From: Don Pearce
Subject: Re: Audio noise in diff amps
Date: Wed, 18 Dec 2002 08:30:46 +0000
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On Wed, 18 Dec 2002 19:25:55 +1100, "Phil Allison"
>"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.
> ....................... Phil
Power is v squared / r.
2.4 uV in 22k ohms is -126dBm = (10 log((2.4e-6)^2 / 22000 + 30), or