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How do I DC adjust an audio signal?

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Sceadwian

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I have a 2volt peak to peak AC waveform coming from an audio source, a standard line-out signal. I'm feeding this signal into an ADC with a 1 megaohm resistance. The ADC only reads from 0 to 5 volts though. How do I DC bias the AC signal? 2.5 volts would be the obvious DC bias, but I don't know how to go about it, help please! =) Any simple theory behind DC biasing (including transitor biasing) would be appreciated as well.
 

Probably the simplest method of introducing dc bias is by connecting two resistor voltage divider between reference and GND and connecting a capacitor between the AC source and the next stage ..
Without going into detailed calculations, you can try 2 x 10kΩ and 100µF ..
Other ideas include opamps ..
See pictures below ..
Regards,
IanP
 

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    Sceadwian

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How would the output resistance of the voltage divider affect the injected AC signal, and what is the effect of increased or reduced values of the capacitor at audio frequencies? I have a 10k pot (basically 5k+5k in your example) and a generic pack of capacitor's from radio shack. I should be able to glub something together that works at least at low frequencies, yes? (100-400hz)
I was planning on generating a 60hz sine wave and feeding that in for a first test. So a 10k pot set at dead center with a 100uf cap fed into the output from the source, should give me some kind of result?
 

10kΩ pot is perfectly fine, and on top of that, if required, with this pot you will be able to "slightly" correct the dc bias ..
I can't see any problems with 60Hz (and with the whole audio frequency range) .. for this frequency 100µF impedance is ≈27Ω, which is negligible comparing with 5kΩ+5kΩ potentiometer ..
Regards,
IanP
 

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    Sceadwian

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What about 47pf for the AC coupling? What would be a good value to use for the resistor network at that point? I have plenty of standard value resistors. Not many capacitors. Is this even possible at 60hz? Also where did you calcuatle the impedance for the frequency as a resistance? I could very much use that.
 

Bear in mind that the role of a coupling capacitor is to separate AC from DC, so its imedance for the lowest frequency should be negligable ..

47pF impedance at 60Hz, for example, is somewhere around 56MΩ (Xc=1/ωC), so with 10kΩ behind it the AC signal will be reduced to almost 0 ..

Try this combination with 100µF (or more) and 10kΩ pot ..

Regards,
IanP
 

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    Sceadwian

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What would be the ideal value capacitor I should use on a standard 2v p/p line-out audio signal that would give me the most signal to start with? At 15khz or less frequency?
 

15kHz is not where the problem lays ..
The difficulty is at the low audio frequency limit, say 20Hz ..
Try to select a capacitor which Zc is below, say 1%, of the lowest impedance in the circuit: output of the audio source and input of the next stage ..
In many cases the output Line impedance is 10kΩ, so aplying the above rule at 20Hz the capacitance will be ≈800µF, and I assume that the input impedance of the next stage (in your case two resistors) >1MΩ --> to ensure this condition you will need a buffer ..
If you allow more realistic limit of 10% you can stay with 100µF coupling cap ..
Regards,
IanP
 

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