[hpsdr] Low noise op amps

[Lyle Johnson]

> If you guys decide that one of the lower cost op amps will do, does that 
> mean that the LT1128s will _/not/_ work on an updated Janus board, e.g., 
> the layout changes? I’m wondering if ordering the fancy ones would be 
> throw away if the design went with the less expensive ones. 

No, the LT1128 and the less expensive parts are pin compatible.  The 
essential difference is noise, with the LT parts rated at about 1.1nV 
per root Hz, while the more expensive parts are in the 3 to 4 nV range. 
  The former is in the range of thermal noise at room temperature of a 
50 to 100 ohm resistor.  Since the input impedances (and probably drive 
impedance from, say, a QSD) are much higher, it seems to me that the 
ambient (thermal) noise will likely mask the advantage of the LT parts. 
  AKM also show the use of the higher noise parts in their application 
circuit for 600-ohm systems, so it seems likely in our typical 
applications that the LT1128's noise performance simply isn't needed.

But I could easily be wrong, and besides that, I don't want to toss out 
my LT1128s either :-)

73,

Lyle KK7P


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