Am 07.11.2015 um 17:24 schrieb Colin Law:
>>
>> *BUT* this only works when the host ground is *not connected* to earth,
>> not even by a noise filter.
>> As soon it is, your circuit isn't symetrical
>> anymore as one of your two data wires (the "ground") is tighter coupled
>> to earth than the other, and subsequently picks up noise more eagerly.
>> Which destroys the loop's cancellation effect.
>
> Don't follow that. We are talking about series mode inductive loop
> pickup not common mode (or at least I am). Provided it is only
> earthed at one point (which should be the case for all 1-wire
> circuits) I don't see it makes any difference to the series mode
> pickup. I am not an expert in this area by any means though so am
> happy to be further educated.
>
My expert times in Kirchhoff exercises are a bit dated, too.
noise
------>
__ +---------+
1.5k pullup / \ | |
+-----/\/\--------(----------------+--+ |
| | \__/ | |
/|\ | / |
| | | |5v ------> \ load |
\|/ | __ / |
| v / \ | |
+---------+-------(----------------+--+ |
| \__/ | |
| === ===
| | |
+------------------------+---------+
The cable capacitance on the ground wire directly shortens the noise
source while the cable capacitance on the DQ wire only shortens the
noise source through the pullup resistor.
So for the same influx, the noise voltage on the ground line is much
smaller than on the DQ line.
Kind regards
Jan
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