[time-nuts] Frequency divider design critique request
bruce.griffiths at xtra.co.nz
Wed Jul 16 00:59:27 EDT 2008
John Miles wrote:
> Right; when the thumbswitches are toggled, the RC integrators will slow down
> the edges into pins 9-11. Sometimes CMOS parts will latch up or otherwise
> fail to reliably with slow edges -- it probably comes down to the
> "complementary" thing, where both halves of a totem pole can turn on
> erratically during a slow transition.
> I have seen large capacitors used on TTL MUXes for EMI suppression, but
> never on CMOS. I have a feeling 100 pF would be safer and still adequate.
> Probably no big deal, but if you want to be anal about it, it could pay to
> check the edge-time specs for the HC family.
> -- john, KE5FX
During a transition a CMOS inverter acts as an amplifier, the gain can
be high with a buffered input where 2 or more inverters are cascaded.
Thus any noise riding on a slowly slewing input can be amplified
considerably as the input signal passes through Vcc/2 where a CMOS
inverter small signal gain is maximum.
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