On Mar 24, 2006, at 7:41 AM, Steve Meier wrote:
Ideally true but in reality not true. Put you oscilloscope probe
tip to
the ground plane at a point further then the probes ground. More often
then not you will see some sort of noise. If the ground had zero
resistence and an infinite supply of available electrons this noise
wouldn't be there.
Not true. This usually isn't due to resistance. Taking skin effect
into account, at 100 MHz a copper ground plane has a surface
resistance of ~3 milliohms/square. On the other hand, a 3 cm diameter
loop has an impedance of ~50 ohms. Now this is just dimensional
analysis, but the mutual impedances that cause the crosstalk tend to
be proportional to these numbers. So at this frequency you can expect
crosstalk due to mutual resistance to be something like 80 dB below
the mutual inductance crosstalk for circuits of typical dimensions.
The reason you see this on the scope is that the ground lead -> probe
-> probe tip -> ground lead loop picks up the induction from loops in
the circuit.
Since ground is the return path of most circuits
(obviously not differential circuits) and two or more circuits can
have
return paths that cross each other on the ground there will be cross
talk on the ground plane. In the case of digital circuits which often
have high frequency clocks that clock itself can propigate through the
ground and power and into the analog circuits where it can then be
amplified causing distortion.
Propagate through ground? No! The fields in a copper ground plane are
very low. The interference propagates through fields surrounding the
circuitry. The most common problem is coupled loops, as in your probe
experiment.
I wish EE profs would teach their students that the user of
Kirchoff's Voltage Law is pledging to account for all of the
induction in the circuit.
So it is often recomended using two grounds which are connected at a
single point. One for analog one for digital. The question is often
where and how to connect the two grounds.
Wherever you have net current flow between the circuitry on the
separate grounds, you want them connected at that point to provide a
return current path. Wherever wish to accurately carry a single ended
voltage between the ground systems, you want them connected at that
point to provide a return reference. If you're serious about EMI,
only differential signals may be allowed to cross between the ground
systems without a return connection between the grounds at the point
where they cross. Even there, you should be careful about common mode
excitation and CMRR: it may be better to provide a common mode return
connection.
The purpose of this is to collapse the loops that are the source of
the trouble. The return current has a strong tendency to flow as
close to the signal current as it can, so giving it a way to be
really close to the signal can make the effective loop area very
small. By reciprocity, similar considerations apply to voltage
transport.
Alexander Graham Bell attempted to patent the "return next to signal"
idea in 1878, but in 1881 the US Patent Office ruled that David
Brooks had beaten him to it (although Bell was successful at
obtaining a patent in England).
If coupling is through mutual resistance, the "single point ground"
idea is a good one, because in this case the coupling must occur
through a common conductor. For precision measurements below ~1kHz,
this may be the main story. But for mutual inductance coupling, no
contact between circuits is required. It's usually better then to
"let ground abound": you're much more likely to get into trouble by
being too clever.
John Doty Noqsi Aerospace, Ltd.
[EMAIL PROTECTED]