I'm generating sine waves by using a micro-controller and DDS module. The output sine normally swings always positive between 0 to around 1V. But I want to generate a real alternating sine with symmetric swing.
If I AC couple via a scope the sine output becomes absolutely symmetric. For instance if the sine wave Vpp is 1.04V. Scope's AC coupling results a swing between -520mV to +520mV.
But if I myself AC couple with an RC high pass filter with 22uF cap and a 1Meg resistor the scope in its DC setting shows a sine with around 10mV error between the swings I observe a swing between -512mV to +528mV.
What could be the reason and how can I fix this to a symmetric swing?
Unfortunately using such large cap for AC coupling to obtain frequency independent solution has problem. I first observed this after sampling the data in plot. This new problem is not related to leakage or DDS offset ect. The problem is the response time is too long when switching from one frequency to another. By response time I mean the time taken for the sine to take its final form when the DDS sine output freq. is varied.
I simulated the circuit to show what I mean see green plot decaying(for this case not that bad but depending on when it is varied and freq. change that decay varies a lot):
Above freq. is switched from 0.1Hz to 1Hz. Response is even worse for different frequencies. I don't need a crazy fast response but definitely this is not reliable. And here from a real data acquisition shows the bottom of the sine wave how it is varying for many seconds. I also observed more weird transitions for different frequency changes:
Basically the above RC AC coupling will not work for me.
So I think my last solution is to remove the offset by a circuit like this:
If I use the above technique there is no issue with the response time. But I need to find a way to fine tune the 1V by a voltage divider and rely on Vref will be stable and DDS have same offset. Anything can be improved for this above circuit?