Logic Datasheet Question

here's the link to the datasheet i was looking at:

V_IH is the input level at which the output of a logic gate goes high.

For a supply voltage of 4.5V 3.15V is the lowest voltage which can be guaranteed to generate a high at the output. 2.4V is tytpically adequate, which is to say that with about half the examples of the circuit that you buy, 2.4V will be enough.

If you go to the V_IL specification you will be told that 1.35V at the input will never give you a high at the output (for a syupply voltage of 4.5V) and that 2.1V is - in half the cases - enough to give you a low at the output, and either a high or something in the middle in the rest of the cases.

You need to settle down for a minute and think about what the specification is telling you.

-- Bill Sloman, Nijmegen

The rule of thumb for CMOS is...

Below 0.3*VDD = a zero

Above 0.7*VDD = a one

BUT... there will be considerable "overlap" current unless you are within less than a VTH from either rail. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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"For a supply voltage of 4.5V 3.15V is the lowest voltage which can be guaranteed to generate a high at the output. 2.4V is tytpically adequate"

I must be missing something hear because... how does that make sense?

3.15 is the lowest which can be guaranteed a high at the output.... but typically people ignore this and use 2.4? Which is way lower than 3.15? Why would the typical value used, be soooo much lower than the lowest guaranteed voltage? Why would the manufacture even say something like what your implying.... you're saying that TI is saying, we guarantee anything equal to and above 3.15V to work... but typically people ignore that and use 2.4V

???????????????????????????

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

thanks Jim, but my confusion is really with trying to understand why the datasheet calls out the typical value as less than its minimum value (for V_IH).... it don't make sense to me

[snip]

Elaborating:

INPUT Below 0.3*VDD = a zero at INPUT = a one OUTPUT, Guaranteed

INPUT Above 0.7*VDD = a one at INPUT = a zero OUTPUT, Guaranteed

Typicals can get confusing...

Everything perfect (with VDD=+5V)...

2.6V Input will give a zero output

2.4V Input will give a one output ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

e

This isn't about what people do - which is jam the inputs as close to the positive or negative rail as they can get them - but about what the parts do in response to a particular input voltage.

Typical values aren't used. They just tell yu what the part does as the input voltage swings from one rail to the other.

No.

-- Bill Sloman, Nijmegen

The manufacturer guarantees that any input voltage above 3.15 volts will be considered a high. However, the actual switching threshold on most parts will be around 2.4 volts. This is _not_ a recommendation to drive the chip with 2.5 volts, and expect that it will respond as if that is a high.

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Peter Bennett, VE7CEI  
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