LM358 strange behavior !

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Hello,

Excuse my english, I'm French ...

The LM 358 is a mono-voltage amplifier capable of approaching its negative  
supply (there is a NPN transistor in the output stage). I had to build a mo
no-voltage subtractor (a voltage substractor built with a LM358 powered wit
h 0v-12v supply)  and with the consent of the electronic simulator ISIS I b
uilt (wired) such a subtractor. I was surprised that it could not entirely  
do the job. For  some subtractions, like 2v - 1.8v for example, the substra
ctor said 0.62v instead of 0.2v. But for some other, it answered the right  
thing, wich was 0.3v (a voltage lower than 0.62v) sometimes. So it was able
 to go under 0.62v !

Why does a 2v - 1.8v substraction gave 0.62v instead of 0.2v !


Resistors, all identical, were 2.5k one, so I do not suspect bias currents  
problem. I tried many other chip, it didn't change anything. I changed the  
0v suply into a -12v one and the substractor worked well for every input vo
ltage.


Does someone have an explanation?  

cdlt.

Michel.

Re: LM358 strange behavior !
On Sun, 4 Nov 2012 01:01:55 -0800 (PST), snipped-for-privacy@gmail.com
wrote:

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supply (there is a NPN transistor in the output stage). I had to build a
mono-voltage subtractor (a voltage substractor built with a LM358 powered with
0v-12v supply)  and with the consent of the electronic simulator ISIS I built
(wired) such a subtractor. I was surprised that it could not entirely do the
job. For  some subtractions, like 2v - 1.8v for example, the substractor said
0.62v instead of 0.2v. But for some other, it answered the right thing, wich was
0.3v (a voltage lower than 0.62v) sometimes. So it was able to go under 0.62v !
Quoted text here. Click to load it
problem. I tried many other chip, it didn't change anything. I changed the 0v
suply into a -12v one and the substractor worked well for every input voltage.
Quoted text here. Click to load it

---
Can you post a schematic, please?

--  
JF

Re: LM358 strange behavior !
Le dimanche 4 novembre 2012 11:44:51 UTC+1, John Fields a écrit :
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ve supply (there is a NPN transistor in the output stage). I had to build a
 mono-voltage subtractor (a voltage substractor built with a LM358 powered  
with 0v-12v supply)  and with the consent of the electronic simulator ISIS  
I built (wired) such a subtractor. I was surprised that it could not entire
ly do the job. For  some subtractions, like 2v - 1.8v for example, the subs
tractor said 0.62v instead of 0.2v. But for some other, it answered the rig
ht thing, wich was 0.3v (a voltage lower than 0.62v) sometimes. So it was a
ble to go under 0.62v !
Quoted text here. Click to load it
ts problem. I tried many other chip, it didn't change anything. I changed t
he 0v suply into a -12v one and the substractor worked well for every input
 voltage.
Quoted text here. Click to load it


Ok. It is the basic soustractor schématic (not the instrumentation one wi
th 3 amplifiers), with unity gain, visible here :

http://crteknologies.fr/electronique/cours/ao_soustracteur.gif

In my case, R1=R2=R3=R4=2.5 k, and Us = U2 - U1.

Michel.


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Re: LM358 strange behavior !
snipped-for-privacy@gmail.com wrote:
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supply (there is a NPN transistor in the output stage). I had to build a
mono-voltage subtractor (a voltage substractor built with a LM358 powered with
0v-12v supply)  and with the consent of the electronic simulator ISIS I built
(wired) such a subtractor. I was surprised that it could not entirely do the
job. For  some subtractions, like 2v - 1.8v for example, the substractor said
0.62v instead of 0.2v. But for some other, it answered the right thing, wich was
0.3v (a voltage lower than 0.62v) sometimes. So it was able to go under 0.62v !
Quoted text here. Click to load it
problem. I tried many other chip, it didn't change anything. I changed the 0v
suply into a -12v one and the substractor worked well for every input voltage.
Quoted text here. Click to load it
amplifiers), with unity gain, visible here :
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   Not strange at all.
   The common mode input voltage, U is equal to 0.5 * U2.
   With the negative supply pin of the op-amp being at zero volts, many  
common op-amps do not operate with that input.
   Also, a number of common op-amps cannot drive the output so close to  
its negative supply.
   Now..if you use a properly selected rail-to-rail op-amp..


Re: LM358 strange behavior !
On Sun, 04 Nov 2012 23:25:48 -0800, the renowned Robert Baer

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The LM358 has an input CM range that is guaranteed to go down to the
negative rail over the full temperature range. One of the things that
makes it so useful for an extremely cheap and common op-amp (it's half
an LM324). So, it's not that.  

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The LM358 is guaranteed to drive the output within 20mV of the
negative rail (5mV typically) over the full temperature range, with a
<10%K load (critically important, but perhaps not obvious from the
data sheet) connected to the negative rail. The subtractor has < 10K
of load but does not have that configuration of loading on the output.
That's his problem.  


Best regards,  
Spehro Pefhany
--  
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: LM358 strange behavior !
wrote:
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y
o

His R1-R2 combination presents 5k from -Vin to output, so he'd need to
sink 320uA for his example (Vin+ = 2.0v, Vin- = 1.8v, Vout = 200mV).

R1=R210%0k would go a long way toward fixing that.

--
Cheers,
James Arthur

Re: LM358 strange behavior !
On Mon, 5 Nov 2012 06:49:47 -0800 (PST), snipped-for-privacy@yahoo.com
wrote:

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Which overwhelms the 50uA nominal internal sinking capacity.  

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Sure, and make the other two 100K as well to kill most of the offset
due to bias current.  


I suppose one could do something like this and keep the 2K5 resistors:

                      |\|
                     -|-\
                      |  >-----+----
                     -|+/      |
                      |/|      |
                               |
                               |
                               |
                               |
                       1M8     |
                       ___   |/
                 +12 -|___|--|        2PC4081R
                             |>
                               |
                               |
                              ===
                              GND

LOL.  



Re: LM358 strange behavior !
On Mon, 05 Nov 2012 17:12:24 -0500, Spehro Pefhany

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Beta limit! Beta limit!

But with a 2:1 beta spread, it's perfectly reasonable.

A depletion mosfet would be fun, too. Or a better opamp. Or a negative
supply for a better opamp.


--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: LM358 strange behavior !
wrote:
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).
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LOL!

I nearly suggested a JFET load, Vgs=0v.  Bigger resistors was easier
though.

--
Cheers,
James Arthur

Re: LM358 strange behavior !
snipped-for-privacy@yahoo.com a écrit :
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Hey, a resistor to a negative supply would be nice too :-)


--  
Thanks,
Fred.

Re: LM358 strange behavior !
On Nov 5, 6:10 pm, Fred Bartoli <" "> wrote:
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y
t
lf
o
a
t.
o
0mV).
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1R
r

Sure, if you've got one. (a negative supply, that is.)

Bob Pease did a neat trick--or was it Jim Williams?--using the e-b
junction of a powered optoisolator as a negative current generator.
The phototransistor served as a solar cell, to pull an LM358 output
all the way down.  Cute.

--
Cheers,
James Arthur

Re: LM358 strange behavior !
snipped-for-privacy@yahoo.com a écrit :
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Was joking of course. If you have a neg supply then just use it for the  
opamp.

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I think it was Bob Pease but I doubt this has enough current for an LM358.


--  
Thanks,
Fred.

Re: LM358 strange behavior !
On Nov 5, 6:24 pm, Fred Bartoli <" "> wrote:
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t>
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any
he
hat
half
 to
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h a
he
0K
put.
 to
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200mV).
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s:
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081R
=
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ier

Missed it--whoosh.  I often don't use negative supplies for V- for
other reasons, like, it might be dirty, or to save power, or reduce
loading on V-, etc.

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.

Could be.  If an ordinary phototransistor isolator has a current
transfer ratio of 20%, and the phototransistor has a hfe of ~100, that
suggests i.b ~= 0.2%.  So, 10mA into the IRLED could make 10 or 20uA.

--
Cheers,
James Arthur

Re: LM358 strange behavior !
On Tue, 06 Nov 2012 00:10:19 +0100, Fred Bartoli <" "> wrote:

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*If* there's no latchup hazard in the opamp. I never pull below ground
any pin on those ancient National designs.


--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: LM358 strange behavior !
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:
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81R
er

That's not a hazard, it's a feature--crowbar.  LM34, right?

Grins,
James

Re: LM358 strange behavior !
On Mon, 5 Nov 2012 19:43:40 -0800 (PST), snipped-for-privacy@yahoo.com
wrote:

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Yes. Read the tiny footnotes, and never power them from more than 5
volts.

We're using LM50's lately, because you can pull down to ground safely
and still measure -40C.


--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: LM358 strange behavior !
On Mon, 05 Nov 2012 17:22:38 -0800, John Larkin


I never pull below ground any pin on those ancient National designs.

More's the pity.

--  
JF

Re: LM358 strange behavior !
On Tue, 06 Nov 2012 18:14:52 -0600, John Fields

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Why? Do you enjoy chips doing weird things, latching up?

If you're not 100% sure where substrate current is going to go,
pulling pins below ground is hazardous. LM324, LM339, 4000A cmos,
LM35, lots of ADCs and DACS, were notorious. Lots of new designs still
have latchup problems; it must be very hard to fix, since so many
designs have this hazard.

When you put hundreds, sometimes 1000+ parts on a board, you've got to
be paranoid about risk, or it will never work.



--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: LM358 strange behavior !
On Tue, 06 Nov 2012 17:30:06 -0800, John Larkin

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---
Whoosh!!!

.            +--------+
.            |  +V    |
.            |   |   [R]
.Vin>---[R]--+--|-\   |
.               |  >--+-->-Vin
.           +---|+/
.           |    |
.          GND  -V <--- Below ground

"I never pull below ground any pin on those ancient National designs."

;)

--  
JF

Re: LM358 strange behavior !
On Wed, 07 Nov 2012 08:29:09 -0600, John Fields

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I meant, obviously, pulling a signal pin of a single-supply chip below
its substrate ground.


--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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