# Opamps DC Offset Errors

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

1. ### masa6614 Thread Starter Active Member

Nov 9, 2008
48
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.

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Last edited: Mar 17, 2009
2. ### SgtWookie Expert

Jul 17, 2007
22,201
1,806
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
48
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