HC14 (and HC132) Schimitt inv threshold variations

[Styx]

If you look at the trip threasholds for the Schmitt triggers, over temperature they do not vary that much (~100mV). BUT the biggest variant is between chips

reading the datasheets it puts the min threshold at say 1.2V and the max at 3.1V

IF you get a 1.2V chip then across temp it would be 1.1 to 1.3V
but the next chip could be at 3.1V!!!

is there any hysteresis chip with a more consistant level? (comparator-based schmitt not really an option)

 

[Hero999]

A 555 timer?
http://www.kpsec.freeuk.com/555timer.htm#buffer

 

[audioguru]

Cmos Schmitt-trigger inputs are not designed for accuracy, only for speeding up the transitions of slow signals. The thresholds also change a lot with the supply voltage.

You could put two inverters in series then add positive feedback with two resistors. It will have threshold voltages near half the supply voltage.

 

[user1453]

this may be what you are after

http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/schmitt.html#c2

 

[Styx]

this may be what you are after

http://hyperphysics.phy-astr.gsu.edu/HBASE/electronic/schmitt.html#c2

Unfortunetly comparator-based Schmitt's take too much relestate on my board.
I have this board working with HC14-based schmitts (just to add some delays into the cct). But as I found there is quite a big spread between chips.

They are ok for timing, just not at a production level it seems (some form of calibration would be needed which is not really an option)


So the only solutions seem to be
1) put a CPLD down (clocking at 10MHz, after ~1us of delay in a few places) and create a simple state-machine, you can still just about get 20pin PLCC CPLD (lattice's GAL chips seem the way)

2) this "feedback" idea of AudioGuru's
@ AudioGuru, so just to clarify

Yes the threshholds vary with supply voltage as well, but that is fairly firm.
So you say two Schmitt's in series and then a feedback across both of them? If you say this five a level ~ 50% of the rail then at least that is more consistant, I will look into this thanks

 

[audioguru]

Don't use two Schmitts in series. Use ordinary inverters like from a 74HC04. Then a 10k resistor from the output to the input for positive feedback and a 2k input resistor. It will switch with input voltages of about 2.0V and 3.0V when there is a 5.0V supply.

 

[Dr.EM]

I often use that (especially as I don't have the integrated ones). We were taught it in electronics and it works rather well.

 

[Roff]

Two HC04 sections and two resistors probably takes up as much room as a 555. Plus, if you're using them as delays, the low input resistance is inconvenient.
What was wrong with Hero's suggestion?

 

[Styx]

Two HC04 sections and two resistors probably takes up as much room as a 555. Plus, if you're using them as delays, the low input resistance is inconvenient.
What was wrong with Hero's suggestion?

The main thing that is wrong with a dedicated comparator schmitt is the PCB footprint.
I have to use -55C to +125C components which isolates me to DIP's for a few things, one of those few things is comparators (logic is avail in SOIC), and the one in particular is the LM119. and this is only a dual-package.

Since I need 3off of these "delays" I would need 2 extra DIP14 packages for the space of one SOIC-14 (I already have one dual-comparator package, both comparator setup as schmitt)

I am actually tempted to go for the GAL route and create a state-machine to get my timing

thanks anyway

 

[Roff]

The main thing that is wrong with a dedicated comparator schmitt is the PCB footprint.
I have to use -55C to +125C components which isolates me to DIP's for a few things, one of those few things is comparators (logic is avail in SOIC), and the one in particular is the LM119. and this is only a dual-package.

Since I need 3off of these "delays" I would need 2 extra DIP14 packages for the space of one SOIC-14 (I already have one dual-comparator package, both comparator setup as schmitt)

I am actually tempted to go for the GAL route and create a state-machine to get my timing

thanks anyway

**broken link removed** (CMOS) is theoretically available in military grade, in a TSSOP package. It is still, of course, only one per package.

 

[Hero999]

If you need a dual chip, the TS556 is also available in military grad.