I am currently reading a book on VLSI, the authors of which are university lecturers. One presented as a project scientist and the other one is proudly credited with having over five years of industrial experiences, and it certainly shows.
On differential signalling they state that , given the source and load impedances are equal, both conductors are induced with identical electromagnetic interference, and since the receiving circuitry only detects the difference between the two conductors, the technique rejects the noise.
Now, this is not only a very lousy description, as it implies that somehow keeping both the source and load impedances equal is the thing to do for digital signal transmission, but also clearly shows that the authors desperately lack knowledge on CMRR.
As we all know keeping the source and load impedances equal gives you the perfect power transfer but with 6dB signal amplitude attenuation, and this is certainly not the way to transmit digital signals let alone analogue. And for CMRR you certainly do not want high source impedance either.
I am not aware of any benefit from silver paint, it may kill germs.John, are you saying that painting my ICs with magic silver audio paint doesn't keep the electron in the signal path and doesn't prevent errors? All that money... Now, I'll have to buy mercury filled speaker cables.
Exactly. What you need is the special green Sharpie that you run all around the edge of your CDs to stop bad photons getting in.John, are you saying that painting my ICs with magic silver audio paint doesn't keep the electron in the signal path and doesn't prevent errors? All that money... Now, I'll have to buy mercury filled speaker cables.
After re-visiting my post it seems I have been a bit economical with what's been said by the authors and missed an important point. They do mention that the differential scheme works on twisted pair lines like RS232, but they also include audio which is not correct, unless of course, and as you mentioned, one decides to transmit audio over 35,000ft using wires at 20kHz. At the same time I am also partially wrong with my comments...................
I know from my experience teaching about balanced interfaces and CMRR that the subject is widely misunderstood by professors and in many textbooks. In the example cited earlier, equal induced common-mode voltages (presumably from an external magnetic or electric field) cannot be assured by simply matching impedances - cable construction (such as twisting conductors) is also a major factor - as is the form of shielding, if used. A good example is foil shielding with a "drain" wire - which has been shown to be a serious problem at audio frequencies when current flows in the shield.
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