14 Oct
2025
14 Oct
'25
10:13 p.m.
On 2025Oct 14,, at 1:42 PM, Rob Jarratt <robert.jarratt(a)ntlworld.com> wrote:
I don’t think I’ve ever seen a comparator datasheet that explicitly laid out the
input-to-output function - contrast with other device datasheets with detailed truth
tables galore.
The comp. datasheets always seem to assume “everybody knows that”. You can figure it out
if you look at some of the example circuits or squint closely at just the right parameters
in the specs and graphs or trace the operation through the internal schematic if present.
In the absence of that, a lot of people seem to (wrongly) assume that “well, + > –
would be 1, so transistor ON”.
The other way of looking at it, is it’s the 'same direction' of behaviour as an op
amp, but without the upper drive-high output transistor.
-----Original
Message-----
From: Brent Hilpert via cctalk <cctalk(a)classiccmp.org>
There’s some confusion here somewhere.
Those input V's would imply the comp. output should be loZ to Vsupply– pin,
around –12V; not hiZ, +7.5V.
Oh my! I have clearly got my understanding the wrong way around, not sure how I did that
because I read the datasheet carefully. Somehow, I got confused. I re-measured and found
1IN+=5.5V, 1IN-=9.4V, Power OK=6.7V, but GND (on the 393) is -13V. So as you say Power OK
should be -13V and AC OK H would be asserted. I guess this must mean that the comparator
itself is faulty. I have some 393s, so I will replace it and see what happens.