CN038x High precision analog/digital conversion: ADI's convenient practical circuit

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This article presents an analog-to-digital conversion reference design using the AD7124-4, a low power, low noise, fully featured 24-bit A/D converter.

Features of CN038x Precision Analog-to-Digital Converters

In order to incorporate information such as temperature and pressure into the system, it is necessary to convert that information from analog values to digital values.

And to capture accurate data, not only the sensor but also the analog front end must be highly accurate.

High-resolution A/D converters are used for high-precision analog-to-digital conversion, but there are various design issues such as noise and drift.

Analog Devices offers reference designs using the AD7124-4 24-bit A/D converter to help you address a variety of design challenges.

This article describes three reference designs for the AD7124-4:

CN0381/CN0383 4-Wire and 3-Wire RTD Measurement Systems
CN0382 4-20mA Transmitter for Temperature and Pressure
CN0384 Thermocouple Measurement System

What is 24-bit resolution?

Before we dive into the reference design, let's think about how much resolution 24 bits is.

Taking full scale as 1.0, LSB (smallest value) = 1/2^𝑛= 1/2^24 = 1/16,777,216 = 0.000,000,059,604,...

For example, if a truck weighs approximately 3,900 kg, the LSB when converted with 24 bits is 0.244 g.

So, with 24-bit resolution, you can tell whether a truck driver is chewing one piece of gum or half a piece of gum.

What is the AD7124-4, a 24-bit A/D converter with low power consumption, low noise, and all functions?

The AD7124-4 is a low power, low noise, complete analog front end for precision measurement applications.

The product includes a 24-bit Σ-Δ analog-to-digital converter (ADC) that can be configured for four differential inputs or seven single-ended or pseudo-differential inputs. A low-noise gain stage allows even small amplitude signals to be interfaced directly to the A/D converter.

For details on the AD7124-4, it is also introduced in the [AD7124 series] Σ-Δ type A/D converter that achieves low power consumption.

1. CN0383 4-wire and 3-wire RTD measurement system

RTDs are sensors commonly used to measure temperature in industrial applications.

They are made of pure metals such as platinum, nickel, and copper, and their resistance changes with changes in temperature.
RTDs have high accuracy and excellent stability when compared to other types of temperature sensors.
Using 4-wire or 3-wire connections eliminates resistance errors when using long cables.

CN0383 is a 4-wire or 3-wire RTD measurement system reference design.
The error is less than 1°C over the entire temperature range, and power down is available as an optional feature.
CN0383 consists of the AD7124-4 evaluation board EVAL-AD7124-8SDZ and EVAL-SDP-CB1Z.

EVAL-AD7124-8SDZ Block Diagram (Source: Excerpt from EVAL-AD7124-8SDZ User Guide)

By changing the configuration of the analog input circuit, the AD7124-4 supports 4-wire and 3-wire input.
The software can also be changed between 4-wire and 3-wire systems using the evaluation board's SPI communication.

4-wire RTD analog input configuration (left), 3-wire RTD analog input configuration (right)

Setting change by AD7124-4 EVAL+SOFTWARE

2.CN0382 4-20mA transmitter for temperature and pressure

The CN0382 is an isolated smart field instrument that can connect to various types of analog sensors, including temperature sensors (RTDs, thermocouples) and bridge pressure sensors.

The instrument communicates via a 4 mA to 20 mA analog output and a HART® (Highway Addressable Remote Transducer) interface.
HART® is a bidirectional digital communication that modulates a 1 mA peak-to-peak Frequency Shift Key (FSK) signal onto a standard 4 mA to 20 mA analog current signal.
The HART® interface provides remote calibration, fault investigation, and process variable transmission for applications such as temperature and pressure control.
The hardware of CN0382 consists of DEMO-AD7124-DZ.

DEMO-AD7124-DZ Block Diagram (Source: Excerpt from CN0382 Circuit Note)

3. CN0384 Thermocouple measurement system

The CN0384 is a highly integrated, high-precision, low-power thermocouple measurement system.

The AD7124-x 24-bit resolution A/D converter includes a low-noise programmable gain array (PGA) that allows small amplitude signals to be interfaced directly to the ADC.

For thermocouple measurements using this system, the overall system accuracy is ±1°C over the measurement temperature range -50°C to +200°C.
Power options also make the AD7124-x suitable for low power applications.

CN0384 consists of EVAL-AD7124-8SDZ and EVAL-SDP-CB1Z.