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'[EE]: DC (0 to 0,2V) voltage amplifier with single'
2001\11\02@134605 by wzab

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picon face
Hi All,

I have to amplify (ca. 30 times) a small DC voltage (0 to 0.1V)
in four channels. Generally it is a trivial task, however the amplifier
should be powered from the single +5V source.
The old LM324 is quite good for that purpose (its datasheet, and
its internal structure confirms that it may work with input voltages
around the ground level). The only problem is its output stage. The lower
PNP in final voltage follower will not work until Uout is higher than ca.
1V.
My idea is to add just a pull-down resistor (ca 3kohm). The whole beast
will drive an AT90S8535 ADC input with high input impedance, so the changes
in amplifier's output impedance should not affect its work. In fact
the feed-back branch may function as pull-down resistor if built with
small resistances, so the final circuit may look simple like this:

     |\                R1 = ca. 3 kohm
     | \ 1/4 LM324     R2 = ca. 100 ohm
In ---+ +\
     |   >-----+-Out   It seems, that this circuit should work without
  +--+ -/      |       any problems, however I fill a little uncomfortable
  |  | /       |       with such "abusing" of LM324's output stage.
  |  |/   R1   |       Does anybody knows about any problems which may
  +------====--+       come out there when Uout is near to 0 ?
  |
  [] R2
  []
  |
 --- GND
--
                               TIA & Regards,
                             Wojciech M. Zabolotny
       http://www.ise.pw.edu.pl/~wzab  <--> spam_OUTwzabTakeThisOuTspamise.pw.edu.pl

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2001\11\02@140732 by Spehro Pefhany

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At 07:45 PM 11/2/01 +0100, you wrote:
>Hi All,
>
>I have to amplify (ca. 30 times) a small DC voltage (0 to 0.1V)
>in four channels. Generally it is a trivial task, however the amplifier
>should be powered from the single +5V source.
>The old LM324 is quite good for that purpose (its datasheet, and
>its internal structure confirms that it may work with input voltages
>around the ground level). The only problem is its output stage. The lower
>PNP in final voltage follower will not work until Uout is higher than ca.
>1V.

No, if you read the data sheet (!), with a 5V supply and a 10K load to ground
the output is guaranteed to get down to 20mV, with 5mV as typical. It's a
well designed output stage (with the exception of crossover distortion,
which can be fixed in those applications that need it with a load resistor).

BTW, it's not the PNP that does it, it's that 50uA NPN current sink
connected directly to the output (if you are analyzing the internal
schematic).

{Quote hidden}

This will reduce the output swing of the LM324 output stage because of
the loading. I'd be a bit careful on this point, as the range is not
guaranteed as far as I can see when using a 5V supply. With a light (>10K)
load, 3V should be ok, though. Suggest increasing your values by 10:1
and adding a 1K in series with the non-inverting input.

Best regards,

>
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2001\11\04@203509 by Russell McMahon

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I'm not too happy with the 324's performance near ground.
With ground referenced input signals you may find the overall performance
imperfect for low signal levels.
One solution is to provide an input  pullup from  a higher voltage through a
higher value resistor. This provides a known positive input offset which can
be subtracted from the final result.

eg if R in -ve is Rin and input pullup is Rpullup, bias voltage is Vpullup,
then effective input voltage from the pullup is
Vpullup x Rin/(Rin + Rpullup). Multiply this by G (here = 30) to get the
constant output pullup voltage. Vpullup can just be Vcc but variations in
Vcc will be reflected in the accuracy of Voffset. A stabilised offset
voltage helps. Even a zener derived Vpullup may be OK IF you can measure the
zero offset at startup with software in the absence of input voltage.


     Russell McMahon
_____________________________



{Original Message removed}

2001\11\04@215926 by Jim

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Are there any "*chopper stabilized" op amps
available any more?

*chopper stabilized: An 'op amp' which first converts
the DC voltage to be amplified to AC, amplifies that,
then converts the AC back into the DC. This is done
because low-level DC amps with their inherent DC
offset and drift can present terms that are factors
greater then the signal/voltage to be amplified.

This technique was also used extensively in the days
of vacuum tubes to build accurate, low-level "operational
amplifiers" using devices the were severely subject to
DC drift and offset.

Jim

{Original Message removed}

2001\11\05@012126 by Spehro Pefhany

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At 08:57 PM 11/4/01 -0600, you wrote:
>Are there any "*chopper stabilized" op amps
>available any more?

Sure, but they are generally not required at relatively high
signal levels like 100mV unless your accuracy requirements are very
high (eg. load cells).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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