EVAL-CN0326-PMDZ: pH Sensor with Temperature Compensation

This design provides a complete solution for pH sensor with temperature compensation. The circuit has three critical stages: the pH probe buffer, the ADC, and the digital and power isolator.

The AD8603, a precision micro power (50 μA maximum) and low noise (22 nV/√Hz) CMOS operational amplifier configured as a buffer to the input of one of the channels of the AD7793.The AD8603 has a typical input bias current of 200 fA that provides an effective solution to the pH probe that has high internal resistance.

The pH sensing and temperature compensation system is based on the AD7793, 24-bit sigma-delta (Σ-Δ) with. It has three differential analog inputs and has an on-chip, low noise, programmable gain amplifier (PGA) that ranges from unity gain to 128. The AD7793 consumes only a maximum of 500 μA making it suitable for any low power applications. It has a low noise, low drift internal band gap reference and can accept external differential reference. The output data rate from the part is software programmable and can be varied from 4.17 Hz to 470 Hz.

The ADuM5401, quad-channel digital isolator with an integrated dc-to-dc converter provides the digital signal and power isolation between the microcontroller and the AD7793 digital lines. The iCoupler chip-scale transformer technology is used to isolate the logic signals and the power feedback path in the dc-to-dc converter.

* This board is designed to interface through the EVAL-ADICUP360 which is an Arduino and PMOD compatible form factor development board. The board uses an open source Eclipse based Interactive Development Environment (IDE) which can be downloaded free of charge. The platform contains many hardware and software example projects, to make it easier for customers to prototype/create systems and solutions using the complete ecosystem provided.

The EVAL-ADICUP360 uses the ADuCM360 Arm Cortex-M3 processor as the main device on board. It has integrated dual 24-bit sigma delta converters on chip, which make it extremely flexible for both analog and digital application.