AD4000 driver

Device driver for Analog Devices Inc. AD4000 series of ADCs and similar devices.

Supported devices

• AD4000

• AD4001

• AD4002

• AD4003

• AD4004

• AD4005

• AD4006

• AD4007

• AD4008

• AD4010

• AD4011

• AD4020

• AD4021

• AD4022

• ADAQ4001

• ADAQ4003

• AD7685

• AD7686

• AD7687

• AD7688

• AD7690

• AD7691

• AD7693

• AD7942

• AD7946

• AD7980

• AD7982

• AD7983

• AD7984

• AD7988-1

• AD7988-5

Wiring connections

Devices of the AD4000 series can be connected to the SPI host controller in a few different modes.

CS mode, 3-wire turbo mode

Datasheet “3-wire” mode is what most resembles standard SPI connection which, for these devices, comprises of connecting the controller CS line to device CNV pin and other SPI lines as usual. This configuration is (misleadingly) called “CS Mode, 3-Wire Turbo Mode” connection in datasheets. NOTE: The datasheet definition of 3-wire mode for the AD4000 series is NOT the same of standard spi-3wire mode. This is the only connection mode that allows configuration register access but it requires the SPI controller to support the SPI_MOSI_IDLE_HIGH feature.

Omit the adi,sdi-pin property in device tree to select this mode.

                                    +-------------+
+ ----------------------------------| SDO         |
|                                   |             |
|               +-------------------| CS          |
|               v                   |             |
|    +--------------------+         |     HOST    |
|    |        CNV         |         |             |
+--->| SDI   AD4000   SDO |-------->| SDI         |
     |        SCK         |         |             |
     +--------------------+         |             |
               ^                    |             |
               +--------------------| SCLK        |
                                    +-------------+

CS mode, 3-wire, without busy indicator

Another wiring configuration supported as “3-wire” mode has the SDI pin hard-wired to digital input/output interface supply (VIO). In this setup, the controller is not required to support SPI_MOSI_IDLE_HIGH but register access is not possible. This connection mode saves one wire and works with any SPI controller.

Set the adi,sdi-pin device tree property to "high" to select this mode.

                                     +-------------+
                +--------------------| CS          |
                v                    |             |
VIO   +--------------------+         |     HOST    |
 |    |        CNV         |         |             |
 +--->| SDI   AD4000   SDO |-------->| SDI         |
      |        SCK         |         |             |
      +--------------------+         |             |
                ^                    |             |
                +--------------------| SCLK        |
                                     +-------------+

Alternatively, a GPIO may be connected to the device CNV pin. This is similar to the previous wiring configuration but saves the use of a CS line.

                                     +-------------+
                +--------------------| GPIO        |
                v                    |             |
VIO   +--------------------+         |     HOST    |
 |    |        CNV         |         |             |
 +--->| SDI   AD4000   SDO |-------->| SDI         |
      |        SCK         |         |             |
      +--------------------+         |             |
                ^                    |             |
                +--------------------| SCLK        |
                                     +-------------+

CS mode, 4-wire without busy indicator

In datasheet “4-wire” mode, the controller CS line is connected to the ADC SDI pin and a GPIO is connected to the ADC CNV pin. This connection mode may better suit scenarios where multiple ADCs can share one CNV trigger.

Set adi,sdi-pin to "cs" to select this mode.

                                    +-------------+
+ ----------------------------------| CS          |
|                                   |             |
|               +-------------------| GPIO        |
|               v                   |             |
|    +--------------------+         |     HOST    |
|    |        CNV         |         |             |
+--->| SDI   AD4000   SDO |-------->| SDI         |
     |        SCK         |         |             |
     +--------------------+         |             |
               ^                    |             |
               +--------------------| SCLK        |
                                    +-------------+

IIO Device characteristics

The AD4000 series driver supports differential and pseudo-differential ADCs.

The span compression feature available in AD4000 series devices can be enabled/disabled by changing the _scale_available attribute of the voltage channel. Note that span compression configuration requires writing to AD4000 configuration register, which is only possible when the ADC is wired in 3-wire turbo mode, and the SPI controller is SPI_MOSI_IDLE_HIGH capable. If those conditions are not met, no _scale_available attribute is provided.

Besides that, differential and pseudo-differential voltage channels present slightly different sysfs interfaces.

Pseudo-differential ADCs

Typical voltage channel attributes of a pseudo-differential AD4000 series device:

Voltage Channel Attributes	Description
in_voltage0_raw	Raw ADC output code.
in_voltage0_offset	Offset to convert raw value to mV.
in_voltage0_scale	Scale factor to convert raw value to mV.
in_voltage0_scale_available	Toggles input span compression

Differential ADCs

Typical voltage channel attributes of a differential AD4000 series device:

Voltage Channel Attributes	Description
in_voltage0-voltage1_raw	Raw ADC output code.
in_voltage0-voltage1_scale	Scale factor to convert raw value to mV.
in_voltage0-voltage1_scale_available	Toggles input span compression

SPI offload support

To be able to achieve the maximum sample rate, the driver can be used with SPI offload engines such as the one usually present in AXI SPI Engine, to provide SPI offload support.

To keep up with SPI offloading transfer speeds, the ADC must be connected either in 3-wire turbo mode or in 3-wire without busy indicator mode and have SPI controller CS line connected to the CNV pin.

When set for SPI offload support, the IIO device will provide different interfaces.

• Either in_voltage0_sampling_frequency or in_voltage0-voltage1_sampling_frequency file is provided to allow setting the sample rate.

• IIO trigger device is not provided (no trigger directory).

• timestamp channel is not provided.

Also, because the ADC output has a one sample latency (delay) when the device is wired in “3-wire” mode and only one transfer per sample is done when using SPI offloading, the first data sample in the buffer is not valid because it contains the output of an earlier conversion result.