Digital Converter

A digital converter: a device that transforms an electrical (analog) signal into a digital signal.

An Acquisition System Designed for the Electrical Prospection of Soil
Nicolas Florsch, Frédéric Mühlach, in
Everyday Applied Geophysics 1
, 2018

6.3.3 The analog–digital converter
An ADC is an electronic device that transforms analog signals into digital data.

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The ADC symbol

The precision of an ADC depends on its resolution (or bit count). For a material with a 10bit conversion (Arduino), the analog quantities are coded following 210 states, i.e. values ​​from 0 to 1023. With a reference of 5 V for Vref + and 0 V for Vref −, the quantum* is equal to 4.88 mV.

*The quantum corresponds to the smallest input voltage change that will lead to an increment or decrement of the output conversion digital value.

The perfect components do not exist, and several parameters affect the quality of the conversion. In practice, chips are characterized by more or less significant discrepancy errors (offset error, gain error, INL, DNL), which result from several factors such as the amplification of the signals, temperature, etc. These errors should be taken into account when choosing a component: the closer it is to its ideal model, the higher its price will probably be.

The ADC inputs can generally be used in two ways:

As single-ended inputs, the tension is measured with only one wire, using the mass of the assembly as a reference point. Disturbances are recorded together with the signal;

in differential mode, the tension measured is that between two wires; the interferences, present on both paths, are thus disregarded.

, The measurement in differential mode

The differential mode is recommended for long lines or lines that are likely to present interferences. This is why we will use it to take our measurements.

The analog inputs of the Arduino model may work in this mode. Despite this, their resolution and absolute accuracy are relatively limited. Therefore, we need to use specialized chips.

Offered in a surface-mounted device format, this material is first of all designed for professional technology integrators that pay attention to the place occupied by a circuit in a given volume. Several companies have made most of the craze started by the makers to provide to their customers these components in a “Breakout Board” format, which is more adapted for the connector boards used in our experiments.

Thus, a company called Adafruit offers the ADS1115 converter (16 bits of resolution) in its “Breakout Board” version:

, The Breakout Board version of the ADS1115 converter

The ADS1115 chip is an ADC that integrates four inputs, of which two may be multiplexed to work in differential mode. The 16-bit resolution is adapted for electrical sounding. The maximum operative speed is 860 SPS (samples per second). Four modules may be addressed by using the I2C protocol. The chip also includes a stage that can amplify the gain of the signals to be measured (FPGA). This converter is perfectly adapted for our project.