[sdiy] mixer

[Scott Bernardi]

Yes, that's what I was saying - it always adds.  It's best to do without it if
possible. If you're using low input resistances anyway, it would take a pretty
big capacitor to filter the noise.
Also, if you just _have_ to use large input resistances (like with a high source
output impedence) you're better off using a FET input opamp even though they
have higher voltage noise, rather than a bipolar type.
A long time ago in my engineering days I wrote an article on opamp noise
calculations that got published in a British electronics engineering magazine.
If I can find the silly thing I can make it available.   National Semiconductor
also has an application note AN-104 "Noise Specs Confusing?".

"jh." wrote:

> > Also, tricks used for error cancellation in precision circuits, like
> having a
> > resistor in series with the + input that is the same as the equivalent
> resistance
> > seen by the - input (eg, Rf || Rin) is actually detrimental, because there
> is no
> > phase cancellation of noise between the + and - inputs - everything adds
> as RMS
> > (the square root of the sum of the squares of all the individual noise
> sources).
>
> This "trick" is for reducing offset voltage comming from input bias current,
> not for noise reduction. I think it even *adds* noise, so better bypass it
> with a capacitor.
>
> JH.

--
Scott Bernardi
sbernardi at home.net