Fully Automatic High Performance Conductivity Measurement System (CN0359)
The circuit shown in Figure 1 is a completely self-contained, microprocessor controlled, highly accurate conductivity measurement system ideal 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 yields an accuracy of better than 0.3% over a conductivity range of 0.1 μS to 10 S (10 MΩ to 0.1 Ω) with no calibration requirements.
Automatic detection is provided for either 100 Ω or 1000 Ω platinum (Pt) resistance temperature devices (RTDs), allowing the conductivity measurement to be referenced to room temperature.
The system accommodates 2- or 4-wire conductivity cells, and 2-, 3-, or 4-wire RTDs for added accuracy and flexibility.
The circuit generates a precise ac excitation voltage with minimum dc offset to avoid a damaging polarization voltage on the conductivity electrodes. The amplitude and frequency of the ac excitation is user-programmable.
An innovative synchronous sampling technique converts the peak-to-peak amplitude of the excitation voltage and current to a dc value for accuracy and ease in processing using the dual, 24-bit Σ-Δ ADC contained within the precision analog microcontroller.
The intuitive user interface is an LCD display and an encoder push button. The circuit can communicate with a PC using an RS-485 interface if desired, and operates on a single 4 V to 7 V supply.
Figure 1. High Performance Conductivity Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown)
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