E
[email protected]
- Jan 1, 1970
- 0
Hello,
We have a circuit that generates signals with an extremely slow
rise/fall time (milliseconds). This circuit should feed a digital
IO sampler that has a CMOS input (AHCT245/645...), and it's known
that slow rise/fall times are bad for CMOSes (forcing them to spend
long time in a conducting state, thus generating power and heat
that may cause damage).
The common solution for this problem is placing a shcmidt trigger
(74VHC14) on the circuit's output, to feed a fast signal to the CMOS.
However, one engineer presented the following appnote: "Implications of
Slow or Floating CMOS Inputs"
(http://focus.ti.com/lit/an/scba004c/scba004c.pdf) and claims that this
article doesn't mention that Schmidts themselves don't suffer from the
slow input problem, and that a Schmidt may be damaged from inputs as
slow as 1 ms. He proposes to use a comparator (that has hystheresis) as
the most robust design. On the contrary, many people say that the
Schmidt is enough, and 74VHC14'th data sheet poses no limit on the rise
time.
What is your opinion ? It's an important subject that surely raises
in many designs.
Eli
We have a circuit that generates signals with an extremely slow
rise/fall time (milliseconds). This circuit should feed a digital
IO sampler that has a CMOS input (AHCT245/645...), and it's known
that slow rise/fall times are bad for CMOSes (forcing them to spend
long time in a conducting state, thus generating power and heat
that may cause damage).
The common solution for this problem is placing a shcmidt trigger
(74VHC14) on the circuit's output, to feed a fast signal to the CMOS.
However, one engineer presented the following appnote: "Implications of
Slow or Floating CMOS Inputs"
(http://focus.ti.com/lit/an/scba004c/scba004c.pdf) and claims that this
article doesn't mention that Schmidts themselves don't suffer from the
slow input problem, and that a Schmidt may be damaged from inputs as
slow as 1 ms. He proposes to use a comparator (that has hystheresis) as
the most robust design. On the contrary, many people say that the
Schmidt is enough, and 74VHC14'th data sheet poses no limit on the rise
time.
What is your opinion ? It's an important subject that surely raises
in many designs.
Eli