[sdiy] The TL072 , part 2

[Seb Francis]

Ian Fritz wrote:
>
> 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!
>

While I agree 100% that TL072 is a bad choice for precision CV summer, 
the input offset voltage spec (which is what was being talked about) 
doesn't make any difference in this case as it is simply cancelled by 
the tuning offset.  What is important here is the temperature 
coefficient of the input offset voltage.  And this is the same across 
all grades of TL072.

So to answer Raph's question: for SDIY use it is almost certainly not 
worth paying extra for a higher grade TL072, although for some 
applications it is worth paying extra for a different opamp altogether.

I tend to use MXL1013/LT1013 for precision CV summers.

Seb