Analog DevicesEVAL-CN0398-ARDZ

Reference Design: Soil Moisture and pH Measurement System with Temperature Compensation

This EVAL-CN0398-ARDZ is a single-supply, low power, high precision complete solution for soil moisture and pH measurements, including temperature compensation. The circuit is optimized for use with capacitive soil moisture sensors that are insensitive to water salinity and do not corrode over time. The circuit also measures soil pH and that function is suitable in a variety of applications.

Block Diagram:  

The system is divided into three independent measurement front ends: pH, soil moisture, and temperature. After signal conditioning, the three channels share an AD7124-8, 24-bit sigma-delta (Σ-Δ) ADC. The AD7124-8, is a low power, low noise, completely integrated analog front end for high precision measurement applications. The device contains a low noise, 24-bit Σ-Δ analog-to-digital converter (ADC), and can be configured to have 8 differential inputs or 15 single-ended or pseudo-differential inputs. The on-chip gain stage ensures that signals of small amplitude can be interfaced directly to the ADC. The AD7124-8 uses the ADR3433,a low cost, low power, high precision CMOS voltage reference, featuring ±0.1% initial accuracy, low operating current, and low output noise.

The ADR3433 provides both the 3.3 V reference and also the AVDD supply to the AD7124-8. The ADP7118-2.5 is a CMOS, low dropout (LDO) linear regulator which provides the power supply for the moisture sensor. The input to the ADP7118-2.5 is selectable via the jumper P10 as either 5 V or 7 V to 12 V coming from the Arduino-compatible platform board. The output of the ADP7118-2.5 is also selectable as 3.3 V or 5 V using the jumper on P8. The circuit uses the ADA4661-2, precision op amp to buffer the high impedance pH probe output and to drive the ADC. The ADA4661-2 is a dual, precision, rail-to-rail input/output amplifier optimized for low power, high bandwidth, and wide operating supply voltage range.

Features:

• Complete Soil Moisture, pH, and Temperature Measurement

• Low Power

• Arduino-Compatible Form Factor

Applications:

• Instrumentation & Measurement

• Electronic Test & Measurement

• Smart agriculture

Functional Test Setup:

Compatible FPGA Boards:

• List of Intel boards with the Arduino connector

Related ADI parts:   

• AD7124-8 : 8-Channel, Low Noise, Low Power, 24-Bit, Sigma-Delta ADC with PGA and Reference

• ADR3433: Micro-Power, High-Accuracy 3.3V Voltage Reference

• ADA4661-2: 18 V, Precision, 725 μA, 4 MHz CMOS RRIO Operational Amplifier

• ADP7118-2.5: 20 V, 200 mA, Low Noise, CMOS LDO Linear Regulator

Reference Documentation:

• List of ADI Arduino Shields