--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0-only
+
+=========================
+IIO Abstractions for ADCs
+=========================
+
+1. Overview
+===========
+
+The IIO subsystem supports many Analog to Digital Converters (ADCs). Some ADCs
+have features and characteristics that are supported in specific ways by IIO
+device drivers. This documentation describes common ADC features and explains
+how they are supported by the IIO subsystem.
+
+1. ADC Channel Types
+====================
+
+ADCs can have distinct types of inputs, each of them measuring analog voltages
+in a slightly different way. An ADC digitizes the analog input voltage over a
+span that is often given by the provided voltage reference, the input type, and
+the input polarity. The input range allowed to an ADC channel is needed to
+determine the scale factor and offset needed to obtain the measured value in
+real-world units (millivolts for voltage measurement, milliamps for current
+measurement, etc.).
+
+Elaborate designs may have nonlinear characteristics or integrated components
+(such as amplifiers and reference buffers) that might also have to be considered
+to derive the allowed input range for an ADC. For clarity, the sections below
+assume the input range only depends on the provided voltage references, input
+type, and input polarity.
+
+There are three general types of ADC inputs (single-ended, differential,
+pseudo-differential) and two possible polarities (unipolar, bipolar). The input
+type (single-ended, differential, pseudo-differential) is one channel
+characteristic, and is completely independent of the polarity (unipolar,
+bipolar) aspect. A comprehensive article about ADC input types (on which this
+doc is heavily based on) can be found at
+https://www.analog.com/en/resources/technical-articles/sar-adc-input-types.html.
+
+1.1 Single-ended channels
+-------------------------
+
+Single-ended channels digitize the analog input voltage relative to ground and
+can be either unipolar or bipolar.
+
+1.1.1 Single-ended Unipolar Channels
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+ ---------- VREF -------------
+ ´ ` ´ ` _____________
+ / \ / \ / |
+ / \ / \ --- < IN ADC |
+ \ / \ / \ |
+ `-´ `-´ \ VREF |
+ -------- GND (0V) ----------- +-----------+
+ ^
+ |
+ External VREF
+
+The input voltage to a **single-ended unipolar** channel is allowed to swing
+from GND to VREF (where VREF is a voltage reference with electrical potential
+higher than system ground). The maximum input voltage is also called VFS
+(Voltage input Full-Scale), with VFS being determined by VREF. The voltage
+reference may be provided from an external supply or derived from the chip power
+source.
+
+A single-ended unipolar channel could be described in device tree like the
+following example::
+
+ adc@0 {
+ ...
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ channel@0 {
+ reg = <0>;
+ };
+ };
+
+One is always allowed to include ADC channel nodes in the device tree. Though,
+if the device has a uniform set of inputs (e.g. all inputs are single-ended),
+then declaring the channel nodes is optional.
+
+One caveat for devices that support mixed single-ended and differential channels
+is that single-ended channel nodes also need to provide a ``single-channel``
+property when ``reg`` is an arbitrary number that doesn't match the input pin
+number.
+
+See ``Documentation/devicetree/bindings/iio/adc/adc.yaml`` for the complete
+documentation of ADC specific device tree properties.
+
+
+1.1.2 Single-ended Bipolar Channels
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+ ---------- +VREF ------------
+ ´ ` ´ ` _____________________
+ / \ / \ / |
+ / \ / \ --- < IN ADC |
+ \ / \ / \ |
+ `-´ `-´ \ +VREF -VREF |
+ ---------- -VREF ------------ +-------------------+
+ ^ ^
+ | |
+ External +VREF ------+ External -VREF
+
+For a **single-ended bipolar** channel, the analog voltage input can go from
+-VREF to +VREF (where -VREF is the voltage reference that has the lower
+electrical potential while +VREF is the reference with the higher one). Some ADC
+chips derive the lower reference from +VREF, others get it from a separate
+input. Often, +VREF and -VREF are symmetric but they don't need to be so. When
+-VREF is lower than system ground, these inputs are also called single-ended
+true bipolar. Also, while there is a relevant difference between bipolar and
+true bipolar from the electrical perspective, IIO makes no explicit distinction
+between them.
+
+Here's an example device tree description of a single-ended bipolar channel::
+
+ adc@0 {
+ ...
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ channel@0 {
+ reg = <0>;
+ bipolar;
+ };
+ };
+
+1.2 Differential channels
+-------------------------
+
+A differential voltage measurement digitizes the voltage level at the positive
+input (IN+) relative to the negative input (IN-) over the -VREF to +VREF span.
+In other words, a differential channel measures the potential difference between
+IN+ and IN-, which is often denoted by the IN+ - IN- formula.
+
+1.2.1 Differential Bipolar Channels
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+ -------- +VREF ------ +-------------------+
+ ´ ` ´ ` / |
+ / \ / \ / --- < IN+ |
+ `-´ `-´ | |
+ -------- -VREF ------ | |
+ | ADC |
+ -------- +VREF ------ | |
+ ´ ` ´ ` | |
+ \ / \ / \ --- < IN- |
+ `-´ `-´ \ +VREF -VREF |
+ -------- -VREF ------ +-------------------+
+ ^ ^
+ | +---- External -VREF
+ External +VREF
+
+The analog signals to **differential bipolar** inputs are also allowed to swing
+from -VREF to +VREF. The bipolar part of the name means that the resulting value
+of the difference (IN+ - IN-) can be positive or negative. If -VREF is below
+system GND, these are also called differential true bipolar inputs.
+
+Device tree example of a differential bipolar channel::
+
+ adc@0 {
+ ...
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ channel@0 {
+ reg = <0>;
+ bipolar;
+ diff-channels = <0 1>;
+ };
+ };
+
+In the ADC driver, ``differential = 1`` is set into ``struct iio_chan_spec`` for
+the channel. Even though, there are three general input types, ``differential``
+is only used to distinguish between differential and non-differential (either
+single-ended or pseudo-differential) input types. See
+``include/linux/iio/iio.h`` for more information.
+
+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. That leads
+to a setup that is equivalent to a pseudo-differential channel. Thus,
+differential unipolar setups can often be supported as pseudo-differential
+unipolar channels.
+
+1.3 Pseudo-differential Channels
+--------------------------------
+
+There is a third ADC input type which is called pseudo-differential or
+single-ended to differential configuration. A pseudo-differential channel is
+similar to a differential channel in that it also measures IN+ relative to IN-.
+However, unlike bipolar differential channels, the negative input is limited to
+a narrow voltage range (taken as a constant voltage) while only IN+ is allowed
+to swing. A pseudo-differential channel can be made out from a differential pair
+of inputs by restricting the negative input to a known voltage while allowing
+only the positive input to swing. Sometimes, the input provided to IN- is called
+common-mode voltage. Besides, some parts have a COM pin that allows single-ended
+inputs to be referenced to a common-mode voltage, making them
+pseudo-differential channels. Often, the common mode input voltage can be
+described in the device tree as a voltage regulator (e.g. ``com-supply``) since
+it is basically a constant voltage source.
+
+1.3.1 Pseudo-differential Unipolar Channels
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+ -------- +VREF ------ +-------------------+
+ ´ ` ´ ` / |
+ / \ / \ / --- < IN+ |
+ `-´ `-´ | |
+ --------- IN- ------- | ADC |
+ | |
+ Common-mode voltage --> --- < IN- |
+ \ +VREF -VREF |
+ +-------------------+
+ ^ ^
+ | +---- External -VREF
+ 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 often 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.
+
+The voltage measured from IN+ is relative to IN- but, unlike differential
+channels, pseudo-differential setups are intended to gauge single-ended input
+signals. To enable applications to calculate IN+ voltage with respect to system
+ground, the IIO channel may provide an ``_offset`` sysfs attribute to be added
+to ADC output when converting raw data to voltage units. In many setups, the
+common-mode voltage input is at GND level and the ``_offset`` attribute is
+omitted due to being always zero.
+
+Device tree example for pseudo-differential unipolar channel::
+
+ adc@0 {
+ ...
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ channel@0 {
+ reg = <0>;
+ single-channel = <0>;
+ common-mode-channel = <1>;
+ };
+ };
+
+Do not set ``differential`` in the channel ``iio_chan_spec`` struct of
+pseudo-differential channels.
+
+1.3.2 Pseudo-differential Bipolar Channels
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+ -------- +VREF ------ +-------------------+
+ ´ ` ´ ` / |
+ / \ / \ / --- < IN+ |
+ `-´ `-´ | |
+ -------- -VREF ------ | ADC |
+ | |
+ Common-mode voltage --> --- < IN- |
+ \ +VREF -VREF |
+ +-------------------+
+ ^ ^
+ | +---- External -VREF
+ External +VREF
+
+A **pseudo-differential bipolar** input is not limited by the level at IN- but
+it will be limited to -VREF or to GND on the lower end of the input range
+depending on the particular ADC. Similar to their unipolar counter parts,
+pseudo-differential bipolar channels ought to declare an ``_offset`` attribute
+to enable the conversion of raw ADC data to voltage units. For the setup with
+IN- connected to GND, ``_offset`` is often omitted.
+
+Device tree example for pseudo-differential bipolar channel::
+
+ adc@0 {
+ ...
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ channel@0 {
+ reg = <0>;
+ bipolar;
+ single-channel = <0>;
+ common-mode-channel = <1>;
+ };
+ };
projects / thirdparty / kernel / linux.git / commitdiff
