AW[n]: Re[2]: LF411 Op-Amp dilemma

[brad sanders]

>i have downloaded the datasheets for tl071 and tl081. and both have Vn =
>18nV/^Hz and In = 0.01 pA/^Hz. so i ask why the 071 have lower noise than
>the 081?

Probably "guaranteed noise specs" and 1/f curves. The AD711 is also a
"low noise" upgrade to the TL081, and it has an eInoise at 10KHz of
about 16nV/^Hz - vs 13nV/^Hz in the TL081!

A very relevant factor with FET opamps is 1/f curve. All opamps
increase in noise at lower frequencies. The point where noise begins
rising is known as the "1/f curve." Ideally noise will start
increasing below "audible frequency," like 10Hz - but most FET opamps
begin doing so at 1KHz or higher. If you'll look at detailed noise
specs in the National Semi databook (for example), you'll see the
TL081 has about 13nV/^Hz noise density at 10KHz - but as much as FORTY
nV/^Hz below ten Hz!


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