STM32 ADS7828 ADC Library

C++ Library for interfacing the ADS7828 AD-Converter with STM32 microcontrollers like the STM32F103C8 using HAL

Features

• Read all Channel Combinations, Single-Ended and Differential
• Read Digit Values or Voltages
• Internal 2.5V or External Manual Voltage Reference, switchable at runtime
• Power Down Modes with implicit switching
• Fixed Scaling for Voltage Divider applications
• Averaging of the last N values for every channel (dynamic or static storage options)

Usage

Includes and Compilation

Include the Library via

#include "ADS7828.hpp"

Make sure to define which STM32 controller you are using! This is relevant for the selection of the HAL Library. You can select the MCU family with the -D STM32F1 flag while compiling. Or simply define it at the start of your code with #define STM32F1. Change accordingly for your STM32!

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Init

Create an ADS7828 object with an initialized I2C handle and the device address.

ADS7828 adc = ADS7828(&hi2c1, 0x48);

You can specify an external reference voltage in the constructor:

ADS7828 adc = ADS7828(&hi2c1, 0x48, ref_voltage);

⚠️ This changes the Power Down Mode implicitly, see Power Down Mode

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Reading a Channel

The ADS7828 has 8 Channels in total. You can read the digit value of each channel combination by calling

float digit = adc.read_digit(ADS7828_CHANNEL channel);

With 12 Bits resolution the returned value is between 0 and 4095.

⚠️ Average filters may apply when enabeled, see Moving Average

The ADS7828 supports two types of readings:

• Single-Ended: Reads the Channel Voltage with reference to COM
• Differential: Reads the Voltage between two channels

All supported measurements are specified in the ADS7828_CHANNEL enum, you can choose between:

ADS7828_CHANNEL	IN+	IN-
CHANNEL_X_COM	CHX	COM
CHANNEL_0_1	CH0	CH1
CHANNEL_2_3	CH2	CH3
CHANNEL_4_5	CH4	CH5
CHANNEL_6_7	CH6	CH7
CHANNEL_1_0	CH1	CH0
CHANNEL_3_2	CH3	CH2
CHANNEL_5_4	CH5	CH4
CHANNEL_7_6	CH7	CH6

Reading the voltage instead of the digit value is also possible by calling

float voltage = adc.read_voltage(ADS7828_CHANNEL channel);

This function converts from digits to volts by mapping the digit to the reference voltage, thus the result depends on your reference voltage!

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Scaling

If you want to scale the voltage reading every time you call read_voltage you can set a fixed scaling factor. This is especially useful when working with voltages dividers, as the ADC voltage gets converted to the actual voltage at the divider. To set a scaling factor for a certain Channel, call

adc.set_scaling(ADS7828_CHANNEL channel, float scaling);

To get the current scaling factor, call

float cur_scaling = adc.get_scaling(ADS7828_CHANNEL channel);

To reset the scaling back to 1, you can call

adc.reset_scaling(ADS7828_CHANNEL channel);

or for all channels

adc.reset_scaling();

⚠️ Keep in mind, that CHANNEL_0_1 and CHANNEL_1_0 for example have different scaling factors!

⚠️ Scaling only applies to the Voltage Reading, not to the Digit Reading!

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Moving Average Filter

You have the option to enable averaging of the last n values for every channel seperately by calling

adc.set_averaging(ADS7828_CHANNEL channel, uint8_t n);

The averaging will be applied directly to the digit value, so that get_digit returns the average instead of the last value. If you want to reset the last n values to 0, call

adc.clear_averaging(ADS7828_CHANNEL channel);

To disable averaging call

adc.disable_averaging(ADS7828_CHANNEL channel);

You can choose the way the last values are stored. Generally, the last digits have to be held in an array of at least size n. There are two memory usage options available with a define in the header:

• Dynamic: The array gets dynamically allocated by calling new/delete
• Static: You can choose the maximum number of values with ADS7828_AVG_MAX, for every channel one array of that size is allocated at compile time

To choose, use #define ADS7828_DYNAMIC_MEM for dynamic allocation. Otherwise static allocation is used.

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Reference Voltage

All measurements done by the ADS7828 are with reference to the specified reference voltage. There are two types of operation:

• Internal Reference: The ADC uses the internal voltage source of 2.5V as reference
• External Reference: The ADC uses an external voltage source as reference Using the internal source means that an input voltage from 0 - 2.5V is mapped to 0 - 4095. The ADC supports external references from 50mV to 5V, which changes the LSB/voltage resolution. See the datasheet for more information.

If you are using an external voltage you have to set the voltage, either in the constructor or by calling

adc.set_ref_voltage_external(float voltage);

⚠️ This changes the Power Down Mode implicitly, see Power Down Mode

⚠️ Changing from internal to external reference and vice versa takes some time, measurements less than 1ms after the switch might be inaccurate!

Changing back to the internal reference can ONLY be done by calling

adc.set_ref_voltage_internal();

⚠️ This changes the Power Down Mode implicitly, see Power Down Mode

⚠️ Changing from internal to external reference and vice versa takes some time, measurements less than 1ms after the switch might be inaccurate!

Changing the reference voltage to the correct value ensures that read_voltage returns the right voltages. Furthermore, the Power Down Mode is changed accordingly.

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Power Down Mode

You can change the Power Down Mode by calling

adc.set_power_mode(ADS7828_PD_MODE mode);

There are 4 different modes to choose from.

ADS7828_PD_MODE	Power Mode
POWER_DOWN	Power Down Between A/D Converter Conversions
REF_OFF	Internal Reference Voltage OFF and A/D Converter ON
REF_ON_AD_OFF	Internal Reference ON and A/D Converter OFF
REF_ON_AD_ON	Internal Reference ON and A/D Converter ON

As you can see, only certain modes can be used with external/internal references. Therefore, switching is done implicitly when changing the type of reference.

• Internal Reference 🠦 Switch to REF_ON_AD_ON
• External Reference 🠦 Switch to REF_OFF

To update the mode, an I2C command has to be send to the ADC. When calling the function, this is done instantly by sending a random read request. However, if you do not want to change the mode until you yourself call the next read request, you can disable the command transmit by calling

adc.set_power_mode(ADS7828_PD_MODE mode, bool update_now = false);

⚠️ Changing from internal to external reference and vice versa takes some time, measurements less than 1ms after the switch might be inaccurate!