Re: [PATCH v3 5/5] iio: adc: ad7192: Add AD7194 support

[David Lechner <dlechner@baylibre.com>]

On Fri, Feb 16, 2024 at 8:22 AM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Thu, 15 Feb 2024 11:13:19 -0600
> David Lechner <dlechner@baylibre.com> wrote:
>

...

> >
> > Tables 22, 23 and 24 in the AD7194 datasheet show that this chip is
> > much more configurable than AD7192 when it comes to assigning
> > channels. There are basically no restrictions on which inputs can be
> > used together. So I am still confident that my suggestion is the way
> > to go for AD7194. (Although I didn't actually try it on hardware, so
> > can't be 100% confident. But at least 90% confident :-p)
>
> You would have to define a channel number for aincom.  There is an explicit
> example in the datasheet of it being at 2.5V using a reference supply.
>
> I wonder what expectation here is.  Allways a reference regulator on that pin, or
> an actually varying input? Maybe in long term we want to support both
> options - so if aincom-supply is provided these are single ended with
> an offset, but if not they are differential channels between channel X and
> channel AINCOM.
>
> Note though that this mode is described a pseudo differential which normally
> means a fixed voltage on the negative.
>
> So gut feeling from me is treat them as single ended and add an
> aincom-supply + the offsets that result if that is provided in DT and
> voltage from it is non 0.

Calling AINCOM a supply doesn't sound right to me since usually this
signal is coming somewhere external, i.e. you have a twisted pair
connected to AIN1 and AINCOM going to some signal source that may be
hot-pluggable and not known at compile time. As an example, if AINCOM
was modeled as a supply, then we would have to change the device tree
every time we changed the voltage offset on the signal generator while
we are testing using an evaluation board.