Adding/Subtracting DC Offset to Sine Wave: Circuit Needed

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Discussion Overview

The discussion revolves around the addition and subtraction of a DC offset to sine waves in a circuit context. Participants explore methods to achieve this using capacitive coupling and operational amplifiers, focusing on the technical details and implications of each approach.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about adding a 0.4V DC signal to a 2V peak sine wave and subtracting the same DC signal from a 180-degree phase-shifted sine wave.
  • Another participant suggests using capacitive coupling, explaining that the AC signal passes through a capacitor while blocking DC, and discusses the importance of selecting appropriate resistor and capacitor values based on signal characteristics.
  • A different approach using operational amplifiers is proposed, detailing a configuration that sums AC and DC signals without AC coupling, emphasizing the role of feedback resistors.
  • One participant attempts to clarify the calculation of resistor and capacitor values based on the frequency of the AC signal, indicating a potential misunderstanding of the relationship between frequency and component values.
  • Concerns are raised about the safety of working with AC mains voltage, with a participant confirming their sine wave source is from an IC, not mains power.
  • A participant expresses uncertainty about which method (capacitive coupling or op-amp) would yield more accurate results for their project, highlighting the importance of this step.
  • Another participant encourages self-research and consideration of the advantages and disadvantages of each method, emphasizing the educational aspect of the inquiry.

Areas of Agreement / Disagreement

Participants present multiple competing views on the best method for adding/subtracting DC offsets, with no consensus reached on which approach is superior. There is also uncertainty regarding the calculations for component values and their implications.

Contextual Notes

Participants express varying levels of understanding regarding circuit design principles, with some calculations and assumptions remaining unverified. The discussion includes considerations of frequency response and component selection that are not fully resolved.

kiamzattu
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Hello

I have a question regarding DC offset addition. I have a sine wave of Peak amplitude 2V. Now i have a DC signal of 0.4v. How do i add this DC signal to my sine wave?
Similarly I'm having another sine wave of 2Vpeak but phase shifted by 180 from the first one. Now again i have 0.4v DC signal which needs to be subtracted from this sine wave.

Can anyone provide a schematic or a link to any circuit which can do the above job? Please
 
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anbullet said:
Hello

I have a question regarding DC offset addition. I have a sine wave of Peak amplitude 2V. Now i have a DC signal of 0.4v. How do i add this DC signal to my sine wave?
Similarly I'm having another sine wave of 2Vpeak but phase shifted by 180 from the first one. Now again i have 0.4v DC signal which needs to be subtracted from this sine wave.

Can anyone provide a schematic or a link to any circuit which can do the above job? Please

Depending on frequencies and a bunch of other considerations, the easiest way to do it is via capacitive coupling. The AC signal will go through a capacitor, but the cap blocks DC. So say you have your signal comiong in the left side of the schematic, going through a horizontal capacitor, and to one side of a vertical resistor. The other side of the resistor goes to your + or - 0.4Vdc supply. The junction between the cap and the resistor is your AC+DC signal.

Now, you have to think about the high-pass characteristic of that circuit, and ensure that you are getting your AC signal through, and also not loading it too much with the resistor, right?

Quiz Question -- Can you show us how you would go about calculating the R and C values for this circuit, based on your signal source characteristics and the AC frequencies involved?

You can also do this with opamps or other transistor buffer circuits, if the simple RC isolation/combiner is too limited for your application.
 
Here is the opamp way. Get a good opamp. ground the + input to common (or use a 330 ohm resistor to reduce input bias current offsets). Attach two 1 k resistors to the - input. Tie the ac signal to one resistor and the dc signal to the other. Tie another 1k resistor from opamp output back to - input (feedback resistor). Output = sum of ac plus dc, times -1. The - input is called a summing junction. No ac coupling required. If you want an uninverted output, follow it with a gain of -1 inverter.
 
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Thanks for the replies

@Berekman's Quiz question

Is it like f=(1/2*pi*R*C), where f is the value of the frequency that the circuit has to allow.
In my case source is 0f 50 Hz, so i should substitute 50 in place of f and calculate values for R and C.
now that i have only one equation, i should have any of the values as a fixed one and calculate the other.

Am i right? Correct me if I'm not

@ Bob
Thanks a lot. I have a small doubt though. The - input is from the source so i should ground my other terminal of the source and also the + input to this ground,right?
 
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anbullet said:
Thanks for the replies

@Berekman's Quiz question

Is it like f=(1/2*pi*R*C), where f is the value of the frequency that the circuit has to allow.
In my case source is 0f 50 Hz, so i should substitute 50 in place of f and calculate values for R and C.
now that i have only one equation, i should have any of the values as a fixed one and calculate the other.

Am i right? Correct me if I'm not

Yes, correct. 50Hz is a very low frequency, however, so it will take large capacitors to pass that frequency adequately. You are not working with AC Mains 220Vrms, 50Hz, are you? That can be quite dangerous for someone like you with only a very basic knowledge of electricity.
 
No am not. My Sine wave is from an IC only. just 2V peak. Which of the method you suggest would be more accurate? The one with capacitors or with the Op-amp because this is one of the crucial steps in my project and i don't want it be less approximate.
 
anbullet said:
No am not. My Sine wave is from an IC only. just 2V peak. Which of the method you suggest would be more accurate? The one with capacitors or with the Op-amp because this is one of the crucial steps in my project and i don't want it be less approximate.

You need to figure this one out on your own, anbullet. It's for school -- you need to do the work. What are the advantages and disadvantages of each approach? It's your assignment, do some work, and tell us your thoughts.
 
Yeah i will. Just got overboard. sorry
 

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