Using 230-12V transformer as sine wave source

[savvej]

I am using atmega 32 adc for data capturing. And sending the captured data via serial port to PC for plotting.I am using sine wave source as the transformer secondary 12V output with clamping circuit and resistor didvider followed to get output in range of 0-5V(with an additional offset of 1.5 dry cell connected in series with the output).The circuit diagram is attached with the post.

Baud rate for serial communication =38400
No. of reading taken=1024

Now with above said when I do data capturing,what I get is a highly noisy output of the sine wave which is kind of amplitude modulated where in you can see another sine wave(photo of graph attached).Now why is it so?(its not noise because amplitude of sine wave(like carrier amplitude in AM) is quite big and the amplitude of sine wave inside it(you will understand when you see the graph) too is same.

What should I do so as to get a nice Sine wave?

PS. the data plotted in graph(plotted using xmgrace in ubuntu) is for first 100 points
PS. also attached is "data.txt" file incase if you have other plotting software

 

[smanches]

How fast are you capturing the data? Most likely you are not capturing it fast enough and are seeing a composite waveform being created from the data you have. Increase your sampling speed and see what happens.

 

[ericgibbs]

hi,
I would agree with 'smanches' your sampling rate is too slow.

 

[savvej]

Thanks @ smanches and @ericgibbs
Actually sampling and plotting simultaneously (using python was taking most of the time).So now I am not doing realtime plotting instead with data being stored in uC itself and then transferring it to PC after capturing has been completed.
P.S. Attached is the waveform I have got

PS.Actually sampling interval was too big and plotiing was happening as said by above

 

[ericgibbs]

hi,
That looks a lot better.
I find its better to store samples in real time then after the sampling buffer is full, plot the results.

Whats the purpose of the diode.??

 

[savvej]

Whats the purpose of the diode.??

Diode + Cap work as clamping circuit.
By raising V0sin(wt) to V(1+sin(wt)) and then using potential divider to scale i down to around 1.3V AC volts .Since diode is not ideal still there will be some -ve part.so to remove it using the 1.5 V battery

 

[ericgibbs]

hi,
Using LTS shows this results , with and without diode.

 

[savvej]

As your graphs show offset moves from 1.4 to 2.4V so clamping is working.Only that I wanted voltage to be well above zero.Else as you show 1.5V offset would be sufficient.