From: "Phil Allison"
Subject: Re: Audio noise in diff amps
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Wed, 18 Dec 2002 19:25:55 +1100
NNTP-Posting-Date: Wed, 18 Dec 2002 19:14:14 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Don Pearce" wrote in message
> On Wed, 18 Dec 2002 16:13:28 +1100, "Phil Allison"
> >"Glenn Gundlach" wrote in message
> >> I work in video duplication and we're in the process of moving the
> >> equipment to a new building. One of the router inputs is 'black' and
> >> no audio. The router input is a differential opamp with 22K resistors
> >> to the inputs. In the past, the 'black' input was simply left floating
> >> but one of the guys said to short them together. FYI, the noise level
> >> floating is -74 dbm (Fluke 8060) with operating level at +4. SO, the
> >> question is: is it theoretically quieter shorted or floating? Is the
> >> Johnson noise of the input resistors significant? When the inputs
> >> float, those resistors are essentially not there so they can't
> >> contribute to the noise, can they?
> > ** The noise from a 22 kohm resistor in the audio band is about 2.4
> >rms or -110 dbm.
> > .............. Phil
> Make that -126dBm
** Huh ?? 2.4 uV divided by 775 mV = - 110.2 dB.
Note: From Don Pearce, the man who openly promotes flauting basic
electrical safety, any damn thing is possible.