Fully automatic high performance conductivity measurement system

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   Fully Automated High-Performance Conductivity Measurement System Evaluation and Design Support Circuit Evaluation Board CN-0359 Circuit Evaluation Board (EVAL-CN0359-EB1Z) Design and Integration Files Schematics, Source Code, Layout Files, Bill of Materials Circuit Functionality and Benefits The 
  circuit in Figure 1 is a complete, microprocessor-controlled, high-precision conductivity measurement system suitable for measuring the ionic content of liquids, water quality analysis, industrial quality control, and chemical analysis. A carefully selected combination of precision signal conditioning components provides better than 0.3% accuracy over the 0.1 µS to 10 S (10 MΩ to 0.1 Ω) conductivity range with no calibration required. Automatic detection is provided for 100 Ω or 1000 Ω platinum (Pt) resistance temperature detectors (RTDs), allowing conductivity measurements to be made with room temperature as the reference. The system supports two-wire or four-wire conductivity electrodes and two-wire, three-wire, or four-wire RTDs for increased accuracy and flexibility. The circuit generates a precise ac excitation voltage with minimal dc offset to avoid polarization voltage damage on the conductivity electrodes. The amplitude and frequency of the ac excitation are user programmable. An innovative simultaneous sampling technique converts the peak-to-peak amplitude of the excitation voltage and current to dc values, which improves accuracy and simplifies signal processing by the dual-channel 24-bit Σ-Δ ADCs built into a precision analog microcontroller. An LCD display and encoder pushbuttons provide an intuitive user interface. The circuit can communicate with a PC using an RS-485 interface as required and operates from a single 4 V to 7 V supply.  
   Figure 1. High Performance Conductivity Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) Circuit Description The excitation square wave for the conductivity electrode is generated by switching the ADG1419 between +V excitation and −V excitation voltages using the PWM output of the ADuCM360 microcontroller. The square wave must have a precise 50% duty cycle and very low dc offset. Even a small dc offset can damage the conductivity electrode over time. The +V excitation and −V excitation voltages are generated by the ADA4077-2 op amps (U9A and U9B), and the amplitude of these two voltages is controlled by the DAC output of the ADuCM360, as shown in Figure 2.  
   Figure 2. Excitation Voltage Source The ADA4077-2 has a typical offset voltage of 15 µV (A grade), a bias current of 0.4 nA, an offset current of 0.1 nA, an output current of up to ±10 mA, and a dropout voltage of less than 1.2 V. The U9A op amp has a closed-loop gain of 8.33 and converts the ADuCM360 internal DAC output (0 V to 1.2 V) to a +V excitation voltage in the range of 0 V to 10 V. The U9B op amp inverts the +V excitation to produce a −V excitation voltage. R22 is selected so that R22 = R24