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

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Message-ID: <20250118173516.130ca9fb@jic23-huawei>
Date: Sat, 18 Jan 2025 17:35:16 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: David Lechner <dlechner@...libre.com>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, lars@...afoo.de,
 corbet@....net, marcelo.schmitt1@...il.com
Subject: Re: [PATCH v2 1/1] Documentation: iio: Add ADC documentation

On Sat, 18 Jan 2025 11:03:26 -0600
David Lechner <dlechner@...libre.com> wrote:

> On 1/18/25 9:51 AM, Jonathan Cameron wrote:
> > On Wed, 15 Jan 2025 11:23:24 -0600
> > David Lechner <dlechner@...libre.com> wrote:
> >   
> >> On 1/14/25 7:53 AM, Marcelo Schmitt wrote:  
> 
> ...
> 
> >>> +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 equivalent 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.  
> 
> OK, it sounds like you and Marcelo are considering bipolar to mean that the
> difference is bipolar rather than the inputs. In that case, it doesn't seem like
> there would ever be such a thing as unipolar (true) differential.

You could build it, but it would indeed be odd.

  -------- VREF -------
     ´ `       ´ `               +-------------------+
   /     \   /     \   /        /                    |
          `-´       `-´    --- <  IN+                |
   ------    VREF- -----        |                    |
                                |            ADC     |
   -------- VREF -------        |                    |
     ´ `       ´ `         --- <  IN-                |
   /     \   /     \   /        \       VREF         |
          `-´       `-´          +-------------------+
   ------   VREF-  -----                  ^        
                                          |       
                                   External VREF  

Where we constrain the negative to be higher that
the positive and it's that difference we are reading.

I'm stumped on what it is for though.  Maybe if vref is
0 some sort of current measurement?  Or just possibly
something odd with weighing cells (if there is a weird
data input rule, that's normally where I look for it!)


> 
> I was looking at this from the point of view of only the inputs and not the
> difference. I'm seeing that the input voltage can only be positive, so to me
> that would be unipolar.
> 
> So at the very beginning, when we first mention unipolar and bipolar, it would
> be helpful to add a bit making it clear exactly which point in the system we
> are talking about, the input or the output.
> 

Definitely good to have a definition.  I only care about data so the output
was what mattered to me.  Analog side is someone else's problem :)

Jonathan