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| AC and DC Coupling Mode in An Oscilloscope |
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| alsetalokin4017:
--- Quote from: David Hess on November 11, 2018, 11:35:22 pm ---The ground setting on good oscilloscopes also precharges the internal AC coupling capacitor to the average DC level without blowing out the input protection or front end circuits when switching coupling modes or moving the probe. That is why old oscilloscopes controls have the ground selection between the DC and AC selections; some include a special precharge setting or multiple ground settings. --- End quote --- That's right and is an excellent point. My old Tek and HP scopes have input coupling switches that do just that, by switching through the "ground" position when changing from AC to DC or vice versa. But what does the DS1054z do? I seem to recall some instructions somewhere about precharging input caps on the Rigol but I can't find any mention of it in the User's Manual at the moment. In fact I don't even know if the Rigol actually uses an input cap for AC coupling or if it is done somehow in software. In my ancient Link parallel port DSO you can actually hear a relay click when switching coupling, plus one of its coupling caps is shorted so has no effect. |
| w2aew:
--- Quote from: wuiven64 on November 02, 2018, 11:13:27 am ---Hi, I recently using a Tektronix TPS2014 oscilloscope to analyse a square wave. I would like to ask what is the functions of AC and DC coupling mode in an oscilloscope and how do we choose the suitable coupling mode in an oscilloscope to analyse the waveform. Thanks. --- End quote --- I did a video on this about 7 years ago: |
| David Hess:
--- Quote from: alsetalokin4017 on November 12, 2018, 09:45:03 pm ---That's right and is an excellent point. My old Tek and HP scopes have input coupling switches that do just that, by switching through the "ground" position when changing from AC to DC or vice versa. But what does the DS1054z do? I seem to recall some instructions somewhere about precharging input caps on the Rigol but I can't find any mention of it in the User's Manual at the moment. In fact I don't even know if the Rigol actually uses an input cap for AC coupling or if it is done somehow in software. In my ancient Link parallel port DSO you can actually hear a relay click when switching coupling, plus one of its coupling caps is shorted so has no effect. --- End quote --- The Rigol DS1000Z series implements "ground" in a later stage so it has no way to precharge its input AC coupling capacitor. Its input protection is no better and actually worse so it is likely more susceptible to damage under conditions where this would matter. Implementing ground at a later stage also means that the Rigol DS1000Z series does not correctly report its own input noise when ground coupling is selected. An external short is required. Arguably this is irrelevant though since it does not perform RMS noise measurements correctly anyway. |
| vk6zgo:
--- Quote from: David Hess on November 11, 2018, 11:35:22 pm ---The ground setting on good oscilloscopes also precharges the internal AC coupling capacitor to the average DC level without blowing out the input protection or front end circuits when switching coupling modes or moving the probe. That is why old oscilloscopes controls have the ground selection between the DC and AC selections; some include a special precharge setting or multiple ground settings. --- End quote --- Oscilloscopes with a "DC offset" control are often used to look at an AC component on a DC voltage level, without switching them to AC coupling. Many of the 'scopes I used over the years were like that. |
| David Hess:
--- Quote from: vk6zgo on November 13, 2018, 04:59:12 am --- --- Quote from: David Hess on November 11, 2018, 11:35:22 pm ---The ground setting on good oscilloscopes also precharges the internal AC coupling capacitor to the average DC level without blowing out the input protection or front end circuits when switching coupling modes or moving the probe. That is why old oscilloscopes controls have the ground selection between the DC and AC selections; some include a special precharge setting or multiple ground settings. --- End quote --- Oscilloscopes with a "DC offset" control are often used to look at an AC component on a DC voltage level, without switching them to AC coupling. Many of the 'scopes I used over the years were like that. --- End quote --- Offset controls are nice. Most oscilloscope vertical position controls function as limited range offset controls. In the extreme case are oscilloscopes which have differential comparator inputs. A Tektronix 7A13 can operate at 1mV/div over +/-10 volt offset range which is like having a position control with a 20,000 division range. |
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