Coupling mode

What it does

Sometimes you have small waveform variations but they are found at a relatively large and constant offset from zero. We can label the small varying part of the waveform as an Alternating Current (AC) and the relatively steady-state offset as a Direct Current (DC).

By changing your PicoScope Automotive oscilloscope channel’s Coupling mode you can change how you view these AC and DC parts of your waveforms.

With AC coupling, PicoScope rejects very low waveform frequencies, below about 1 Hz. This removes the offset so you can use the channel’s full resolution to accurately measure the AC waveform characteristics. The consequence of AC coupling is that the waveform is centered around zero and you cannot measure the waveform level with respect to ground.

DC coupling does not remove the offset and the waveform retains its level relative to measurement ground. I.e. both the AC and DC parts of the waveform are preserved.

Note: the Coupling mode labels are illustrative and not literal descriptions of the types of waveforms they are meant to be used with: Although the Coupling mode labels are borrowed from AC and DC power source descriptions, you do not have to use AC coupling with only alternating current waveforms and DC coupling with only direct current waveforms.

How it can help you

If the varying part of your waveform is small relative to its offset then the varying part can suffer from a relative lack of resolution as (in DC coupling mode) the input range must be increased to cover both the offset and varying waveform parts (to prevent over-ranging).

By removing the waveform offset using AC Coupling, you can decrease the input range to cover only the varying waveform part and view it with a significant increase in resolution.

In automotive diagnostics, it is useful to first look at your waveform in DC coupling mode to check whether or not the waveform offset is as expected and then change to AC coupling mode, with a decreased range, to examine the varying part’s finer details.

Typical uses are:

  • Alternator testing - use DC coupling to check the overall charging/system voltage and AC coupling to check the smaller ripple for signs of diode or winding defects.
  • Testing certain types of wheel speed sensor - use DC coupling to check for any expected (or not) offsets and AC coupling to check the superposed output signal.
  • Fuel rail pressure sensors in Common Rail systems - use DC coupling to check expected values then AC coupling to view small changes in rail pressure related to injector or control valve operation.

How to use it

Select a channel to open the Channel options panel. Select either AC or DC under Coupling Mode on the Vertical tab.

An animation showing how to use Coupling mode in PicoScope 7 Automotive