# Help with this dc offset circuit

#### Mulo2k2

Joined May 20, 2021
7
Qualcuno può aiutarmi con una spiegazione della funzione di questo circuito.

Translated;
Can anyone help me with an explanation of the function of this circuit

I can't find any article on the web

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#### ericgibbs

Joined Jan 29, 2010
18,827
Hi Mulo,
Welcome to AAC.
Can you post the complete circuit diagram so that we can see how it could be used.?

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#### Mulo2k2

Joined May 20, 2021
7
Yes!

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#### ericgibbs

Joined Jan 29, 2010
18,827
hi M,
The audio output of the OPA is connected via 10uF DC blocking capacitor and connected to the Analog input of an Arduino.
The two 100k's bias the ADC input to Vcc/2, so the AC audio signal can be converted by the ADC.

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Update:

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#### Mulo2k2

Joined May 20, 2021
7
Ok thanks
I'm doing a school project, what do you suggest me to write in the theoretical notes

#### ericgibbs

Joined Jan 29, 2010
18,827
hi M,
What is the actual question in the homework assignment, what does it ask you to explain.??

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#### Mulo2k2

Joined May 20, 2021
7
Describe the Operation of the part of the circuit

#### ericgibbs

Joined Jan 29, 2010
18,827
hi,
OK,
Do you know what the A0 pin is designed to do internally in the Arduino when programmed to be an Analog input pin.?
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#### Mulo2k2

Joined May 20, 2021
7
Y

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#### ericgibbs

Joined Jan 29, 2010
18,827
hi,
From that short clip of the Sketch it appears that the program is designed to measure the frequency of the Audio input signal.

It is possible that the OPA is being used to boost the level of the signal.?
If so the OPA could be set so that that output voltage is about +/-2.5V

As you my know the input pin of an Arduino should be limited to 0V thru +5v

Can you post all of the Sketch code.?

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#### djsfantasi

Joined Apr 11, 2010
9,160
It is possible that the OPA is being used to boost the level of the signal.?
If so the OPA could be set so that that output voltage is about +/-2.5V

As you my know the input pin of an Arduino should be limited to 0V thru +5v
Why +/-2.5V on the OPA output?

Why not 0 - +5V...?

Makes things easier, doesn’t it?

#### ericgibbs

Joined Jan 29, 2010
18,827
hi dj,
I believe the original question is: what is is the function of the 10uF and the two 100k's

I have made it +/-2.5v before the 10uF, so that he can figure out the resultant signal at theA0 input.
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#### Mulo2k2

Joined May 20, 2021
7
The sketch

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#### ericgibbs

Joined Jan 29, 2010
18,827
hi M,
Thanks, that helps.
It is designed to measure frequency of an audio signal.

edited!

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#### ericgibbs

Joined Jan 29, 2010
18,827
hi M,
look at this simulation.
Tell me what the difference is between Vout and the ADC signals.
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#### Mulo2k2

Joined May 20, 2021
7
thank you very very much.
everything is much clearer now

#### ericgibbs

Joined Jan 29, 2010
18,827
hi Mulo,
Thats good to hear. Congratulazioni

I did two more simulations for you in case you had a problem
One without the 10uF and the other without the 100k's.
You can see the problem.!
The ADC input swings +/- 2.5Volts
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