xr2206 D.C offsets?

[Neil Groves]

Hi Guys....

Well i just joined this what looks to be an amazing site, having stumbled across it in the bing search engine whilst looking for the xr2206 function generator chip, i have just built from a kit using this I.C a function generator, having looked at my waveforms on the scope i notice they all have a D.C bias/offset of a couple of volts, i thought A.C (audio) signals bounced around 0 Volts and had no D.C offset? i intend to use the generator as an audio source but am a little concerned that my output has a D.C offset, can anyone explain this to me please?

I used to work as an electronics technician 11 years ago, and am very rusty since i have only just got around to setting up the 'Lab' again.

thanks in anticipation

Neil.

 

[moffy]

Not having looked at the datasheet I assume you are using a single supply? If so then the xr2206 is just self biasing. If you want no offset then pass the output through a cap.

 

[Neil Groves]

What do i do if i want to feed the output into a filter to check the frequency response which is why initially built it? do i need a D.C offset or do i throw it an A.C Coupled signal? what do i google to read up on this stuff?

thanks

Neil.

 

[carbonzit]

What do i do if i want to feed the output into a filter to check the frequency response which is why initially built it? do i need a D.C offset or do i throw it an A.C Coupled signal?

Pretty sure a DC offset may throw things off, so you probably want to AC couple it. Size of capacitor will depend on frequency. What's your frequency range? (You can use different size caps for different Fs.)

 

[Neil Groves]

Pretty sure a DC offset may throw things off, so you probably want to AC couple it. Size of capacitor will depend on frequency. What's your frequency range? (You can use different size caps for different Fs.)

I'm building a 3 channel light modulator for music so i'm dealing with 20Hz-20Khz, i guess it's just a case of experimenting and see what works?

Neil.

 

[moffy]

Since you have the DC you can use it to advantage and use a 47u electrolytic cap. Just bias the other side down to ground with say a 10k resistor.

 

[carbonzit]

Since you have the DC you can use it to advantage and use a 47u electrolytic cap. Just bias the other side down to ground with say a 10k resistor.

Couple questions here:

1. How would this be "using [the DC] to advantage"? You're just blocking it, aren't you?
2. Why bias the far side of the cap to ground with a resistor? Won't it function just fine simply being in series with the output?

 

[Neil Groves]

Couple questions here:

1. How would this be "using [the DC] to advantage"? You're just blocking it, aren't you?
2. Why bias the far side of the cap to ground with a resistor? Won't it function just fine simply being in series with the output?

I will find time hopefully to experiment today, if i find anything interesting i will let you know.

thanks for your help guys

Neil.

 

[moffy]

An electrolytic capacitor has polarity i.e. + and - sides. You have to ensure that the + side is always more positive than the negative. The output bias from the xr2206 ensures this so that you can use a polar electrolytic rather than a non polar cap such as mylar or ceramic. Just compare the size and cost of a 47u electro with any of the non polar types! So in this case you can make the offset work to your advantage.

 

[Neil Groves]

Not having looked at the datasheet I assume you are using a single supply? If so then the xr2206 is just self biasing. If you want no offset then pass the output through a cap.

I think that was my question moffy, if i am using this unit to test filter responses in the audio frequency range, do i use a D.C offset or an A.C offset? what is an audio circuit expecting to see at it's input?

Neil.

 

[alec_t]

do i use a D.C offset or an A.C offset?

Not sure what you mean by an AC offset. If an AC sine-wave is asymmetrical about the 0V axis then it has a DC offset.

what is an audio circuit expecting to see at it's input?

That depends entirely on what the audio circuit is. Some might tolerate a DC offset and others might not.

 

[Neil Groves]

So really a case of suck it and see?

i think what i need to do is monitor the output of a tape deck/c.d player and see where the signal sits, then adjust my test unit to suite.

Neil.

p.s soory about the A.C offset question earlier, it was early hours for me and i hadn't had my coffee....lol

 

[moffy]

Use the cap and resistor as already mentioned.