From: Ryan O'Connor <rocifier@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 8 Feb 2021 17:12:17 -0800
Subject: Re: [EE] Low noise attenuator
Thread-Topic: [EE] Low noise attenuator
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David do you have any theory or practice which can explain this? I would be
interested to read more about why this helps. I'm trying to read up on
Sean's topics and I'm finding it pretty complex. I have more questions than
before, such as what is the relationship between impedence matching to
reduce noise and impedence matching to gain maximum power? Do they happen
to co-inside, or how much out of phase are they (literally can't find
anything on the net about this)

Cheers
Ryan

On Tue, 9 Feb 2021 at 13:57, David VanHorn <microbrix@gmail.com> wrote:

> Use capacitive dividers.
>
> On Mon, Feb 8, 2021, 4:20 PM Sean Breheny <shb7@cornell.edu> wrote:
>
> > A fundamental property of absorbative attenuators (i.e. those which
> > dissipate the power removed from the signal) is that they introduce
> > (1-a)kTB thermal noise in watts, where a is the attenuation factor, k i=
s
> > Boltzman's constant, T is kelvin temperature of the attenuator, and B i=
s
> > the downstream circuit bandwidth in Hz. So, for example, a 10dB
> attenuator
> > (a=3D0.1), introduces 90% of a full equivalent of thermal noise.
> >
> > It is possible to attenuate reflectively (i.e., produce an impedance
> > mismatch) and that can often circumvent this limitation although it
> depends
> > on the source characteristics. There is a common practice in low noise
> > amplifiers call "noise matching" where instead of matching the impedanc=
e
> of
> > the source to that of the amplifier input, a purposeful mismatch is
> > introduced which reduces the contribution of noise due to the real part
> of
> > the amplifier's input impedance. I think the fundamental principle is t=
he
> > same.
> >
> > Sean
> >
> >
> >
> > On Mon, Feb 8, 2021 at 5:00 PM Ryan O'Connor <rocifier@gmail.com> wrote=
:
> >
> > > Hi, I am curious about a general topic which mostly interests me in t=
he
> > > area of sensitive audio signals.
> > >
> > > What options do I have for attenuating a copper line signal while
> > > introducing a very low amount of noise? For example, if I grounded L+=
R
> > > channels using independent resistors would that work? OR do I need a
> > proper
> > > high-pass or low-pass filter design? But then what about
> > > capacitors/inductors or series resistors introducing thermal noise?
> > >
> > > Open to discussion on types of noise as well :)
> > >
> > > Ryan
> > > --
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