Re: [PATCH v2 1/1] Documentation: iio: Add ADC documentation

[Jonathan Cameron <jic23@kernel.org>]

> > > > +1.2.2 Differential Unipolar Channels
> > > > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > > +
> > > > +For **differential unipolar** channels, the analog voltage at the positive input
> > > > +must also be higher than the voltage at the negative input. Thus, the actual
> > > > +input range allowed to a differential unipolar channel is IN- to +VREF. Because
> > > > +IN+ is allowed to swing with the measured analog signal and the input setup must
> > > > +guarantee IN+ will not go below IN- (nor IN- will raise above IN+), most
> > > > +differential unipolar channel setups have IN- fixed to a known voltage that does
> > > > +not fall within the voltage range expected for the measured signal. This leads
> > > > +to a setup that is equivalent to a pseudo-differential channel. Thus,
> > > > +differential unipolar channels are actually pseudo-differential unipolar
> > > > +channels.    
> > > 
> > > I don't think this is equevent to pseudo-differential unipolar. That one has
> > > a common mode voltage supply on the negative input. This one has a full range
> > > signal on the negative input. This is the diagram I was expecting here.
> > > 
> > > ::
> > > 
> > >   -------- VREF -------
> > >     ´ `       ´ `               +-------------------+
> > >   /     \   /     \   /        /                    |
> > >          `-´       `-´    --- <  IN+                |
> > >   ------ GND (0V) -----        |                    |
> > >                                |            ADC     |
> > >   -------- VREF -------        |                    |
> > >         ´ `       ´ `     --- <  IN-                |
> > >   \   /     \   /     \        \       VREF         |
> > >    `-´       `-´                +-------------------+
> > >   ------ GND (0V) -----                  ^        
> > >                                          |       
> > >                                   External VREF  
> > 
> > If it's unipolar, output must be positive which isn't true here.
> > Do we actually see differential unipolar except for the pseudo case with
> > common mode voltage?   Seems like a weird device.  
> 
> I don't think we have differential unipolar with IN- allowed to float (i.e. not
> set to a constant voltage). Haven't seen any of those. Can't think of how we
> would support such thing either. See my reply to David for more thoughts on this.

Agreed it is an odd beast, but that doesn't mean we should necessarily write
text that implies it is always the same as pseudo-differential unipolar.

Meh you do say 'most' so fair enough.


Jonathan