In Vitro Diagnostics

The circuit shown in Figure 1 is a completely isolated low power pH sensor signal conditioner and digitizer with automatic temperature compensation for high accuracy.

The circuit gives 0.5% accurate readings for pH values from 0 to 14 with greater than 14-bits of noise-free code resolution and is suitable for a variety of industrial applications such as chemical, food processing, water, and wastewater analysis.

This circuit supports a wide variety of pH sensors that have very high internal resistance that can range from 1 MΩ to several GΩ, and digital signal and power isolation provides immunity to noise and transient voltages often encountered in harsh industrial environments.

Many important liquid analyses like pH rely on electrochemistry, a branch of chemistry that characterizes the behavior of reduction-oxidation (redox) reactions by measuring the transfer of electrons from one reactant to another. Electrochemical techniques can be used directly or indirectly to detect several important parameters that affect water quality, including chemical indicators, biological and bacteriological indicators and even some low level contaminants like heavy metals. Many of these indicative measurements are pertinent to determining important quality parameters of the tested analyte.

The circuit shown in Figure 1 is a modular sensing platform that allows the user to design a flexible electrochemical water quality measurement solution. Its high level of integration enables an electrochemical measurement platform applicable to a variety of water quality probes including pH, oxidation reduction potential (ORP), and conductivity cells.

The system allows up to four probes to be connected at one time for different water quality measurements.

The AD5940 and AD5941are high precision, low power analog front ends (AFEs) designed for portable applications that require high precision, electrochemical-based measurement techniques, such as amperometric, voltammetric, or impedance measurements. The AD5940/AD5941 is designed for skin impedance and body impedance measurements, and works with the AD8233 AFE in a complete bioelectric or biopotential measurement system. The AD5940/AD5941 is designed for electrochemical toxic gas sensing.

The AD5940/AD5941 consist of two high precision excitation loops and one common measurement channel, which enables a wide capability of measurements of the sensor under test. The first excitation loop consists of an ultra low power, dual-output string, digital-to-analog converter (DAC), and a low power, low noise potentiostat. One output of the DAC controls the noninverting input of the potentiostat, and the other output controls the noninverting input of the transimpedance amplifier (TIA). This low power excitation loop is capable of generating signals from dc to 200 Hz.

The second excitation loop consists of a 12-bit DAC, referred to as the high speed DAC. This DAC is capable of generating high frequency excitation signals up to 200 kHz.

The AD5940/AD5941 measurement channel features a 16-bit, 800 kSPS, multichannel successive approximation register (SAR) analog-to-digital converter (ADC) with input buffers, a built in antialias filter, and a programmable gain amplifier (PGA). An input multiplexer (mux) in front of the ADC allows the user to select an input channel for measurement. These input channels include multiple external current inputs, external voltage inputs, and internal channels. The internal channels allow diagnostic measurements of the internal supply voltages, die temperature, and reference voltages.

The current inputs include two TIAs with programmable gain and load resistors for measuring different sensor types. The first TIA, referred to as the low power TIA, measures low bandwidth signals. The second TIA, referred to as the high speed TIA, measures high bandwidth signals up to 200 kHz.

An ultra low leakage, programmable switch matrix connects the sensor to the internal analog excitation and measurement blocks. This matrix provides an interface for connecting external transimpedance amplifier resistors (R_TIAs) and calibration resistors. The matrix can also be used to multiplex multiple electronic measurement devices to the same wearable electrodes.

A precision 1.82 V and 2.5 V on-chip reference source is available. The internal ADC and DAC circuits use this on-chip reference source to ensure low drift performance for the 1.82 V and 2.5 V peripherals.

The AD5940/AD5941 measurement blocks can be controlled via direct register writes through the serial peripheral interface (SPI) interface, or, alternatively, by using a preprogrammable sequencer, which provides autonomous control of the AFE chip. 6 kB of static random access memory (SRAM) is partitioned for a deep data first in, first out (FIFO) and command FIFO. Measurement commands are stored in the command FIFO and measurement results are stored in the data FIFO. A number of FIFO related interrupts are available to indicate when the FIFO is full.

A number of general-purpose inputs/outputs (GPIOs) are available and controlled using the AFE sequencer. The AFE sequencer allows cycle accurate control of multiple external sensor devices.

The AD5940/AD5941 operate from a 2.8 V to 3.6 V supply and are specified over a temperature range of −40°C to +85°C. The AD5940 is packaged in a 56-lead, 3.6 mm × 4.2 mm WLCSP. The AD5941 is packaged in a 48-lead LFCSP.

Applications

• Electrochemical measurements
• Electrochemical gas sensors
• Potentiostat/amperometric/voltammetry/cyclic voltammetry
• Bioimpedance applications
  
  • Skin impedance
  • Body impedance
• Continuous glucose monitoring
• Battery impedance

The ADT7422 is a high accuracy, digital I²C temperature sensor designed to meet the clinical thermometry specification of the ASTM E1112 standards when soldered onto the final printed circuit board (PCB).

The ADT7422 contains an internal band gap reference, a temperature sensor, and a precision analog-to-digital converter (ADC). The ADT7422 provides a 16-bit temperature result with a resolution of 0.0078°C and an accuracy of up to ±0.1°C across the temperature range of 25°C to 50°C without the need for calibration after the PCB soldering process.

Operating at 3.0 V, the average supply current is typically 210 μA. The ADT7422 has a shutdown mode that powers down the device and offers a shutdown current of typically 2.0 μA at 3.0 V. The ADT7422 is rated for operation over the −40°C to +125°C temperature range.

Pin A0 and Pin A1 are available for address selection and provide four possible I²C addresses for the ADT7422. The CT pin is an open-drain output that becomes active when the temperature exceeds a programmable critical temperature limit. The INT pin is also an open-drain output that becomes active when the temperature exceeds a programmable limit. The INT pin and CT pin can operate in comparator and interrupt event modes.

Product Highlights

1. No calibration or correction required by the user.
2. Low power consumption.
3. Long-term stability and reliability.
4. High accuracy for industrial, instrumentation, and medical applications.

Applications

• Vital signs monitoring (VSM)
• Medical equipment
• Resistance temperature detector (RTD) and thermistor replacement
• Food transportation and storage
• Thermocouple cold junction compensation
• Environmental monitoring and heating, ventilation, and air conditioning (HVAC)
• Laser diode temperature control

The LTC2387-18 is a low noise, high speed, 18-bit 15Msps successive approximation register (SAR) ADC ideally suited for a wide range of applications. The combination of excellent linearity and wide dynamic range makes the LTC2387-18 ideal for high speed imaging and instrumentation applications. No latency operation provides a unique solution for high speed control loop applications. The very low distortion at high input frequencies enables communications applications requiring wide dynamic range and significant signal bandwidth.

To support high speed operation while minimizing the number of data lines, the LTC2387-18 features a serial LVDS digital interface. The LVDS interface has one-lane and two-lane output modes, allowing the user to optimize the interface data rate for each application.

Applications

• High Speed Data Acquisition
• Imaging
• Communications
• Control Loops
• Instrumentation
• ATE

The LTC2662 is a family of five-channel, 16-/12-bit current-source digital-to-analog converters, providing five high-compliance current source outputs with guaranteed 1V dropout at 200mA. The part supports load voltages of up to 32V. There are eight current ranges, programmable per channel, with full-scale outputs of up to 300mA; and the channels can be paralleled to allow for ultrafine adjustments of large currents, or for combined outputs of up to 1.5A.

A dedicated supply pin is provided for every output channel. Each can be operated from 2.85V to 33V, and internal switches allow any output to be pulled to the optional negative supply.

The LTC2662 includes a precision integrated 1.25V reference (10ppm/°C maximum), with the option to use an external reference.

The SPI/Microwire-compatible 3-wire serial interface operates on logic levels as low as 1.71V at clock rates up to 50MHz.

APPLICATIONS

• Tunable Lasers
• Semiconductor Optical Amplifiers
• Resistive Heaters
• Current Mode Biasing
• Proportional Solenoid Drive