Opamps DC Offset Errors

Discussion in 'General Electronics Chat' started by masa6614, Mar 17, 2009.

  1. masa6614

    Thread Starter Active Member

    Nov 9, 2008
    49
    0
    Hi,
    I'm trying to come up with a accuracy range estimate of a measurement circuit. Now my input analog voltage is in the range of 0-5V, and i'm putting it through a buffer and then some further circuitry, but i'm stuck on this first stage, i found this nice app note ( http://ww1.microchip.com/downloads/en/AppNotes/01177a.pdf ), that on page 5, fig 13 is what i'm looking at. Says the error from a buffer is composed of Vost which is shown in Equation 3, I cant seem to figure out where they get ΔVdd, ΔVcm, or where i myself would get them in my circuit. My 10V supply is a Regulator with 5% error, does that mean ΔVdd is 0.5V? what about ΔVcm? Because i added the 1k resistors (1%), would that further errors? any help is appreciated, thanks.
     
    Last edited: Mar 17, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Vcm is the common mode input voltage (average of VPLUS and VMINUS)
    (under CMRR, page 2)

    Since the LT1013 has a minimum input impedance of 70 megohms, neither of the two 1k Ohm resistors shown in your schematic are necessary.

    The LT1013 also has a PSRR of at least 97dB. You'd be hard-pressed to detect that small of a change without some pretty expensive equipment.
     
  3. masa6614

    Thread Starter Active Member

    Nov 9, 2008
    49
    0
    Thanks, still a bit confused??,

    so since my Vminus is Vout (which in turn is Vplus), ΔVcm would should be the avg. (5V + 5V)/2 = 5V?,
    i dont see how they got the (4.3V -(-.03V)) for ^Vcm in example 1?

    and with a PSRR of 100db ~100kV/V
    ΔVdd/PSRR = (0.5V)/(100kV/V) ~ 5uV of error ?
    which all seems small and negligable like you were saying...
     
    Last edited: Mar 17, 2009
Loading...