Part Details for AD8628 by Analog Devices Inc

The total dissolved solids (TDS) present in a water system is composed of inorganic salts and small amounts of organic matter that are dissolved in water, and is an important measure of water quality. TDS can be derived from the electrical conductivity (or conductivity) of the solution by a factor dependent on the properties, temperature and, number of ions present. By measuring the conductivity of the solution, determining the TDS of the system is faster, economical, and less complicated in contrast to the more accurate gravimetric method. The latter method involves evaporating the water and weighing the residue, which is applicable in laboratory settings but impractical in the field.<p>This circuit is a TDS measurement system based on the conductivity of the solution. This design uses a combination of components that allow for single-supply operation, which minimize circuit complexity, making this suitable for low-power and portable instrument applications.<p>The simplest method of measuring the conductivity of the solution uses a 2-wire conductivity cell. Conductivity measurements require temperature compensation for measurements taken at temperatures other than 25˚C (or other reference temperature). This system can reference the conductivity measurement to room temperature using either a 100 Ω or 1000 Ω, 2-wire resistance temperature device (RTD) and can accommodate 2-wire conductivity cells of various cell constants and operating parameters.<p>The capacitance and polarization effects of the electrodes in the conductivity cell require that the excitation signal be a bipolar square wave with a sufficiently high frequency to reduce polarization effects but also with sufficiently long periods to reduce capacitance effects. To avoid damaging the conductivity electrodes, the signal must have a very low to zero dc offset and magnitude.<p>The circuit can measure the range of conductivity values from 1 µS to 0.1 S. A multiplexer switches between seven precision resistors of different values to set the gain when measuring the conductivity probe signal. The system can automatically determine the gain setting of the conductivity measurement through an auto-ranging procedure implemented in software. The system can also be calibrated in the high conductivity range to increase its accuracy.

This circuit uses the ADuC7060 or the ADuC7061 precision analog microcontroller in an accurate thermocouple temperature monitoring application. The ADuC7060/ ADuC7061 integrate dual 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs), dual programmable current sources, a 14-bit digital-to-analog converter (DAC), and a 1.2 V internal reference, as well as an ARM7 core, 32 kB flash, 4 kB SRAM, and various digital peripherals such as UART, timers, serial peripheral interface (SPI), and I2C interfaces.<p>In the circuit, the ADuC7060/ ADuC7061 are connected to a thermocouple and a 100 Ω platinum resistance temperature detector (RTD). The RTD is used for cold junction compensation. As an extra option, the ADT7311 digital temperature sensor can be used to measure the cold junction temperature instead of the RTD.<p>In the source code, an ADC sampling rate of 4 Hz was chosen. When the ADC input programmable gain amplifier (PGA) is configured for a gain of 32, the noise-free code resolution of the ADuC7060/ ADuC7061 is greater than 18 bits.<p>The single edge nibble transmission (SENT) interface to the host is implemented by using a timer to control a digital output pin. This digital output pin is then level shifted externally to 5 V using an external NPN transistor. An EMC filter is provided on the SENT output circuit as recommended in Section 6.3.1 of the SENT protocol (SAE J2716 Standard). The data is measured as falling edge to falling edge, and the duration of each pulse is related to the number of system clock ticks. The system clock rate is determined by measuring the SYNC pulse. The SYNC pulse is transmitted at the start of every packet. More details are provided in the SENT Interface section.

The heart of the solution is the AD7768-1, which is a low power, high performance, Σ-Δ ADC, with a Σ-Δ modulator and digital filter for precision conversion of both ac and dc signals. The AD7768-1 offers the user flexibility to configure and optimize for input bandwidth vs. output data rate (ODR) vs. power dissipation.

The circuit is optimized for the Figaro TGS8100 sensor, which is comprised of a MOS sensing chip and an integrated heater formed on a silicon substrate using MEMS technology. Other MOS sensors can be used by making appropriate changes in the hardware and software.