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'[EE] Op Amp - unused gate - what-to-do?'
2008\01\14@093109 by Rolf

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Hi all.

I am using a quad opamp in a design, but, one of the gates will not be
used. What is the recommended way to 'tie off' the gate. My gut feel is
to tie both inputs and the output to each other, and then through a
1K'ish resistor to ground... This way the gate should not be floating,
and should not be trying to drive the output....

Is this a good solution? what is the recommended practice?

Thanks

Rolf

2008\01\14@094558 by Bob Axtell

face picon face
Rolf wrote:
> Hi all.
>
> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground... This way the gate should not be floating,
> and should not be trying to drive the output....
>
> Is this a good solution? what is the recommended practice?
>
> Thanks
>
> Rolf
>  
I believe National Semi long ago recommended connecting unused inputs to
GND or VCC,
your choice. Outputs need not be tied to anything.

That's what I do.

--Bob A

2008\01\14@094707 by Martin Klingensmith

face
flavicon
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Rolf wrote:
> Hi all.
>
> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground... This way the gate should not be floating,
> and should not be trying to drive the output....
>
> Is this a good solution? what is the recommended practice?
>
> Thanks
>
> Rolf
>  
I think the safest way would be to make a unity-gain buffer out if it
and ground the noninverting input, assuming it is unity gain stable.

-
Martin

2008\01\14@094913 by Michael Rigby-Jones

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> -----Original Message-----
> From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu] On
Behalf
> Of Rolf
> Sent: 14 January 2008 14:31
> To: Microcontroller discussion list - Public.
> Subject: [EE] Op Amp - unused gate - what-to-do?
>
> Hi all.
>
> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel
is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground... This way the gate should not be floating,
> and should not be trying to drive the output....
>
> Is this a good solution? what is the recommended practice?

Tie output to inverting input, and non-inverting input to ground.  You
then have a unity gain buffer which will be fine as long as the op-amp
is stable at unity gain.

Regards

Mike

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2008\01\14@095348 by Martin Klingensmith

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Martin Klingensmith wrote:
{Quote hidden}

Ah, I wasn't thinking. Bob Axtell is certainly correct.
-
Martin

2008\01\14@101332 by John Coppens

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On Mon, 14 Jan 2008 09:30:47 -0500
Rolf <learrspamKILLspamrogers.com> wrote:

> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground...

Much depends on which opamp. Check the datasheet or appnotes for
recomendations. Some opamps do not include gnd in the operational range
if fed from single supply, and should not be connected to gnd. There are
other considerations, like you might want to bias the amp into saturation
to lessen current, but how depends on the IC design.

But probably never connect + and - input together to the output. - yes, +
no.

John
BTW, I am supposing it _is_ about a quad opamp. You talk about gates in
your message.

2008\01\14@103521 by Pablo Ginhson

picon face

I agree with Bob Axtell, I do that frequently.

Pablo Ginhson


> Date: Mon, 14 Jan 2008 09:53:05 -0500> From: .....martinKILLspamspam.....nnytech.net> To: EraseMEpiclistspam_OUTspamTakeThisOuTmit.edu> Subject: Re: [EE] Op Amp - unused gate - what-to-do?> > Martin Klingensmith wrote:> > Rolf wrote:> > > >> Hi all.> >>> >> I am using a quad opamp in a design, but, one of the gates will not be > >> used. What is the recommended way to 'tie off' the gate. My gut feel is > >> to tie both inputs and the output to each other, and then through a > >> 1K'ish resistor to ground... This way the gate should not be floating, > >> and should not be trying to drive the output....> >>> >> Is this a good solution? what is the recommended practice?> >>> >> Thanks> >>> >> Rolf> >> > >> > > I think the safest way would be to make a unity-gain buffer out if it> > and ground the noninverting input, assuming it is unity gain stable.> >> > -> > Martin> > > Ah, I wasn't thinking. Bob Axtell is certainly correct.> -> Martin>

2008\01\14@110507 by Herbert Graf

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face

On Mon, 2008-01-14 at 09:30 -0500, Rolf wrote:
> Hi all.
>
> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground... This way the gate should not be floating,
> and should not be trying to drive the output....
>
> Is this a good solution? what is the recommended practice?

As long as you tie the inputs to one rail you should be fine, either Vcc
or GND, it shouldn't really matter.

Personally I just tie both inputs to ground. Only reason to put the
resistor in is if you want to leave the option open of using that op-amp
in the future, otherwise just tie it to ground. You don't need to tie
the output to anything.

TTYL

2008\01\14@122049 by John Coppens

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On Mon, 14 Jan 2008 09:30:47 -0500
Rolf <learrspamspam_OUTrogers.com> wrote:

> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground...

Much depends on which opamp. Check the datasheet or appnotes for
recomendations. Some opamps do not include gnd in the operational range
if fed from single supply, and should not be connected to gnd. There are
other considerations, like you might want to bias the amp into saturation
to lessen current, but how depends on the IC design.

But probably never connect + and - input together to the output. - yes, +
no.

John
BTW, I am supposing it _is_ about a quad opamp. You talk about gates in
your message.

2008\01\14@122207 by John Coppens

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face
On Mon, 14 Jan 2008 10:28:03 -0500
Spehro Pefhany <@spam@speffKILLspamspaminterlog.com> wrote:

> I think that would be "all", not *some*. unless the op-amp somehow  
> generates a negative voltage internally. Even if the CM range includes  
> ground (or a few
> hundred mV below ground) the Vos could be negative by a few microvolts,
> and the *output* cannot go below ground without a negative supply
> *somewhere*, so the op-amp cannot balance.

There are quite a few opamps that have pnp transistors on the inputs and
allow (slight) negative excursions (LM324 comes to mind). Others have
JFET or MOSFET at the inputs and also work around 0V.

Also balancing is not the point - the OP wanted to know how to tie off an
unused amp.

John

2008\01\14@123337 by Mark Rages

face picon face
On Jan 14, 2008 8:30 AM, Rolf <KILLspamlearrKILLspamspamrogers.com> wrote:
> Hi all.
>
> I am using a quad opamp in a design, but, one of the gates will not be
> used. What is the recommended way to 'tie off' the gate. My gut feel is
> to tie both inputs and the output to each other, and then through a
> 1K'ish resistor to ground... This way the gate should not be floating,
> and should not be trying to drive the output....
>
> Is this a good solution? what is the recommended practice?
>
>


This was posted to geda-user last year.  Unused sections are discussed
on page 8.

http://encon.fke.utm.my/nikd/latest/sloa067.pdf

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
RemoveMEmarkragesTakeThisOuTspammidwesttelecine.com

2008\01\14@142423 by Rolf

face picon face
Mark Rages wrote:
{Quote hidden}

Thanks Mark, Everyone.

I am pleased I asked that question, seems to be more variance in the
answers than I expected !!!

I am going to be using a precision single-supply R-R opamp for a
pressure sensor, but I am not yet certain of the exact part I will use
as I am still uncertain as to what my critical design criteria are.
There are a few devices I am considering that all have the same
footprint so I am working on the board at the same time. I will
double-check with the DataSheets as I go, but, I think I will design the
first board to use the 'Good' solution recommended in that PDF. It makes
sense now to put the Non-Inv input at a mid-level, and tie the output to
the Inv input. Connecting to either rail seems a relatively poor idea as
a general rule unless a particular amp is purpose built for it.

Thanks for the link, it was very interesting reading.

Rolf

2008\01\14@152521 by Andre Abelian

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face
Mark,

It is nice pdf file but in page 8 even correct OP termination shows NO!.
The one shows smart not every time useful. It is good only for prototype
version not for production. Correct termination is the one shows NO!

Andre

{Original Message removed}

2008\01\14@154851 by Spehro Pefhany

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Quoting Andre Abelian <TakeThisOuTaabelianEraseMEspamspam_OUTmason-electric.com>:

> Mark,
>
> It is nice pdf file but in page 8 even correct OP termination shows NO!.
> The one shows smart not every time useful. It is good only for prototype
> version not for production. Correct termination is the one shows NO!
>
> Andre

Not ideal with single-supply or R-R op-amps. Usually there's a voltage
around you can borrow for the "Good" option.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
RemoveMEs...spamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2008\01\14@155531 by peter green

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face
Andre Abelian wrote:
> Mark,
>
> It is nice pdf file but in page 8 even correct OP termination shows NO!.
> The one shows smart not every time useful. It is good only for prototype
> version not for production. Correct termination is the one shows NO!
>
>  
It looks like they are assuming single rail supply in those pics.
Connecting the input of an op-amp buffer running split rail to ground is
the same as connecting an op-amp buffer running single rail to mid supply.



2008\01\14@164415 by Michael Rigby-Jones

picon face


> -----Original Message-----
> From: piclist-bouncesEraseMEspam.....mit.edu [EraseMEpiclist-bouncesspammit.edu] On
Behalf
> Of Andre Abelian
> Sent: 14 January 2008 20:28
> To: Microcontroller discussion list - Public.
> Subject: RE: [EE] Op Amp - unused gate - what-to-do?
>
> Mark,
>
> It is nice pdf file but in page 8 even correct OP termination shows
NO!.
> The one shows smart not every time useful. It is good only for
prototype
> version not for production. Correct termination is the one shows NO!
>

That was my opinion too.  Why would the output of the op-amp going to
one rail cause excessive heating if the output is not driving any
current?  The only cases I can imagine is if the op-amp has a rail-rail
output, but driving the inputs to ground is outside their specification,
or if the op-amp is not stable when configured for unity gain.  These
would generally be exceptional cases though and should be dealt with
accordingly.

Adding numerous zero ohm links is certainly useful during
prototype/development stage where the extra section might turn out be
useful (and can be kludged into operation with some wires soldered on),
but adds unnecessary cost and board space to a finalised design.

Regards

Mike

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2008\01\14@213954 by Marcel Duchamp

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Michael Rigby-Jones wrote:
{Quote hidden}

I'm of the same opinion as well.  Aside from the special cases Mike
mentions (unity gain stable, etc.), it works 100% as expected.

The output will not be at ground necessarily but who cares?  One
polarity of offset will drive the output as low as it can.  The other
polarity of offset will result in the output slightly above ground,
assuming the output can go that far towards ground to start with.

In no case will it
       a) cause current consumption
       b) oscillate
       c) ???

The "smart" case shown is smart only for prototypes and foolish
otherwise, most of the time.  Many of the boards around here are so
tight, it's a difficult job to get room for the silk art let alone six
more components.





2008\01\14@224756 by Spehro Pefhany

picon face
At 09:38 PM 1/14/2008, you wrote:
{Quote hidden}

With due respect, Marcel, I have to disagree. I have seen popular bipolar
R-R output op-amps break into MHz small amplitude oscillations when
driven close to saturation. The phase margin goes to h-e-double-hockeysticks
on a handcart under those conditions. Current consumption is *not* predictable.

You also need to be careful of "solutions" that involve differential input
voltage of more than a few hundred mV-- some op-amps will draw current
from one input to the other under those conditions. None of this is
typically explicitly shown on the datasheet, and just because it's
successful with op-amp type A, it may be a booby-trap with op-amp B. For
example, in many situations it's perfectly acceptable to leave all 3 pins
of the unused section of a 4558 or 1458 open. My company has shipped many
tens of thousands like that, but I don't recommend it in general, because
it doesn't work in general.

It's a bit like driving current into protection networks- if the
requirements are not strict and you don't look too close, you can get
away with it. In more critical applications it can bite one soundly in
the hindquarters.

>Best regards,

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



2008\01\14@231505 by Sean Breheny

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I am puzzled by a number of things in that app note. For one thing, he
says that "comparators' inputs are labeled in the opposite way"
compared with op-amps. I fail to see how this is true. He also talks
about "using a comparator in negative feedback" and then clearly shows
positive feedback.

I'm curious about R-R op-amps going unstable near the rails. It would
seem to me that it cannot really be considered a R-R output op-amp if
that is so (since any time the output gets near the rail, even for a
short time, there may be funky behavior).

Most of the time when I use op-amps, I have a virtual ground (i.e.,
the mid-way point between the positive and negative rails) and I
connect unused op-amps as a unity-gain follower with the non-inverting
terminal either connected to the virtual ground (basically what this
app note says, although with no extra parts since virtual ground is
already present) or to one of the rails.

Sean


On Jan 14, 2008 10:47 PM, Spehro Pefhany <RemoveMEspeffspam_OUTspamKILLspaminterlog.com> wrote:
{Quote hidden}

> -

2008\01\15@002938 by Xiaofan Chen

face picon face
On Jan 15, 2008 5:44 AM, Michael Rigby-Jones
<EraseMEMichael.Rigby-JonesspamspamspamBeGonebookham.com> wrote:
> Adding numerous zero ohm links is certainly useful during
> prototype/development stage where the extra section might turn out be
> useful (and can be kludged into operation with some wires soldered on),
> but adds unnecessary cost and board space to a finalised design.
>

I will say the one says "SMART" will be labled "Brain Dead" in many
practical designs due to cost and board space constraint. And you
will only put optional resistors to more critical places, not an unused
OPAMP pins.

The one marks "Good" is correct assuming single supply. Assuming
dual supply, the one marked "NO" and the one makred "Just as bad"
are both correct.

Xiaofan

2008\01\15@003211 by Marcel Duchamp

picon face
Spehro Pefhany wrote:
>
> With due respect, Marcel, I have to disagree.

I won't argue with the two resistor split rail as far as "works all the
time" goes.

My experience with R-R amps is undoubtedly lacking here too.  I'll keep
your advice in mind and see what happens in the future with those
devices the next chance I get.

LM324's? No problem!  Newer stuff is, well, newer stuff!

2008\01\15@041500 by peter green

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Sean Breheny wrote:
> I am puzzled by a number of things in that app note. For one thing, he
> says that "comparators' inputs are labeled in the opposite way"
> compared with op-amps.
No he says in the particlar schematic he is exibiting they are the
opposite way round, the reason they are the opposite way arround is the
comparator circuit has one less inverting stage.


2008\01\15@042926 by Jinx

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> LM324's? No problem!  Newer stuff is, well, newer stuff!

What turned out to be an op-amp problem drove me batty
yesterday. Making an electret mic pre-amp (part of the ISD
project). Simple enough, but couldn't get rid of a spike that
always happened on the rising quadrant of a wave cycle. Had
the circuit apart, added things, took things away, single-ended
supply, split rail, hit the web, no change. Only when I swapped
the op-amp for a dirty old 741 would it work cleanly. NE5534,
TLC271, TL071, no good. All had the spike. Weird and very
frustrating

2008\01\15@115638 by Marcel Duchamp

picon face
Jinx wrote:
>> LM324's? No problem!  Newer stuff is, well, newer stuff!
>
> What turned out to be an op-amp problem drove me batty
> yesterday. Making an electret mic pre-amp (part of the ISD
> project). Simple enough, but couldn't get rid of a spike that
> always happened on the rising quadrant of a wave cycle. Had
> the circuit apart, added things, took things away, single-ended
> supply, split rail, hit the web, no change. Only when I swapped
> the op-amp for a dirty old 741 would it work cleanly. NE5534,
> TLC271, TL071, no good. All had the spike. Weird and very
> frustrating
>

That sounds like crossover distortion.  It happens when the output
switches from sourcing to sinking current.  LM324's and the like suffer
from this but it's easily cured for the most part.

Try hanging a 1K resistor from the output to the negative rail; this
will make the output try harder and should minimize the spike.

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