Using the ADC Expansion

The ADC (Analog-to-Digital Converter) Expansion allows the user to read and interpret analog voltage, effectively giving the Omega analog inputs. The Expansion has 4 input channels, each with a resolution of 16 bits and a maximum of 860 samples per second.

With the ADC Expansion, a whole world of analog sensors is now open to you and your Omega projects.

You can learn more about the technical specifications of the ADC Expansion in our ADC Expansion hardware overview

The Channels

The ADC Expansion has 4 input channels, they’re available on the header as well as the grove connectors. It’s important to note that each channel should be connected to an input source through either the header or the Grove connector.

Input Header

Input channels are available on the female header on the board, along with 5V power and ground headers, to power the sensors.

ADC-Expansion-illustration-labeled

Make sure to connect the sensors correctly - connect 5V from the sensor to the VCC on the female input and same with GND and Signal connectors.

ADC-Expansion-analog2

Grove Connectors

The ADC Expansion has two analog grove connectors:

ADC-Expansion-analog-grove

Remember, it’s important that a channel should be connected to an input source either through the header or the grove connector. Using both for a single input channel can cause damage to the ADC Expansion.

Digital Grove Connector

The ADC Expansion also includes a digital I2C grove connector, allowing for additional Grove sensors to be connected.

ADC-Expansion-analog-digital-grove

The Address Switch

The address switch changes the I2C device address of the ADC Expansion, allowing users to use two ADC Expansions with a single Omega, providing EIGHT analog input channels

ADC-Expansion-switch

The switch changes the I2C address of the ADC chip on the ADC Expansion. That way, two ADC Expansions can be included on the Omega’s I2C bus

Switch Position	I2C Device Address
0x48 (Farther from Expansion Header)	0x48
0x49 (Closer to Expansion Header)	0x49

Using the Command Line

There is a command line utility that is available to read analog inputs from the Omega’s Linux command line.

Installing the Utility

Before you use your ADC Expansion, you need to install the adc-exp utility:

opkg update
opkg install adc-exp

Command Usage

For a print out of the command’s usage, run it with just a -h argument:

adc-exp -h

In general, the usage is as follows:

adc-exp [options] <channel>

Reading an Input Voltage

To sample and read the voltage on an input channel, run the following:

adc-exp <channel>

Note that this assumes the switch is set to 0x48

For example, if we want to read channel 0:

adc-exp 0

Will return something like

A0 Voltage: 2.12 V

The Switch Option

By default, the adc-exp command assumes the switch is set to 0x48. It’s possible to explicitly specify the switch value:

adc-exp -s <switch value> <channel>

So for example, reading channel 3 when the switch is set to 0x49:

adc-exp -s 0x49 3

Will return something like

A3 Voltage: 3.33 V

Reading All channels

In order to read all channels at once use the following:

adc-exp [-s <switch value>] all

Let’s say the switch is set to 0x49, the command will look like this:

adc-exp -s 0x49 all

And will return something like:

A0 Voltage: 3.30 V
A1 Voltage: 4.99 V
A2 Voltage: 2.53 V
A3 Voltage: 0 V

JSON Output

Sometimes it’s useful to have the output in JSON, so we’ve built in an option:

adc-exp -j [-s <switch value>] <channel>

Let’s say we want to read channel 1 and the switch is set to 0x48, we can run:

adc-exp -j -s 0x48 1

It will return something like:

{"channel":1,"voltage":2.51, "switch":0}

Or, if the switch is set to 0x49 and we want to read channel 3:

adc-exp -j -s 0x49 3

It will return:

{"channel":3,"voltage":4.99, "switch":0x49}