then the impedance of that circuit will shunt the LMP2234 extremely high input impedance, reducing it to that presented by the external circuit.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0
    TI__Guru 51396 points

    Hello Gregory,

    As Thomas mentioned, the internal ESD protection cells have leakages lower than anything you can add externally (economically).

    If the input is truly "high-impedance", if for example, >1Gohm, then adding a few kilo-ohms of series resistance to the Gohms will make little difference to the signal, but it will make a BIG difference in the ESD performance.

    Adding the series resistance helps the internal ESD cells by reducing the peak current the cells need to clamp. Reducing the peak current also reduces the peak voltage across the input.

    it is common to add a 10k-100k resistor in series with the input if the input is expected to go "outside" the board.

    Of course, the resistor needs to be able to handle high voltage. A little 0204 SMT resistor is probably not going to do it...metal film or carbon in a larger packages is good. The capacitance across the resistor (~0.5-1pF) is where a portion of the fast-edge (HBM) ESD event  passes., so it is also a good idea to break the resistor up into several series resistors to divide the capacitance *across* the total resistor by 'n' (two 5k's, three 3k's - actual value or tolerance is not important - what ever is convenient).