AW: Re: Couple EE questions...

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Tue Oct 22 22:00:00 CEST 1996


>
> It does make a diffrence, not only from the obvious heat problems but also
> from
> the noise level.

Noise is an important point indeed. I underestimated the noise of large 
resistors
for a long time. Think of this: Using Low noise opamps, but 500k feedback
resistors all the time.
I was surprised when I looked into an old EMS octave filter bank: 741 
opamps,
but resistors in the 1.5k range for the filters ! These guys knew exactly 
what
they did !

>  By makeing wise selection of the resistors you can balance
> the
> termal noise and the current noise. You can teoretically find a optimal 
source
> resistance to each input of an opamp, if you stick close to this value 
(which
> is op-amp type dependent) you will minimize the noise figure. You migth 
want
> to
> stick in a resistor between the (+) input of the opamp and the ground when
> doing an inverter for this reason among others.

Please correct me if I am wrong, but I think the smaller the resistor, the 
better.
The lower limit would be set by other criteria like power consumption and
output drive capability of the opamp, and of course at a certain point
improovement would become smaller and smaller, but
does the noise floor really rise again at some point? I thought this whole
"noise figure" stuff works the *other* way round: select an *opamp* for a
*given* source impedance, and calculate the loss in SNR compared to
an ideal resistor of given impedance.  But decreasing the impedance
should always improove the SNR, shouldn't it ?

JH.



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