[sdiy] The TL072 , part 2

[Raph]

Thanks David,
 My ultimate goal here is to try to get a handle on whether or not it is
worth paying the extra buck for an op amp that has a 1000 uV offset vs one
that has a 3000 uV one.  

Cheers,
Raph
-----Original Message-----
From: David G. Dixon [mailto:dixon at interchange.ubc.ca] 
Sent: Thursday, February 26, 2009 6:10 PM
To: 'Altitude'; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] The TL072 , part 2

First, common-mode rejection ratio (CMRR):

An ideal op-amp responds only to the differential voltage applied across its
input terminals, and not to common voltages applied to both terminals.
However, in real op-amps, the two inputs are subject to slightly different
gains, and so common-mode voltages are not entirely rejected.  CMRR is
simply the ratio of the gains of the differential and common-mode voltages.
It should be very large.

In synth circuits, it often makes very little difference, because we
typically use op-amps as inverters rather than differential amps.  When one
input is grounded, the common-mode voltage is zero by definition, so there
is nothing to reject, strictly speaking.  CMRR is only important when
op-amps are used as differential amplifiers, but even then I doubt that most
applications in synths are sensitive enough to the small errors associated
with finite CMRR to worry about it too much.

Second, input offset voltage, or Vio:

An ideal op-amp gives an output of exactly zero volts when its input
terminals are shorted together.  Real op-amps do not.   Real op-amps require
some small non-zero differential voltage to give zero output.  This small
input voltage is Vio.  This is typically important for high-gain dc
applications.  I believe it can be safely ignored in most ac applications.
If it is a problem, then one input or the other can be trimmed (i.e., have a
small voltage added to it through a trimmer) to eliminate it.

As usual, if I've put my foot in, one or more of the gurus will correct me!

I also suggest you do a Google search for these terms.  They all have very
informative wiki pages, usually from several sources, some with simulations
you can run to see the effects for yourself.

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Altitude
> Sent: Thursday, February 26, 2009 6:43 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] The TL072 , part 2
> 
> Can someone explain the various specs for the different versions of these
> (or any) opamps and how they relate to analog/synth circuits? Namely:
> Common
> Mode Rejection Ratio and Input Offset Voltage
> 
> Cheers,
> Raph
> 
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