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Voltage offset shifting

bokica1991

Aug 27, 2012
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Hallo everyone,

I have question, for my project I need square wave oscillator (-2.5,2.5V), I generated pwm signal (0.5V) from Arduino Uno, but I can't shift it. I tried to make sumator with PWM signal and -2.5V, but it didn't worked out. Can somebody help. :)

Thank you
 

Gryd3

Jun 25, 2014
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Hallo everyone,

I have question, for my project I need square wave oscillator (-2.5,2.5V), I generated pwm signal (0.5V) from Arduino Uno, but I can't shift it. I tried to make sumator with PWM signal and -2.5V, but it didn't worked out. Can somebody help. :)

Thank you
Only thing I can think of is using a capacitor to decouple a 5V pwm signal from the Arduino... The problem here is that if the PWM value is anything other than 50% the center point of the PWM signal will drift, in theory you could end up with a -1v to 4v PWM signal.
What are you trying to do? Can you provide example code or schematics for what you tried? You may need additional circuitry to push-pull the values.
 

Harald Kapp

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Show us your summing circuit. And tell us what exactly didn't work. Which signal did you expect, which signal did you see instead?


With some assumptions from my side:
A typical OpAmp based summer has a transfer function that creates:
Vout = -Rf/Rin*(Vin1+Vin2+...)​

Therefore with Vin2=-2.5V you will see Vout = -Rf/Rin*(Vin1-2.5V) Assuming for simplicity Rf=Rin this simplifies to Vout=-(Vin1+2.5V), therefore for
Vin1 = 0V -> Vout = -2.5V
Vin1 = 5V -> Vout = -7.5V

I think you expect
Vin1 = 0V -> Vout = -2.5V
Vin1 = 5V -> Vout = 2.5V
The respective transfer function is Vout= Vin1-2.5V.
You can create this function by inverting the output from the arduino:
Vin1 = -Varduino -> Vout = -(Vin+2.5V) = -(-Varduino+2.5V) = Varduino-2.5V

Regards,
Harald
 

duke37

Jan 9, 2011
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Show us your circuit.

If all you wish to do is to make the waveform symmetrical about zero, then feed it through a capacitor with a high resistor to ground.
 

bokica1991

Aug 27, 2012
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here is my circuit


I understand what i should do, but i can't find proper OP Amp.

yes, I want to make from 0 -> -2.5V and from 5V-2.5V
 

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Harald Kapp

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I understand what i should do, but i can't find proper OP Amp.
Sorry, you don't understand fully. Almost any OpAmp will do, but yo will need two of them:
  • OpAmp 1 inverts the output of the arduino
  • OpAmp2 will perform as summer

Harald
 

BobK

Jan 5, 2010
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Harald,

I disagree. That circuit will work just fine. Here is a simulation:

offset.JPG
 

Harald Kapp

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Bob, please compare the phase of the +-2.5V output with the 0...5V input. If I'm not mistaken, this circuit introduces 180° phase shift (inverts). If that is no issue for the OP, you're right.
 

BobK

Jan 5, 2010
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Yes, it does shift the phase, but this circuit will do the same without shifting the phase. And I eliminated the extra supply by using a voltage divider to bias the + input. The same mod can be done to the previous circuit with the same value resistors.

Any linear transformation of one range of voltages to another can be done by a single op amp (as long as the input and output voltages are within its range.)

offsetnoninvert.JPG

Bob
 

Harald Kapp

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O.K., the trick here is the resistive divider from V3 to -5V.
The OpAmp in this circuit works not as summer, but as buffer and amplifier *2.

Nice and simple!
 
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