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

Status
Not open for further replies.
T

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: 76

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.
 

Status
Not open for further replies.

Similar threads

Cookies are required to use this site. You must accept them to continue using the site. Learn more…