[time-nuts] Performance of 74LVC series ICs
kb8tq at n1k.org
Tue Jun 9 07:05:07 EDT 2015
The isolation in the package is likely better than the (practical) layout you will
do to mate up with them. In fact, the single gate stuff probably does a better job
of isolation than the multi gate stuff, simply because you can spread it out on
In the case of dividing by two, there are single gate flip-flops that are Q bar output
rather than Q. That eliminates the second single gate package in this design. Yes, there
are far to many different numbering systems. Finding this or that can be a massive
If power is an issue, the real trick is to find a family that is happy running on a low(er)
supply voltage. Some of this stuff will toggle at 20 MHz with very low supply. Often
the inputs are “high voltage tolerant” even with those low supplies.
> On Jun 8, 2015, at 8:30 PM, Dan Watson <watsondaniel3 at gmail.com> wrote:
> 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
> 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.
> On Mon, Jun 1, 2015 at 6:13 PM, Andy <AI.egrps+tn at gmail.com> wrote:
>> The gates on that page
>> 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.
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