[sdiy] The TL072 , part 2

[David G. Dixon]

Excellent advice!  Thanks, Ian.

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
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Canada
 
Tel 1-604-822-3679
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> -----Original Message-----
> From: Ian Fritz [mailto:ijfritz at comcast.net]
> Sent: Thursday, February 26, 2009 4:39 PM
> To: David G. Dixon; 'Altitude'; synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] The TL072 , part 2
> 
> At 04:10 PM 2/26/2009, David G. Dixon wrote:
> >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.
> 
> 
> True as far as it goes.  The equation is
> Vo_error = Voff ( R2 / R1 + 1 )
> 
> Look what happens when the gain is small.  The offset appears directly at
> the output!
> Vo_error ~= Voff ( 0 + 1 )
> So if you are attenuating to a small signal level, then the offset voltage
> can be relatively large wrt the signal.  This happens in the input summer
> of an expo converter if you use a 100k input resistor and a 2k tempco as
> the feedback resistor.  This is why I always use an OPA2227 or OP27 at
> this
> point.  An offset there leads directly to a temperature drift!
> 
> http://www.ecircuitcenter.com/Circuits/op_voff/op_voff.htm
> http://www.ecircuitcenter.com/Circuits/op_voff/op_voff.htm
> 
>    Ian