high Z-in comparator/follower

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I have a requirement  to insert a comparator/buffer/follower in a very tight
through-hole  design.  A simple schmitt logic block would do, but TinyLogic
isn't on my normal radar.  Even using FETs with a few external components
consumes real estate, and the same goes for op amps - a DIP8 will fit but the
external components kill me. (resistors must lie flat).

The input (logic) signal is <0.3V low and 4.2-ish high, but effectively drives
out through a 100K+ impedance so the follower needs to be high Zin.

I am toying with a bare ICM7555 as a comparator.  Input signal to 2 and 6.
Discharge (7) or Out (3) will serve as output.

Am I barking up the wrong tree?

Repost: high Z-in comparator/follower

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As no-one responded to my original post, I breadboarded this system with
puzzling results.  Basically the ICM7555 works reliably as a window comparator,
except that its first couple of cycles after system power-up appear to stick ON
(aka output low, discharge transistor on) until it feels like turning off.  Once
it gets over this intialisation stage it is great.

I presume that it is due to the flip-flop not being flipped (or should it be
flopped?) for some reason, but I'm buggered if I can see any difference between
those first couple of input transitions and the rest of them when it goes like a
train.

TIA for any constructive suggestions.

Re: Repost: high Z-in comparator/follower
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   The timing capacitor(s) is (are) fully discharged on power up.
Timing is off, until they go through a full cycle.  Look at the
datasheet and you'll see the comparators trip at 1/3 and 2/3 of VCC.


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Re: Repost: high Z-in comparator/follower

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Try the following 7555 configuration Smoky :

Connect pins 1 and 2 to ground.
Connect pin 8 to +ve supply.
Connect your input to pin 8.
Use pin 3 as output.

This uses the RESET function of the 7555.
Pin 4 is a high impedance input with a typical threshold around 0.6V (Intersil).




 


Re: Repost: high Z-in comparator/follower
On Fri, 07 Mar 2008 11:54:10 -0500, "Michael A. Terrell"

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drives
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comparator,
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ON
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Once
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between
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like a
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Capacitors?  What capacitors?  I'm using it as a window comparator.  Would have
used an op amp except this requires less external components and therefore
better fits my v.tight footprint condition. (see O/P)

Input to 2/6 via 100K.  Output drives load circuit.  Discharge terminal pulls
down a LED separate from the main output/load.

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Yep, I know when they trip.  The thing obviously starts up indeterminate when
power is applied to both the 7555 and the source equipment simultaneously.  I
have often been suspicious of the trigger behaviour as described in the various
data sheets.  This behaviour isn't improving my level of suspicion.

The intial state of the F/F on powerup doesn't seem defined, presumably because
that's not pivotal to "normal" uses as a mono/astable etc.  But given that this
chip is (mis-)applied in such a vast number of configurations, maybe powerup
status IS a spec that should be defined.

BTW, tnx for dropping down to "aus.*" from S.E.D.

Re: Repost: high Z-in comparator/follower

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the
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drives
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comparator,
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ON
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Once
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between
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like a
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(Intersil).

CORRECTION !!!
Connect your input to pin 4 (not pin 8 !!!!!)





Re: Repost: high Z-in comparator/follower

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tight
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the
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drives
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comparator,
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stick ON
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Once
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between
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like a
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(Intersil).
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Interesting possibility.  Looking at the truth table on p4 of Intersil's data
sheet, that would seem to work assuming the input impedance is hgh enough -
couldn't see it specified anywhere.

As an aside, I substituted an NE555 in place of the  ICM7555 and the power-up
glitches disappeared.  Now THAT is puzzling.

Re: Repost: high Z-in comparator/follower

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tight
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TinyLogic
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but the
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drives
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6.
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comparator,
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stick ON
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off.  Once
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be
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between
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like a
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(Intersil).
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High Z = low current
Look at page 4, in the table at the top.
"Reset current  10nA  (@25degC)"

Also, from page 6......
"The RESET terminal is designed to have essentially the
same trip voltage as the standard bipolar 555/6, i.e., 0.6V to
0.7V. At all supply voltages it represents an extremely high
input impedance."

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You asked for help, so please try my pin 4 RESET configuration suggestion
with a 7555 and see what happens.

Connect pins 1 and 2 to ground.
Connect pin 8 to +ve supply.
Connect your input to pin 4 - RESET.
Use pin 3 as output.







Re: Repost: high Z-in comparator/follower

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tight
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TinyLogic
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components
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but the
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drives
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6.
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comparator,
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stick ON
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off.  Once
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it be
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between
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goes like a
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(Intersil).
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OK
 
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I follow what you suggest.  I asked for suggestions regarding MY proposed
configuration.  After my first post, the boards were made anyway, and are now
populated with NE/SE555 chips (rather than the CMOS version) and percolating
nicely as required.  To try your configuration would now require hacking a
working board to see if it does what we both expect, which in any event would be
no advance on where we are.

But thanks for your input, it is appreciated.

Re: Repost: high Z-in comparator/follower

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very tight
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TinyLogic
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components
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but the
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effectively drives
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and 6.
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with
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comparator,
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stick ON
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off.  Once
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it be
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difference between
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goes like a
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(Intersil).
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data
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power-up
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be
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Understood.


No worries.








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