[time-nuts] Performance of 74LVC series ICs

David McGaw n1hac at dartmouth.edu
Mon Jun 8 23:16:12 EDT 2015


One thing that is hidden in AC and later CMOS is very tightly controlled 
edge-rate to combat ground bounce.  The original AC components were so 
fast, the ground bounce could be measured in volts and they had to be 
quickly redesigned.

For the D-FF function, you might consider using one section of the dual 
74LVC74.  With the inputs of the unused section connected to ground or 
Vdd, it will draw no power.

David N1HAC


On 6/8/15 8:30 PM, Dan Watson 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
> 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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