What is DC offset block in Oscilloscope
Sometimes we need to zoom in on a particular part of a waveform to see the ripple, noise or signal superimposed on a DC signal. In the image below we may be interested in the ripple waveform on the DC voltage. Simply turning up the sensitivity would push the ripple waveform off the top of the display.



Figure 1. Image source: Wikipedia Ripple.

We have two options:


Add a DC blocking capacitor in series with the probe. This will present the trace as an AC waveform centred around zero volts. There are some problems with this:


You can't take DC measurements of points on the waveform anymore because the DC component has been removed.
The blocking capacitor will attenuate low frequency signals so low-frequency waveforms will be distorted.

Add a DC offset. This is the better option as we can now increase the sensitivity (pushing the ripple off the top of the display) but bring it back down into view again at higher resolution by using the DC offset. We can now examine the trace and take measurements because we (and the oscilloscope) know the offset that has been applied.


For example, if the waveform of Figure 1 has an average of 25 V and a ripple of 5 V peak to peak we might set the vertical sensitivity to 1 V/div and a DC offset of -25 V. Now the ripple would be centred about the zero line and be about &pm;2.5 divisions high.