[hpsdr] Low noise op amps

[Ahti Aintila]

Hi Lyle,

Very good analysis of the noise!  The superior choice would be LT1128A
and we may need it when using lower noise solutions for the QSD as
suggested and tested by Leif Asbrink and Phil Harman.

However, opamps like 5534A and OPA227 will match pretty well with the
present QSD used in the SDR-1000. Instead of the NE5534A my choice
would have been OPA4227 that has 4 x OPA227 in the same 14-lead
package. The noise specs are following:
Input noise voltage density, f = 10 Hz, 3.5 nV/sqrt Hz
                                             f = 100 Hz, 3 nV/sqrt Hz
                                             f = 1 kHz, 3 nV/sqrt Hz
Current noise density,          f = 1 kHz, 0.4 pA/sqrt Hz

My suggestion: keep the layout as is for the LT1128. If anybody wants
to use other solutions, a small "daughter board" may be inserted on
top of the present layout.

73, Ahti OH2RZ


On 05/06/06, Lyle Johnson <kk7p at wavecable.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> > 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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