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What is the function of DC offset block in an oscilloscope?

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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.

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Figure 1. Image source: Wikipedia Ripple.

We have two options:

  1. 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.
  2. 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 ±2.5 divisions high.

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  • \$\begingroup\$ Great feature. Just pointing out that cheaper scopes might not have it \$\endgroup\$ – Scott Seidman Jul 7 '19 at 14:36
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  • when the signal goes off screen, the front end can be saturated and shifting the trace into the viewable area may not prevent this 1st stage high variable gain from saturation.

  • However the ability to apply input DC offset means you can measure DC **“load regulation error”**on a DC regulator at some voltage like 12Vdc with 0.1% resolution for measuring the sag from rising output current due to finite Rout and finite negative feedback gain to reduce this error. Normally load Reg. Errors are <<1% on good regulators.

  • however for PSU ripple measurements, the best Test Engineers will always use an AC-coupled 50 Ohm-terminated transmission line or coax) to DSO to eliminate probe ground inductance but no DC offset needed. This is critical when you get rise times of ripple << 0.1us.

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