Connect with us

LMC6462AIM:- voltage offset issue

Discussion in 'Electronic Design' started by Riscy, Oct 20, 2005.

Scroll to continue with content
  1. Riscy

    Riscy Guest

    I was test a board containing LMC6462AIM as high impedance buffer, +ve
    pin to 1K to 0V (for test), wire link between -ve to output
    (gain=1V/V). Output load 100K. The power is +/-5V from linear bench
    PSU. The room temp is 25C and the PCB is very clean, no residual flux
    or dirts.

    Now using precision DVM, I measure 0.0mV at the +ve pin. I then measure
    the output and found to be -2.1mV.

    According to the datasheet the maximum offset should be 1.2mV and
    typical being 0.6mV, which is why I choose this device. I was
    surprised to measure this. I tried different DVM and the result is the
    same, the voltage offset exceed the manufacture.

    I tried other op-amp within the same batch and it has similar results.
    It must be bad batch but it leave me wonder how they occurs, why
    National fails to rejects them and how to avoid this happening again.
     
  2. Put a scope on it, see if there is a little fuzzy
    oscillation that the DVM is averaging to a dc.

    Conversely, if the dc offset gets better when you
    put the scope on it then you *had* an oscillation.

    If there was an oscillation, try a 100uA to 1mA
    pulldown current on the output stage.
     
  3. Jon

    Jon Guest

    Riscy,
    The input offset voltage Vos is defined as the differential voltage
    that must be applied to the input to achieve 0V output. However, the
    Output voltage that appears with 0V input is equal to:
    ~
    Vo = Vos(1 + Acl)
    ~
    Where Acl is the closed loop gain of the amplifier. In your case, Acl
    = 1. So the maximum output voltage that you can expect is:
    ~
    Vo = 1.2(1+1) = 2.4.
    ~
    This is in the range that you would expect for this device.
    Regards,
    Jon
     
  4. Jon

    Jon Guest

    Riscy,
    Oops! My previous reply was wrong. The equation does not apply to the
    unity gain buffer.
    Regards,
    Jon
     
  5. John Larkin

    John Larkin Guest

    Darn, I was getting ready to jump all over that one.

    John
     
  6. Jon

    Jon Guest

    John,

    You'll probably get your chance after my next post :)

    Regards,
    Jon
     
  7. Riscy

    Riscy Guest

    Jon,
    Just as happen when I read you 1st message...I nearly dropped by coffe
    and missed my heart beat until I saw the 2nd message(!)...whew, you
    really had me knotted.

    By the way, I found there is no ac on waveform, it pure DC.
     
  8. Riscy

    Riscy Guest

    Okay...no response since the question, I try AD822 or similar device
    with lower offset and see the difference AD may have better quality
    control than NSC but that only the guess.
     
  9. In your last post you said there was no AC, only DC. Were you measuring the
    DC output with the precision voltmeter, at the same time that you were
    looking at it with the scope? Pay attention to Tony Williams' post,
    "Conversely, if the dc offset gets better when you put the scope on it then
    you *had* an oscillation."

    You're powering it from a bench supply, presumably several feet of test lead
    away from the circuit. Do you have bypass capacitors around the opamp? The
    LMC6462 has a CMOS output stage; the AD822 is bipolar. That will tend to
    make the LMC6462 less stable, I think, especially if the load is at all
    capacitive.

    Despite your claim that there is no AC, there must be at least some, because
    of noise. The LMC6462 is quite noisy, around 100nV/rt-Hz between DC and 1k.
    The AD822 is quieter, at around 20nV/rt-Hz in that range. But even the
    AD822 is specified to have 2uV of broadband noise between 0.1Hz and 10Hz, so
    the LMC6462 is probably worse; how long did you average your measurement
    over?

    Did you wait for the opamp to temp-stabilize before taking your
    measurements?

    "There is no such thing as DC."
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-