On 01/10/2016 11:32 AM, Attila Kinali wrote:
Moin phk!
On Sat, 09 Jan 2016 22:56:27 +0000
"Poul-Henning Kamp" <p...@phk.freebsd.dk> wrote:
Single gate chips better than multi gate chips.
(no interference through the power supply of the different sub-parts)
Would paralleing multiple gates in the same chip make things
better or worse ?
Good question. I have no idea.
My first guess would be, that it would only give a slight improvement,
if at all.
The reasoning is the following:
Under the assumption that the noise of all gates is ergodic and stationary,
then averaging the outputs of the gates should reduce the output noise.
But the noise is not truly ergodic and there will be coupling between the
gates (both through the power supply and the outputs), that will change the
noise properties of the gates. Which in turn might lead to positive interference
of the noise, instead of averaging out.
But I have to admit that noise in electronic circuits is for me still
something very unintuitive. And I am more often wrong than right, when
it comes to predicting noise behaviour.
Signals couples so nicely through power-pins, as the parasitic
inductance work on them. This is also known as ground-bounce. It will
limit the properties, and already have. In some counters, it was
"convenient" from a layout perspective to use a double-comparator. The
ground-bounce formed one of the main limiting factors, so in the next
generation they used single comparators and a few other tricks and could
half the noise-limits of the counter.
Few "chips" (rather chip packages) includes internal decoupling caps,
but it has started to appear for some of the larger onces as it is the
only way to avoid the issues while pushing speed upwards.
If you feed the same signal to the different XOR gates in the same
package, wiring might cause some spread, but ground-bounce would connect
them such that "late" gates would be encouraged by "early" gates. It
will not be so "independent" anymore.
Sometimes this cross-talk can work for you, but often against you.
Unless you know exactly what you are doing, isolation is a good thing.
Cheers,
Magnus
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