[sci.electronics] Vaguely related to the light bulb problem

hobbit@topaz.rutgers.edu (*Hobbit*) (07/20/89)

What's the correct way to drive TTL from an op-amp?  I'm proposing to do some
kind of threshold detection by sending an op-amp output into a Schmitt; the
op-amp seems unable to pull *low* enough to make the Schmitt input see
a low level.  I suppose I could go out and buy one of those fancy op-amps
that drives all the way to the - rail, but I'd rather have a simple fix I
can do from my junkbox.

The only thing that's worked so far is something like

	        .1 uf
	|\   +---||---+
  opamp	| >--*        *-----> to schmitt input
	|/   +-v^v^v^-+
	        1k or so

... which apparently creates a pulse low enough to kick the schmitt down if
the falling edge is fast enough, but s'pose I want to respond to an output
that drops more slowly??

_H*

tomb@hplsla.HP.COM (Tom Bruhns) (07/22/89)

hobbit@topaz.rutgers.edu (*Hobbit*) writes:
>What's the correct way to drive TTL from an op-amp?  I'm proposing to do some
>kind of threshold detection by sending an op-amp output into a Schmitt; the
>op-amp seems unable to pull *low* enough to make the Schmitt input see
>a low level.  I suppose I could go out and buy one of those fancy op-amps
>that drives all the way to the - rail, but I'd rather have a simple fix I
>can do from my junkbox.
>
Given the constraints, assuming an inversion is ok, put an NPN transistor
in between.  Ground emitter.  R from opamp to base.  R from base to
gnd.  R from collector to +5.  collector to TTL input.  Should be able
to do this with less than 400 microamps from opamp output, at somewhere
around the midpoint of its output voltage range.  If you have a small
enhancement mode mosfet in your junkbox, you could save a couple resistors,
but be careful not to exceed its gate voltage maximums.

Or use CMOS logic (old 4000 series, or 74HC -- not 74HCT series), which
has low input current and Vil threshold spec'd at 1/3 of Vcc.

roy@phri.UUCP (Roy Smith) (07/22/89)

In <Jul.20.00.01.34.1989.25586@topaz.rutgers.edu> hobbit@topaz.rutgers.edu:
> What's the correct way to drive TTL from an op-amp? [...] the op-amp seems
> unable to pull *low* enough to make the Schmitt input see a low level.  I
> suppose I could go out and buy one of those fancy op-amps that drives all
> the way to the - rail

	Not only is the question related to the light bulb problem, but the
answer is too.  In fact, at the risk of being pedantic, I'll just quote my
answer to the light bulb question, since it applies here too.

In article <3868@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>	The problem is that TTL can sink current, but not source it.  With
>TTL you really have to think current sinks instead of voltage sources.

	I'll throw out a few possibilities off the top of my head.  Try
pulling the TTL input down with a resistor to ground.  Or, use a CMOS gate as
a buffer (much higher imput impedence and sufficient output to drive a single
TTL input).  Or, use a low-power TTL part for the first gate after the op-amp.
Or, an op-amp which can sink more current.  Or, a transconductance amplifier
between the op-amp output and the TTL input.

	The bottom line is you don't really want to think about what the TTL
input voltage is.  What you want to think about is if you can sink enough
current.  A TTL input is a current source.  You gotta do something with that
current or the input drags itself high.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"