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# Why is output of op amp sine wave generator acquired a DC level?

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#### treez

##### Guest
Hello,

I have produced a 50KHz sine wave by filtering a 5V, -5V square wave in LTspice.

Despite the output being capacitively coupled, the output sine wave has managed to acquire a DC level of 70mV…….the input square wave has an input DC level of 0mV.

How has the sine wave got this DC level?…here is the schematic….(its also linked below )

Schematic
http://i49.tinypic.com/2953lom.jpg

Here is the output sine waveform, showing its been “Lifted”….

Output waveform
http://i45.tinypic.com/wi4ag.jpg

Here is the LT1006 op amp datasheet:
http://cds.linear.com/docs/Datasheet/1006fa.pdf

View attachment Schematic ...sine wave generator.pdf

- - - Updated - - -

Sorry , i forgot to attach the LTspice file

- - - Updated - - -

Hi, the LTspice is now attached....in txt form , as it wouldnt allow .asc form

#### Attachments

• filtered square wavee.txt
4.2 KB · Views: 69

It could be an offset caused by the bias current to the + input flowing through the 100K resistor and not for the - input of the output amp. Connect the - input to the output via a 100K, rather then a short circuit and see if things improve. Could be a leakage current through the coupling capacitor.
Frank

your offset must be occuring in the last stage only, because like you have pointed out, its all Cap coupled.
Also i calculate/ measure the offset to be 80mV and not 70mV. Am i correct ?

However strangely enough, this offset is a strong function of your frequency. And it does not depend on either the biasing resistor (100K) or on the coupling cap value.
I don't get it either.
sorry....

Treez,

are you aware that you have chosen the worst solution for the lowpass? Four first order RC sections in series.

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