[time-nuts] Performance of 74LVC series ICs

Dan Watson watsondaniel3 at gmail.com
Mon Jun 8 20:30:15 EDT 2015


I have something of a follow up question. How good is the isolation inside
these devices (74LVC, SOT-23 package) between gates?

Let's say I have a 20MHz TCXO. I want to square up the output signal and
divide by two. Easy, just a buffer or inverter and a flip flop. But looking
at the pinout of the 74LVC1G175 (D flip flop) it doesn't have a Q not
output. So now I need a second inverter to make it toggle. The 74LVC2G14
includes two schmitt inverters in the package, but will isolation inside
the device be good enough to use it for two separate functions at 20 and 10
MHz?

Just from a layout perspective using three devices instead of two would be
easier. However the thing will be battery powered, so I'd like to save the
power if possible.


Thanks

Dan

On Mon, Jun 1, 2015 at 6:13 PM, Andy <AI.egrps+tn at gmail.com> wrote:

> The gates on that page
>
>    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html
>
> use bipolar transistors.  The 74LVC parts are CMOS.  There are various
> effects caused by that difference.
>
> And those examples have vastly inferior control over input switching
> levels, compared to just about any well made digital IC from the last half
> century.  (Funny to think that it has been half of a century!)
>
> 2N2222 type transistors might have switching delays upwards of 100 ns
> (depending on load), whereas the LVC parts switch in the 1-5 ns range.
>
>    "On the other hand: A well designed discrete circuit can beat a general
>     purpose integrated circuit in almost all performance measures."
>
> Some performance metrics would be hard to beat with even a well designed
> discrete circuit.  On-die capacitance and inductance tends to be much
> smaller than any discrete circuit can achieve.
>
> Andy
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