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'[PIC]: AD877 with negative input voltage - conclus'
2001\03\05@023310 by Vasile Surducan

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face
I'm not agree, see my conclusion below.

On Fri, 2 Mar 2001, Drew Vassallo wrote:

> >I am attempting to do a A/D conversion to a signal ranging from -1 VDC to
> >+5.5. VDC.  I am using a PIC16F877.  I understand to need to use the VREF,
> >+VREF, and -VREF pins in order to do this.  The problem is that I have not
> >found a good description of how to use/apply these pins.
>
> This may be out of spec.  The analog input minimum voltage spec for Vain and
> Vref- is Vss-0.3 Volts.  I don't know what the chip would do with
> -1.0 V.
>

 Well, I've dig out enough to afirm that 877 AD WORKS OK with negative
input signals. I know many of guru's of this list will cry: IT'S OUT OF
SPECIFICATION. Who care, if works perfectly !
 Here are three important tested conclusion:
1. With 877 suplied from +5.00V and gnd ( single power supply )
  -Vref to -0.5V, +Vref to +4.5V input range is between -0.5 to +4.5V
  all other ad specification respect data sheet information.
  Do not try to increase -vref level, will not work because at
  -0.6V...-0.7V the input protective diodes will go on and some important
  currents will flow on these diodes
  ( so the vss-0.3V is a joke, they have standard silicon protective
  input diodes, maybe schottky)
2. If you need a greater gain that AD has ( 0...5V to 0...1023
  points; 4.8mv resolution )
  you may use +vref and -vref but also to respect the following
  condition:
 a.  +vref - |-vref| >= 1.7V where the |-vref| is the absolute
  value of -vref,
 b. |-vref| < +Vref
 c. |-vref| <= 2.5V, +Vref >= 2.5V but respect a. condition.

 Example: AD will work with 10bit resolution if -vref = 2V, +vref = 4V
 so, for a 0...2V input voltage we have 0...1023 points, 1.9mV
 resolution which is pretty good...
 AD will work with less than 10bit resolution if +vref - |-vref| < 1.7V
 AD will not working if +vref < |- vref|

3.AD could measure input signals between -2.5V to +2.5V without any
 external active components ( operational amplifiers, etc...)
 But I want to hear first your solutions, gentlemans...

Cheers,
Vasile

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2001\03\05@081832 by Bob Ammerman

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>   Well, I've dig out enough to afirm that 877 AD WORKS OK with negative
> input signals. I know many of guru's of this list will cry: IT'S OUT OF
> SPECIFICATION. Who care, if works perfectly !

****IT'S OUT OF SPECIFICATION****

Yeah, it works perfectly:

on the one chip you tested
at room temperature
at Vdd = 5.00V
etc.

(btw: does responding to this bait make me a guru?)

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Quote hidden}

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2001\03\05@090455 by Olin Lathrop

face picon face
>   Well, I've dig out enough to afirm that 877 AD WORKS OK with negative
> input signals. I know many of guru's of this list will cry: IT'S OUT OF
> SPECIFICATION. Who care, if works perfectly !

Whoever has to use it would.  Just because it appears to work on one PIC at
a particular temperature doesn't mean it will work on other PICs, or the
same PIC at a different temperature.  You also don't know if you are causing
hidden damage that will catch you later.  An obvious possibility is blowing
the protection diodes.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\03\05@093002 by art

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face
Bob
What kind of fish is a guru is it a kind of guppy?
also be careful of giving short snappy answers that another sign of being a
guru.

Regards Art.

{Original Message removed}

2001\03\05@112257 by Dan Michaels
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Art wrote:
>Bob
>What kind of fish is a guru is it a kind of guppy?
>also be careful of giving short snappy answers that another sign of being a
>guru.
>
>> (btw: does responding to this bait make me a guru?)
>>


If both gurus and guppies rise to the bait uniformly,
then how can you tell them apart?

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2001\03\05@140555 by hard Prosser

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OK , it works.
But be very careful. Diode forward voltage drops with temperature and
schotty diode leakage increases dramatically with temperature. If the
application is going to have to work at anything over 20C I'd strongly
recommend a level shift of some sort and possibly a voltage divider to
ensure the input voltage stays within limits.
For a one-off try - great. For production -sorry!

Richard P





I'm not agree, see my conclusion below.

On Fri, 2 Mar 2001, Drew Vassallo wrote:

> >I am attempting to do a A/D conversion to a signal ranging from -1 VDC
to
> >+5.5. VDC.  I am using a PIC16F877.  I understand to need to use the
VREF,
> >+VREF, and -VREF pins in order to do this.  The problem is that I have
not
> >found a good description of how to use/apply these pins.
>
> This may be out of spec.  The analog input minimum voltage spec for Vain
and
> Vref- is Vss-0.3 Volts.  I don't know what the chip would do with
> -1.0 V.
>

 Well, I've dig out enough to afirm that 877 AD WORKS OK with negative
input signals. I know many of guru's of this list will cry: IT'S OUT OF
SPECIFICATION. Who care, if works perfectly !
 Here are three important tested conclusion:
1. With 877 suplied from +5.00V and gnd ( single power supply )
  -Vref to -0.5V, +Vref to +4.5V input range is between -0.5 to +4.5V
  all other ad specification respect data sheet information.
  Do not try to increase -vref level, will not work because at
  -0.6V...-0.7V the input protective diodes will go on and some important
  currents will flow on these diodes
  ( so the vss-0.3V is a joke, they have standard silicon protective
  input diodes, maybe schottky)
2. If you need a greater gain that AD has ( 0...5V to 0...1023
  points; 4.8mv resolution )
  you may use +vref and -vref but also to respect the following
  condition:
 a.  +vref - |-vref| >= 1.7V where the |-vref| is the absolute
  value of -vref,
 b. |-vref| < +Vref
 c. |-vref| <= 2.5V, +Vref >= 2.5V but respect a. condition.

 Example: AD will work with 10bit resolution if -vref = 2V, +vref = 4V
 so, for a 0...2V input voltage we have 0...1023 points, 1.9mV
 resolution which is pretty good...
 AD will work with less than 10bit resolution if +vref - |-vref| < 1.7V
 AD will not working if +vref < |- vref|

3.AD could measure input signals between -2.5V to +2.5V without any
 external active components ( operational amplifiers, etc...)
 But I want to hear first your solutions, gentlemans...

Cheers,
Vasile

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2001\03\06@015931 by Vasile Surducan

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On Mon, 5 Mar 2001, Bob Ammerman wrote:

> >   Well, I've dig out enough to afirm that 877 AD WORKS OK with negative
> > input signals. I know many of guru's of this list will cry: IT'S OUT OF
> > SPECIFICATION. Who care, if works perfectly !
>
> ****IT'S OUT OF SPECIFICATION****
>
> Yeah, it works perfectly:
>
> on the one chip you tested
> at room temperature
> at Vdd = 5.00V
> etc.
>
> (btw: does responding to this bait make me a guru?)
>
> > 3.AD could measure input signals between -2.5V to +2.5V without any
> >   external active components ( operational amplifiers, etc...)
> >   But I want to hear first your solutions, gentlemans...
> >

Sorry, but when I do an affirmation it means I have made some test:
 on more than one chip ( tested on three )
 at different temperature than room one ( do you haven't there
 climatic chambers ? ) but not at +50C, neither at -20C where any
 operational amplifiers or AD needs to pray to God...( and
 this ir REAL and not taken from specifications )
 in standard TTL range power supply ( 4.75...5.25V )

But I hear no comments about my last affirmation (+/-2.5V ) which
*is in specification* ,of course, with a trick.

by, Vasile

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2001\03\06@025643 by Vasile Surducan

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On Mon, 5 Mar 2001, Olin Lathrop wrote:

> >   Well, I've dig out enough to afirm that 877 AD WORKS OK with negative
> > input signals. I know many of guru's of this list will cry: IT'S OUT OF
> > SPECIFICATION. Who care, if works perfectly !
>
> Whoever has to use it would.  Just because it appears to work on one PIC at
> a particular temperature doesn't mean it will work on other PICs, or the
> same PIC at a different temperature.  You also don't know if you are causing
> hidden damage that will catch you later.  An obvious possibility is blowing
> the protection diodes.
>
 Blowing protection diodes is impossible since I have use input
protection resistors. And I've tested on different "human" temperatures
(0 to 30C )
Of course is possible to don't work on different pics or
different pics from same family.

Vasile

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