Part Details for AD712 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 is an integrated thermocouple measurement system based on the AD7124-4/AD7124-8 low power, low noise, 24-bit, Σ-Δ analog-to-digital converter (ADC), optimized for high precision measurement applications. Thermocouple measurements using this system show an overall system accuracy of ±1°C over a measurement temperature range of −50°C to +200°C . Typical noise free code resolution of the system is approximately 15 bits.

This 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, Σ-Δ, analog-to-digital converter (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 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.

RTDs are frequently used sensors for temperature measurements. A RTD is made from a pure metal (examples include platinum, nickel, or copper), which has a predictable change in resistance as the temperature changes and can typically measure up to +850°C. RTDs are capable of high accuracy and good stability when compared with other types of temperature sensors such as thermistors, thermocouples, and semiconductor (IC) temperature sensors. The most widely used RTDs are platinum Pt100 and Pt1000. These sensors are categorized by their nominal resistance at 0°C. There are several industry standards that define the tolerance and accuracy limits of a Platinum RTD sensor.

Thermistors are inexpensive, nonlinear resistors, whose resistance is a strong function of temperature. Precision applications most often employ thermistors with a negative temperature coefficient (NTC). Positive temperature coefficient (PTC) thermistors are also available but are typically used in safety applications or as resettable fuses. The PTC thermistor exhibits only a slight change in resistance with respect to temperature until its Curie point (or switching point) is reached, above which an increase in resistance of several orders of magnitude occurs over a few tens of °C. The switching point for a PTC thermistor is typically between 60°C and 120°C. In contrast, NTC thermistors are usable over a much larger temperature range, typically −50°C to +150°C, suitable for a wide range of applications, including industrial automation, instrumentation, and medical.