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### Topic: How to eliminate the offset of a signal (Read 3651 times)previous topic - next topic

#### zaciatok

##### Jun 26, 2013, 04:47 pm
Hello,
I would like to ask you very kindly how I can eliminate the offset of a signal. My signal is oscillating around 1.4 volts so the maximal displacement of my signal is 1.45 volts and the minimal 1.35volts. I need that signal oscillating around zero so maximal displacement 0.05 volts and minimal -0.05. In other words +/- 0.05 volts. How Can I do that ? Could you help me with this problem please?.
Have a nice day.

#### johnwasser

#1
##### Jun 26, 2013, 05:44 pm
To eliminate the DC component of an AC signal, run it through a capacitor.
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#### PaulS

#2
##### Jun 27, 2013, 10:07 pm
Quote
To eliminate the DC component of an AC signal, run it through a capacitor.

Buy a spare Arduino, though, before you run the negative voltage into the one you have. You'll need the spare.

#### zaciatok

#3
##### Jun 28, 2013, 09:06 pm
Hello,
I have tried capacitors but, it is not working. I dont really understand why! I have used 1uF and10uF electrolitic capacitors, but it is not solving the problem, the signal is still oscillating around 1.4volts.
Could you explain why the condensator is not solving that problem? . I have connected the signal to the capacitor positive leg and the other to the ground? is it correct ? or how should I connect the capacitor???
Best regards.

#4
For example

#### zoomkat

#5
##### Jun 28, 2013, 09:41 pm
Quote
how should I connect the capacitor???

How do you have it mounted now? You might run your signal thru a resistor (to limit the charging swings of the capacitor) to the end device, and connect the capacitor (+ to the signal and - to ground) between the resistor and the end device.
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#### retrolefty

#6
##### Jun 29, 2013, 07:03 pm
Using a series cap will not let you be able to read a value for the signal when it's below 0vdc, the count will remain 0 for all values less then 0. It won't damage the chip as the signal never gets .5vdc below ground or greater, but again it won't be measurable as only the portion more positive then ground will give valid values. So if you are trying the measure the peak to peak value of the signal a simple series cap will not provide a solution.

Perhaps if you tells us what properties of the signal you are trying to measure we might be able to give suggestions.

Lefty

#### mirith

#7
##### Jul 02, 2013, 09:15 pm
Another option is a unity Summing Op-amp design with the signal on one leg and -1.40V on another.  Though you do need to account for the inverting nature of the op-amp circuit.

Though I do believe you can (at least on some of the atmel chips) use 2 ADCs to create a differential input and read below 0V.  I could be wrong though.

#### retrolefty

#8
##### Jul 04, 2013, 03:24 pm

Another option is a unity Summing Op-amp design with the signal on one leg and -1.40V on another.  Though you do need to account for the inverting nature of the op-amp circuit.

Though I do believe you can (at least on some of the atmel chips) use 2 ADCs to create a differential input and read below 0V.  I could be wrong though.

Yes that is wrong. At no point can any input pin be subjected to a voltage of greater then negative .5 volts or more relative to the chips common ground potential, as that would cause the pin's negative clamping protection diode to conduct, causing damage to the chip if the source impedance of the signal allows too much current to flow.

Lefty

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