Hi David, thank you for your suggestions. I think I've applied most of them and will soon send a v2 only with the docs. Replying here mostly on the comments I didn't comply with. On 12/18, David Lechner wrote: > On 12/18/24 8:38 AM, Marcelo Schmitt wrote: > > ADCs can have different input configurations such that developers can get > > confused when trying to model some of them into IIO channels. > > ... > > > > Add documentation about common ADC characteristics and IIO support for them. > > ... > > +In the ADC driver, `differential = 1` is set into `struct iio_chan_spec` for the > > +channel. See ``include/linux/iio/iio.h`` for more information. > > + > > +1.2.2 Differential Unipolar Channels > > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > To be consistent with the other sections, move unipolar before bipolar. I had differential unipolar before differential bipolar on a preliminary version (not sent to the mailing list), but it lead to a much more intricate explanation of differential unipolar. That's why I prefer to keep the Differential Unipolar section after Differential Bipolar. > > > + > > +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. > > The diagrams are really helpful, so please add a diagram in this section as well. There is no diagram for Differential Unipolar. What would be the Differential Unipolar diagram is the diagram for Pseudo-Differential Unipolar. Having Differential Unipolar section here also makes it closer to the Pseudo-Differential Unipolar diagram. > ... > > + > > +1.3.1 Pseudo-differential Unipolar Channels > > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > + > > +:: > > + > > + -------- +VREF ------ +-------------------+ > > + ´ ` ´ ` / | > > + / \ / \ / --- < IN+ | > > + `-´ `-´ | | > > + --------- IN- ------- | ADC | > > The bottom rail should be GND, not IN-. Typically, the common mode voltage is > VREF / 2. In other words it is halfway between the two rails. IN- may be above GND (e.g. at VREF / 2 as a typical common mode voltage). In that case, the minimum voltage for IN+ (i.e. the bottom rail) would be VREF / 2. The generic constraint would be that IN+ does not fall below IN-. See bipolar/unipolar configuration section of AD4170 datasheet page 46. https://www.analog.com/media/en/technical-documentation/data-sheets/ad4170-4.pdf On that example, VREF is actually a differential voltage reference that is 2.5V nominal voltage (halfway between 5V AVDD and 0V AVSS). If IN+ is allowed to go below IN-, then this becomes Pseudo-differential Bipolar. > > > + | | > > + Common-mode voltage --> --- < IN- | > > + \ +VREF -VREF | > > + +-------------------+ > > + ^ ^ > > + | +---- External -VREF > > This is unipolar, so would not expect -VREF here. I think IN- could in theory be negative (bellow GND) if the ADC inputs are true bipolar inputs. Though, I have never seen such thing so can't say for sure. Anyway, -VREF is not doing anything on this setup so I omitted it in v2. > > > + External +VREF > > + > > +A **pseudo-differential unipolar** input has the limitations a differential > > +unipolar channel would have, meaning the analog voltage to the positive input > > +IN+ must stay within IN- to +VREF. The fixed voltage to IN- is sometimes called > > +common-mode voltage and it must be within -VREF to +VREF as would be expected > > +from the signal to any differential channel negative input. > > + > > +In pseudo-differential configuration, the voltage measured from IN+ is not > > +relative to GND (as it would be for a single-ended channel) but to IN-, which > > +causes the measurement to always be offset by IN- volts. To allow applications > > +to calculate IN+ voltage with respect to system ground, the IIO channel may > > +provide an `_offset` attribute to report the channel offset to user space. > > In some chips though, the common mode voltage may be GND. (Example is AD7944 > that calls this "ground sense"). So in that case, there is no common mode > supply or ``_offset`` attribute. > Added a comment about it omitting the ``_offset`` in those cases. My understanding is that, because the common mode voltage (or ground sense in AD7944's case) is at GND, the ``_offset`` is always zero and that's why the ``_offset`` attribute is not needed in that case. Whenever the common mode voltage is at something other than GND, we would need ``_offset`` to be able to get the voltage relative to GND. Oh well, that's just another way of saying what you already told me I guess.
