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LMV771: Specs and Distribution, offset voltage and bias current

Part Number: LMV771
Other Parts Discussed in Thread: OPA350

Dear Specialists,

My customer is considering LMV771.
He has question about deviation of some specs.

Could you please advise?

---questions---

Acccording to the Lecture Manual of TI Precision Labs of Op Amps(P.5),
typ value of Offset Voltage and TCVos is within +/- one sigma.
The device is OPA350.

Could you please see attached file.

(1)Is the bias current specification ± sigma like the offset voltage?

In case of LMV771, Vos(typ) is 0.3V, TCVos(typ) is 0.3V,Input offset current(typ) is -0.1pA and Input offset current is 0.004pA,
these specs are ± sigma. Are these correct?

(2)Which is LMV771, CMOS or Bipolar?

A post of E2E listed below says, calucration to total Vos is different from CMOS and Bipolar.

CMOS is root mean square, Bipolar is SUM.

(3)When I also consider bias bias current, could I think same as (2).

---

I appreciate your great help in advance.

Best regards,
Shinichi

Offset Voltage Specs and Distribution.pdf

  • Shinichi,

    Please see the answers below:

    (1) Is the bias current specification ± sigma like the offset voltage?  

    Yes.  IB=0.1pA represents one-standard deviation at Vs=2.7V (see below) but it is NOT tested in production.  IB also varies significantly over supply voltage; for example, IB typical is 0.23pA at Vs=5V

    In case of LMV771, Vos(typ) is 0.3V, TCVos(typ) is 0.3V,Input offset current(typ) is -0.1pA and Input offset current is 0.004pA,

    these specs are ± sigma. Are these correct?

    Yes, except that at Vs=2.7V TCVos typical is 0.45uV/C - see below.

     

    (2) Which is LMV771, CMOS or Bipolar?

    LMV771 is CMOS input op amp - most bipolar op amp would have 1nA<IB<1uA

     

    A post of E2E listed below says, calculation to total Vos is different from CMOS and Bipolar.

    CMOS is root mean square, Bipolar is SUM.

    Yes, that is correct. Since in CMOS op amps Vos and TCVos are NOT correlated you must use root-mean square function to add them up (like vectors).

    In bipolar op amps, TCVos=~Vos[@25C]/300C so they are correlated quantities and for that reason you need to add them up linearly.

    (3) When I also consider bias bias current, could I think same as (2).

    I am not sure what exactly do you want to add in case of the input bias current?  But whatever it is, if the quantities are correlated to one another, you must add them up linearly and if they are NOT corraleted you must add them up using root-mean square function.

  • Hi Marek

    Thank you for your reply.

    Before sending to the customer, I'd like to confirm.
    Could you please advise?

    ---questions---

    (1) Despite Vos at 2.7V is 0.3V means one sigma, max value is 0.85V, less than three sigma(0.9V=0.3*3).

    How can I think about it?

    I think max value should be around three sigma(0.9V)

    (2)You mentioned LMV771 is CMOS input OP Amp.

    In this case, which way should be used for Vos calcurate, RMS or SUM?

    ---

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi
  • Shinichi,

    You are correct that typically maximum values is set by 3 or even 4 sigma but not always.  Since all units get tested for Vos in production, marketing may decide on tighter limit at the expense of lower yield.  It seems that the reason for Vos thighter limit at 25 deg C was driven by desire to set LMV771 maximum Vos limit over temperature at +/-1mV.

    When it comes to Vos, only in bipolar input op amps there is correlelation between Vos and Vos drift.  Therefore, since LMV771 is CMOS, Vos and drift are uncorrelated and you must use RMS to add them up.

  • Hi Marek

    Thank you for your reply.

    I understand the situation.

    I'll share this information with him.

    If he has an additional question, I'll consult you.

    I appreciate your great help.

    Best regards,

    Shinichi