[linrad] Re: Homebrew I/Q chains and Op-amps, compiling Linrad

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Roger and all,

> Around 2000 or so when I built my simple EME receiver to use with Linrad,  
> it looked like the AD797s had the best noise specs and so this is what I  
> used.
> 
> Now it seems that some of the cognoscenti tout the LT1128A and don't even  
> mention the AD797.
> 
> The typical noise at 1 kHz for the AD797 is 0.9 nV/sqrt[Hz] and that for  
> the LT1128 is 0.85, and max figures are 1.2 for the AD797 and 1.1 for the  
> LT1128, so the LT1128 is ever so slightly better in this respect.
> 
> The gain bandwidth of the AD797 is 110 MHz and that of the LT1128 is 20 I  
> think;  I guess that doesn't matter for our baseband amps.
It all depends on where the performance limiting factors are.

The low end is noise, but there is a high end also and 
second order distortion is usually the interference that
limits dynamic range. I think the AD797 has much lower
harmonic distortion than the LT1128. The LT1028 is better,
probably because it is faster.

> The slew rate of the AD797 is 20 V/usec and of the LT1128 is 5V/usec.
> 
> So my question is:
> 
> For use as the 'last stage' of amplification before the soundcard for a  
> system sampling at up to 192 kHz, are these two chips pretty much  
> equivalent given the [to me] tiny differences in noise, or is the LT1128  
> clearly superior, and if so, why.  If the LT1128 would have advantanges  
> under some conditions, for some projects, what would those conditions or  
> projects be?
> 
> If there is not a big difference in performance for this purpose, why is  
> the LT1128 currently the 'ultra low noise op amp of choice'?  I don't  
> think it is cost, as from Digikey they both cost 8-9 USD in single  
> quantities.

In case the signal source has a low output impedance, the noise
voltage becomes more important. On the other hand, if you need
a large output swing, linearity is more important. Those who
like the LT1128 might use it on a 3 ohm source (vinyl player)

Maybe a LT1128 followed by an AD797 that has only a small gain
(and takes care of the differential amplifier task that
references the output to the soundcard reference point)

> PS I got a really pleasant surprise yesterday when I tried to compile and  
> run Linrad 02.14 on an old P3 with a pre-Fedora version of Redhat Linux  
> installed and everything compiled and ran perfectly, including XLinrad!!
Linrad should be backwards compatible all the way to 
the 2.2 kernels and RedHat 6.1;-)

73

Leif / SM5BSZ

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