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'[EE]: Unused pins on a comparator'
2002\03\03@123042 by Brian Kraut

picon face
This reminds me of something I came accross recently on an LM339
comparator.  I have a book that says to ground all unused pins of the
comparator.  I would think that with both input pins grounded that the
output would be unstable.  It seems to make more sense to me to ground
one input and connect the output to VCC to get a stable output.  Anyone
not agree?

Dave Dilatush wrote:

{Quote hidden}

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2002\03\03@124505 by Spehro Pefhany

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At 12:34 PM 3/2/02 -0800, you wrote:
>This reminds me of something I came accross recently on an LM339
>comparator.  I have a book that says to ground all unused pins of the
>comparator.  I would think that with both input pins grounded that the
>output would be unstable.  It seems to make more sense to me to ground
>one input and connect the output to VCC to get a stable output.  Anyone
>not agree?

I'd suggest following the original manufacturer's data-sheet
recommendation, which is to ground all 3 pins of unused comparators
(that includes the output).

(In unusual circumstances, there may be some benefit in low-level
reduced noise or current consumption with biasing it one way or the
other, but I'd prefer to see ~100mV to do it, not Vcc).

Connecting an open-collector output to Vcc is not a very good idea, but
you probably didn't actually mean to write that!

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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2002\03\03@124507 by Dave Dilatush

picon face
Brian Kraut wrote...

>This reminds me of something I came accross recently on an LM339
>comparator.  I have a book that says to ground all unused pins of the
>comparator.  I would think that with both input pins grounded that the
>output would be unstable.  It seems to make more sense to me to ground
>one input and connect the output to VCC to get a stable output.  Anyone
>not agree?

The idea of grounding all unused pins--both inputs AND the output--of an
unused LM339 section works because the LM339 output is open-collector;
it can only pull down.  You wouldn't want to do this with any device
having an active-high output (such as an opamp), but doing it with an
LM339 comparator is OK.  This applies only to an LM339 whose negative
supply terminal is grounded; obviously, if it is tied to a negative
voltage, the unused output should be tied there, not to ground.

For the LM339, connecting one input to ground and the other input to VCC
would be pointless because the LM339's input common-mode range does not
include VCC; and when either of the inputs is outside the input CMR, the
output is indeterminate.

Dave

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2002\03\03@125543 by Brian Kraut

picon face
O.K.  Obviously the open collector tied to ground will be a determined
output, but I guess it can keep oscillating between open collector and
ground internally.  I guess that must be O.K. though.  I thought that
maybee it was a typeo in my book.  This is probably the only practical
soultion since grounding one input and having one at some less than Vcc
voltage divider would be impractical.  The output will probably stay one
way or the other with both inputs grounded from tollerences in the part
anyway.

Dave Dilatush wrote:

{Quote hidden}

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2002\03\03@132524 by Dave Dilatush

picon face
Brian Kraut wrote...

>O.K.  Obviously the open collector tied to ground will be a determined
>output, but I guess it can keep oscillating between open collector and
>ground internally.

I think that's what's called a "distinction without a difference".

The bottom line, when dealing with unused opamp sections or comparator
sections, is twofold:

1.  Keep both inputs within their specified common-mode range.

2.  Keep the output from flopping around.

Whatever satisfies both of these requirements, works.

Dave

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2002\03\03@132731 by Spehro Pefhany

picon face
At 05:43 PM 3/3/02 +0000, you wrote:

>For the LM339, connecting one input to ground and the other input to VCC
>would be pointless because the LM339's input common-mode range does not
>include VCC; and when either of the inputs is outside the input CMR, the
>output is indeterminate.

Hmmm.. "indeterminate" or "unspecified"?  ;-)

From the internal schematic, it looks like tying both pins
to a voltage >> Vcc (but < 36V) will lead to a predictable result
(output low and and some  current saving). There's a range
around Vcc where the current sources are shutting down that won't be
so predictable if both inputs are in that same range, but if one pin
is within the common-mode range, the results from the other pin being
-0.3 < Vin < 36V should not be different from what you'd expect,
regardless of supply voltage.

(BTW, their assumption that ground = negative supply was corrected on
later data sheets, I guess some people didn't realize the reasoning)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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2002\03\03@132944 by Brian Kraut
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Works for me.  Thanks.

Dave Dilatush wrote:

{Quote hidden}

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2002\03\03@133531 by Olin Lathrop

face picon face
> This reminds me of something I came accross recently on an LM339
> comparator.  I have a book that says to ground all unused pins of the
> comparator.  I would think that with both input pins grounded that the
> output would be unstable.  It seems to make more sense to me to ground
> one input and connect the output to VCC to get a stable output.  Anyone
> not agree?

I don't like tying both input pins together because, as you say, it could be
unstable.  You are counting on the offset voltage to drive the output one
way.

However, I would definitely NOT tie one to power and the other to ground.
Depending on the input stage, that could draw excessive current or even
destroy the chip.  This is less likely to be a problem on comparators
because these are usually designed for a large differential mode input.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam.....embedinc.com, http://www.embedinc.com

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