CN0398 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.
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.
The chip select is configurable to be routed to 3 general purpose I/O pins of the board, digital pins 8, 9 and 10, allowing multiple boards using SPI communications protocol to be stacked up.
As default, shunt is placed across pins 1 and 2 of connector P5 of EVAL-CN0398-ARDZ shield board which associates to the used of GPIO pin 10 of EVAL-ADICUP360.
If the moisture sensor requires a 5V power supply place the jumper as seen on the image below
The EVAL-CN0398-ARDZ shield board comes with a BNC connector J1 for pH sensors as shown on the image below
The output of the pH sensor is bipolar and gives a maximum signal of ±414 mV at 25°C. The AD7124-8 operates from a single power supply therefore the pH probe should be biased above ground so that it is within the acceptable common-mode range of the AD7124-8. One of the integrated features of the AD7124-8 is its internal bias voltage generator that sets the common mode voltage of a channel to AVDD/2, or 1.65 V. This bias voltage from the ADC is applied to the pH probe shield and sets the output of the sensor 1.65 V ±414 mV at 25°C. For information about pH calibration, visit CN0398 Circuit Note.
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