Voltage offset shifting

[bokica1991]

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]

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]

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]

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]

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

 

[Harald Kapp]

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]

Harald,

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

 

[Harald Kapp]

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]

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.)



Bob

 

[Harald Kapp]

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!