# Forum: Analog Circuits pure Despair: strange OpAmp behaviour.

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Ok, here we go:

I am building a SoundCard DAC for controlling my Laser.
The SC outputs from 0.7 to 3.1V whereby 2.55V is the "middle".
An Amplifier should convert this to a differential signal with 10V p-p.

currently not soldered anything to the SoundCard; im using a USB plug
for power and for testing the opamp's stage behavior, im was using a
1MOhm potentiometer to adjust any signal voltage between +5 and -5V.
now, after i thought the high resistance of it could be the error cause,
i replaced it with 3x 10k R's at which i can get 4 different voltages
for testing.

I attached my circuit plan, its exactly how i builded it. between
+5V/GND and -5V/GND are a 100µF elko each for stabilising, otherwise
currently no capacitors are used, except the ones for the ICL7660
negative voltage converter.

So i did a row of different measurements on different setups. Results
are attached to this Post, too.


  1 | 2 | 3 | 4 -5V : -4.82 -4.80 +4.84 +4.95 A : -1.55 -1.55 +0.94 +1.88 B : +1.71 +1.71 -1.03 -2.06 +5V : +4.99 +4.99 -4.74 -4.76 [ADD] -vref + -0.94 = 3.49 -vref + +1.03 = 1.52 [SUBST] -4.81 + -(0) = -2.41 -4.81 + -(GND) = -4.76 -4.81 + -(-vref) = -2.27 -1.55 + -(0) = -0.58 -1.55 + -(GND) = -1.16 -1.55 + -(-vref) = +1.39 +1.71 + -(0) = +0.64 +1.71 + -(GND) = +1.29 +1.71 + -(-vref) = +3.84 +4.99 + -(0) = +2.50 +4.99 + -(GND) = +4.96 +4.99 + -(-vref) = +4.95 +vref + -(0) = +1.28 +vref + -(GND) = +2.56 +vref + -(-vref) = +4.95 0 + -(+vref) = -2.55 GND + -(+vref) = -2.54 -vref + -(+vref) = -4.76 0 + -(0) = 0.00 GND + -(0) = 0.00 -vref + (-0) = -1.28 


In the table, the 1st collumn is just measured with multimeter on the
Vin 3*10k divider, withoud any load. second one is the divider voltages
feed into a voltage-follower opamp circuit, 3rd one is fed into
inverting-amplifier with 1x gain and 4rd finaly with 2x gain.

So this is strange. the "real" voltages on the divider are coming out
unaltered with a impendance amplifier / follower; but are changing
extreme when i use something with gain.
Also in the last test setup, the substractor, when i feed in 2.55V i got
the half out, without any other voltage is going in which could alter it
(0 in the table means unconnected cable)
On the other side, whenever i do operations with buffer stabilised
negative n/o directly from LM336 reference circuit Vref ±2.55V Voltage;
the output never differs from ideal theory values!

What the...?!

I never get "zero-level" signal output on 2.55V "offset-level" input. SC
is a CM106 so the Vref is set right. OPA's are rail2rail and able to
deliver Vout = Vsupply - 0.01V !
Here's a bigger scan:
http://www.abload.de/image.php?img=image0022ff8c.jpg

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Whats the problem ?
You wonder, why a voltage divider built up from 10k resistors
is not stable if a load of 10k is attached ?

Use voltage followers for Va und Vb, and 3 and 4 will work.

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I always got told the resistance of an opamp is very high an practically
no current i flowing from the inputs?

I tried it with a 1MOhm divider bevore. why it didn's work there?
And will it do when i use SoundCard Oputput's (after Chip) directly?

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Correct, the INPUT of the amplifier has a virtually infinite resistance.
But the additional resistors that you connect from the input to the
OUTPUT of the amplifier draw current from your voltage divider, thus
changing the voltage.

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@Xero,

you entirely forget, that the inverting OPamp circuit is creating a
"virtual ground" at the inverting input. So, connecting an inverting
OPamp with a 10k input impedance to your "3 x 10k" voltage divider means
to connect an additional load of 10k to signal ground, which erodes the
potentials of your voltage divider, of course.

Kai Klaas

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So it can even change the potential when i use a 1Mohm Poti for
dividing?

because it even then got wrong...

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>So it can even change the potential when i use a 1Mohm Poti for
>dividing?

Yes, even more, because then the 10k input impedance of inverting OPamp
is even more loading the voltage divider. The voltage divider must have
an impedance that is way smaller then the input impedance of inverting
OPamp. So, connecting an inverting OPamp to a voltage divider is not a
good idea in your example. But a voltage follower with its nearly
infinite input impedance could be used.

Show us in a bit more detail, what exactly you want to implement. An
inverting OPamp could be used, eventually, for your purpose, but not in
the way you intend right now. Is it a pot you want to buffer?

Kai Klaas

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Is your soundcard able to put out DC? Is there no highpass?

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Ok... you all was right :<

Have used an follower as buffer and done the Tests now. its working!

What i want to build? The Soundcard-DAC for Lasercontrolling!

I didn't drawed my full circuit to scan yet, but can do it.
Merely it converts single ended signal from soundcard, which swings from
0.7V to 3.8V with 2.55V as middle. thus need to be converted to
differential output with 10V p-p, also +5V to -5V

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@ sssss

We desolder the output coupler wires for that

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