Chemical Sensing Technology

The circuit in Figure 1 is a complete single-supply, low noise LED current source driver controlled by a 16-bit digital-to-analog converter (DAC). The system maintains ±1 LSB integral and differential nonlinearity and has a 0.1 Hz to 10 Hz noise of less than 45 nA p-p for a full-scale output current of 20 mA.

The innovative output driver amplifier eliminates the crossover nonlinearity normally associated with most rail-to-rail input op amps that can be as high as 4 LSBs or 5 LSBs for a 16-bit system.

This industry-leading solution is ideal for pulse oximetry applications where 1/f noise superimposed on the LED brightness levels affects the overall accuracy of the measurement.

Total power dissipation for the three active devices is less than 20 mW typical when operating on a single 5 V supply.

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.

Gas detection instruments are used in a wide range of applications ranging from home air quality measurement devices to industrial solutions for detecting toxic gases. Many of these instruments use electrochemical gas sensors. This sensor technology requires specialized front-end circuitry for biasing and measurement.

By utilizing built-in diagnostics features (such as impedance spectroscopy or bias voltage pulsing and ramping) it is possible to inspect sensor health, compensate for accuracy drift due to aging or temperature, and estimate the remaining lifetime of the sensor right at the edge of the sensor network without user intervention. This functionality allows smart, accurate sensor replacement at the individual edge nodes. An integrated, ultra low power microcontroller directly biases the electrochemical gas sensor and runs onboard diagnostic algorithms.

The circuit shown in Figure 1 shows how an electrochemical gas sensor is connected to the potentiostat circuit and how it is biased and measured. Common 2-lead, 3-lead, and 4-lead electrochemical gas sensors can be used interchangeably. The integration of this signal chain dramatically reduces cost, size, complexity, and power consumption at the sensor node.

The AD4000/AD4004/AD4008 are high accuracy, high speed, low power, 16-bit, Easy Drive, precision successive approximation register (SAR) analog-to-digital converters (ADCs) that operate from a single power supply, VDD. The reference voltage, V_REF, is applied externally and can be set independent of the supply voltage. The AD4000/AD40004/AD4008 power scales linearly with throughput.

Easy Drive features reduce both signal chain complexity and power consumption while enabling higher channel density. The reduced input current, particularly in high-Z mode, coupled with a long signal acquisition phase, eliminates the need for a dedicated ADC driver. Easy Drive broadens the range of companion circuitry that is capable of driving these ADCs (see Figure 2 in the data sheet).

Input span compression eliminates the need to provide a negative supply to the ADC driver amplifier while preserving access to the full ADC code range. The input overvoltage clamp protects the ADC inputs against overvoltage events, minimizing disturbances on the reference pin and eliminating the need for external protection diodes.

Fast device throughput up to 2 MSPS allows users to accurately capture high frequency signals and to implement oversampling techniques to alleviate the challenges associated with antialias filter designs. Decreased serial peripheral interface (SPI) clock rate requirements reduce digital input and output power consumption, broadens digital host options, and simplifies the task of sending data across digital isolation. The SPI-compatible serial user interface is compatible with 1.8 V, 2.5 V, 3 V, and 5 V logic by using the separate VIO logic supply.

APPLICATIONS

• Automated test equipment
• Machine automation
• Medical equipment
• Battery-powered equipment
• Precision data acquisition systems
• Instrumentation and control systems

The AD549 is a monolithic electrometer operational amplifier with very low input bias current. Input offset voltage and input offset voltage drift are laser trimmed for precision performance. The ultralow input current of the part is achieved with Topgate™ JFET technology, a process development exclusive to Analog Devices, Inc. This technology allows fabrication of extremely low input current JFETs compatible with a standard junction isolated bipolar process. The 10¹⁵ Ω common-mode impedance, which results from the bootstrapped input stage, ensures that the input current is essentially independent of the common-mode voltage.

The AD549 is suited for applications that require very low input current and low input offset voltage. It excels as a preamp for a wide variety of current output transducers, such as photo-diodes, photomultiplier tubes, or oxygen sensors. The AD549 can also be used as a precision integrator or low droop sample and hold. The AD549 is pin compatible with standard FET and electrometer op amps, allowing designers to upgrade the performance of present systems at little additional cost.

The AD549 is available in a TO-99 hermetic package. The case is connected to Pin 8 so that the metal case can be independently connected to a point at the same potential as the input terminals, minimizing stray leakage to the case. The AD549 is available in four performance grades. The J, K, and L versions are rated over the commercial temperature range of 0°C to +70°C. The S grade is specified over the military temperature range of −55°C to +125°C and is available processed to MIL-STD-883B, Rev. C. Extended reliability plus screening is also available. Plus screening includes 168 hour burn-in, as well as other environmental and physical tests derived from MIL-STD-883B, Rev. C.

Product Highlights

1. The AD549 input currents are specified, 100% tested, and guaranteed after the device is warmed up. They are guaranteed over the entire common-mode input voltage range.
2. The AD549 input offset voltage and drift are laser trimmed to 0.50 mV and 15 μV/°C (AD549K), and to 1 mV and 20 μV/°C (AD549J)..
3. A maximum quiescent supply current of 700 μA minimizes heating effects on input current and offset voltage.
4. AC specifications include 1 MHz unity gain bandwidth and 3 V/μs slew rate. Settling time for a 10 V input step is 5 μs to 0.01%.

Applications

• Electrometer amplifier
• Photodiode preamp
• pH electrode buffer

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 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 AD7175-2 is a low noise, fast settling, multiplexed, 2-/4- channel (fully/pseudo differential) Σ-Δ analog-to-digital converter (ADC) for low bandwidth inputs. It has a maximum channel scan rate of 50 kSPS (20 µs) for fully settled data. The output data rates range from 5 SPS to 250 kSPS.

The AD7175-2 integrates key analog and digital signal conditioning blocks to allow users to configure an individual setup for each analog input channel in use. Each feature can be user selected on a per channel basis. Integrated true rail-to-rail buffers on the analog inputs and external reference inputs provide easy to drive high impedance inputs. The precision 2.5 V low drift (2 ppm/°C) band gap internal reference (with output reference buffer) adds embedded functionality to reduce external component count.

The digital filter allows simultaneous 50 Hz/60 Hz rejection at 27.27 SPS output data rate. The user can switch between different filter options according to the demands of each channel in the application. The ADC automatically switches through each selected channel. Further digital processing functions include offset and gain calibration registers, configurable on a per channel basis.

The device operates with a 5 V AVDD1, or ±2.5 V AVDD1/AVSS, and 2 V to 5 V AVDD2 and IOVDD supplies. The specified operating temperature range is −40°C to +105°C. The AD7175-2 is in a 24-lead TSSOP package.

Applications

• Process control: PLC/DCS modules
• Temperature and pressure measurement
• Medical and scientific multichannel instrumentation
• Chromatography

The AD7768/AD7768-4 are 8-channel and 4-channel, simultaneous sampling sigma-delta (Σ-Δ) analog-to-digital converters (ADCs), respectively, with a Σ-Δ modulator and digital filter per channel, enabling synchronized sampling of ac and dc signals.

The AD7768/AD7768-4 achieve 108 dB dynamic range at a maximum input bandwidth of 110.8 kHz, combined with typical performance of ±2 ppm integral nonlinearity (INL), ±50 μV offset error, and ±30 ppm gain error.

The AD7768/AD7768-4 user can trade off input bandwidth, output data rate, and power dissipation, and select one of three power modes to optimize for noise targets and power consumption. The flexibility of the AD7768/AD7768-4 allows them to become reusable platforms for low power dc and high performance ac measurement modules.

The AD7768/AD7768-4 have three modes: fast mode (256 kSPS maximum, 110.8 kHz input bandwidth, 51.5 mW per channel), median mode (128 kSPS maximum, 55.4 kHz input bandwidth, 27.5 mW per channel) and low power mode (32 kSPS maximum, 13.8 kHz input bandwidth, 9.375 mW per channel).

The AD7768/AD7768-4 offer extensive digital filtering capabilities, such as a wideband, low ±0.005 dB pass-band ripple, antialiasing low-pass filter with sharp roll-off, and 105 dB attenuation at the Nyquist frequency.

Frequency domain measurements can use the wideband linear phase filter. This filter has a flat pass band (±0.005 dB ripple) from dc to 102.4 kHz at 256 kSPS, from dc to 51.2 kHz at 128 kSPS, or from dc to 12.8 kHz at 32 kSPS.

The AD7768/AD7768-4 also offer sinc response via a sinc5 filter, a low latency path for low bandwidth, and low noise measurements. The wideband and sinc5 filters can be selected and run on a per channel basis.

Within these filter options, the user can improve the dynamic range by selecting from decimation rates of ×32, ×64, ×128, ×256, ×512, and ×1024. The ability to vary the decimation filtering optimizes noise performance to the required input bandwidth.

Embedded analog functionality on each ADC channel makes design easier, such as a precharge buffer on each analog input that reduces analog input current and a precharge reference buffer per channel reduces input current and glitches on the reference input terminals.

The device operates with a 5 V AVDD1A and AVDD1B supply, a 2.25 V to 5.0 V AVDD2A and AVDD2B supply, and a 2.5 V to 3.3 V or 1.8 V IOVDD supply (see the 1.8 V IOVDD Operation section for specific requirements for operating at 1.8 V IOVDD).

The device requires an external reference; the absolute input reference voltage range is 1 V to AVDD1 − AVSS.

Applications

• Data acquisition systems: USB/PXI/Ethernet
• Instrumentation and industrial control loops
• Audio test and measurement
• Vibration and asset condition monitoring
• 3-phase power quality analysis
• Sonar
• High precision medical electroencephalogram (EEG)/electromyography (EMG)/electrocardiogram (ECG)

The AD7988-1/AD7988-5 are 16-bit, successive approximation, analog-to-digital converters (ADC) that operate from a single power supply, VDD. The AD7988-1 offers a 100 kSPS throughput, and the AD7988-5 offers a 500 kSPS throughput. They are low power, 16-bit sampling ADCs with a versatile serial interface port. On the CNV rising edge, they sample an analog input, IN+, between 0 V to V_REF with respect to a ground sense, IN−. The reference voltage, REF, is applied externally and can be set independent of the supply voltage, VDD.

The SPI-compatible serial interface also features the ability to daisy-chain several ADCs on a single 3-wire bus using the SDI input. It is compatible with 1.8 V, 2.5 V, 3 V, or 5 V logic using the separate supply, VIO.

The AD7988-1/AD7988-5 generics are housed in a 10-lead MSOP or a 10-lead LFCSP with operation specified from −40°C to +125°C.

Applications

• Automated test equipment
• Data acquisition systems
• Medical instruments
• Machine automation

The ADA2200 is a synchronous demodulator and configurable analog filter designed to perform precision magnitude and phase measurements in low power, sensor signal conditioning and data acquisition applications for the industrial, medical, and communications markets. Implemented with ADI’s patent pending sampled analog technology (SAT), the ADA2200 is an analog input, sampled analog output device that includes an analog domain, low-pass 1/8x decimation finite impulse response (FIR) filter, a configurable infinite impulse response (IIR) filter, mixer with 0°/90° phase selection, reference clock and ADC driver output. Utilizing sampled analog technology, the signal processing is performed entirely in the analog domain by charge sharing among capacitors, which eliminates the effects of quantization noise and rounding errors and reduces downstream ADC sample rates while also offloading computationally heavy tasks from the digital processor or micro-controller.

The ADA2200 acts as a precision filter when the demodulation function is disabled. The filter has a programmable bandwidth and tunable center frequency. The filter characteristics are highly stable over temperature, supply, and process variation.

Single-ended and differential signal interfaces are possible on both input and output terminals, simplifying the connection to other components of the signal chain. The low power consumption and rail-to-rail operation is ideal for battery-powered and low voltage systems.

The ADA2200 can be programmed over its SPI-compatible serial port or can automatically boot from the EEPROM through its I2C interface. On-chip clock generation produces a mixing signal with a programmable frequency and phase. In addition, the ADA2200 synchronization output signal eases interfacing to other sampled systems, such as data converters and multiplexers.

The ADA2200 is available in a 16-lead TSSOP package. Its performance is specified over the industrial temperature range of −40°C to +85°C. Note that throughout this data sheet, multifunction pins, such as SCLK/SCL, are referred to either by the entire pin name or by a single function of the pin, for example, SCLK, when only that function is relevant.

Applications

• Synchronous demodulation
• Sensor signal conditioning
• Lock-in amplifiers
• Phase detectors
• Precision tunable Filters
• Signal recovery
• Control systems

The ADA4661-2 is a dual, precision, rail-to-rail input/output amplifier optimized for low power, high bandwidth, and wide operating supply voltage range applications.

The ADA4661-2 performance is guaranteed at 3.0 V, 10 V, and 18 V power supply voltages. It is an excellent selection for applications that use single-ended supplies of 3.3 V, 5 V, 10 V, 12 V and 15 V, and dual supplies of ±2.5 V, ±3.3 V, and ±5 V. It uses the Analog Devices, Inc., patented DigiTrim^® trimming technique, which achieves low offset voltage. Additionally, the unique design architecture of the ADA4661-2 allows it to have excellent power supply rejection, common-mode rejection, and offset voltage when operating in the common-mode voltage range of −V_SY + 1.5 V to +V_SY − 1.5 V.

The ADA4661-2 is specified over the extended industrial temperature range (−40°C to +125°C) and is available in 8-lead MSOP and 8-lead LFCSP (3 mm × 3 mm) packages.

Applications

• Current shunt monitors
• Active filters
• Portable medical equipment
• Buffer/level shifting
• High impedance sensor interfaces
• Battery powered instrumentation

The ADuCM355 is an on-chip system that controls and measures electrochemical sensors and biosensors. The ADuCM355 is an ultralow power, mixed-signal microcontroller based on the Arm^® Cortex^™-M3 processor. The device features current, voltage, and impedance measurement capability.

The ADuCM355 features a 16-bit, 400 kSPS, multichannel successive approximation register (SAR) analog-to-digital converter (ADC) with input buffers, built-in antialias filter (AAF), and programmable gain amplifier (PGA). The current inputs include three transimpedance amplifiers (TIA) with programmable gain and load resistors for measuring different sensor types. The analog front end (AFE) also contains two low power amplifiers designed specifically for potentiostat capability to maintain a constant bias voltage to an external electrochemical sensor. The noninverting inputs of these two amplifiers are controlled by on-chip, dual output digital-to-analog converters (DACs). The analog outputs include a high speed DAC and output amplifier designed to generate an ac signal.

The ADC operates at conversion rates up to 400 kSPS with an input range of −0.9 V to +0.9 V. An input mux before the ADC allows the user to select an input channel for measurement. These input channels include three external current inputs, multiple external voltage inputs, and internal channels. The internal channels allow diagnostic measurements of the internal supply voltages, die temperature, and reference voltages.

Two of the three voltage DACs are dual output, 12-bit string DACs. One output per DAC controls the noninverting input of a potentiostat amplifier, and the other controls the noninverting input of the TIA.

The third DAC (sometimes referred to as the high speed DAC) is designed for the high power TIA for impedance measurements. The output frequency range of this DAC is up to 200 kHz.

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

The ADuCM355 integrates a 26 MHz Arm Cortex-M3 processor, which is a 32-bit reduced instruction set computer (RISC) machine. The Arm Cortex-M3 processor also has a flexible multichannel direct memory access controller (DMA) supporting two independent serial peripheral interface (SPI) ports, universal asynchronous receiver/transmitter (UART), and I²C communication peripherals. The ADuCM355 has 128 kB of nonvolatile flash/EE memory and 64 kB of single random access memory (SRAM) integrated on-chip.

The digital processor subsystem is clocked from a 26 MHz on-chip oscillator. The oscillator is the source of the main digital die system clock. Optionally, a 26 MHz phase-locked loop (PLL) can be used as the digital system clock. This clock can be internally subdivided so that the processor operates at a lower frequency and saves power. A low power, internal 32 kHz oscillator is available and can clock the timers. The ADuCM355 includes three general-purpose timers, a wake-up timer (which can be used as a general-purpose timer), and a system watchdog timer.

The analog subsystem has a separate 16 MHz oscillator used to clock the ADC, DACs, and other digital logic on the analog die. The analog die also contains a separate 32 kHz, low power oscillator to clock a watchdog timer on the analog die. Both the 32 kHz oscillator and this watchdog are independent from the digital die oscillators and system watchdog timer.

A range of communication peripherals can be configured as required in a specific application. These peripherals include UART, I²C, two SPI ports, and general-purpose input/output (GPIO) ports. The GPIOs, combined with the general-purpose timers, can be combined to generate a pulse-width modulation (PWM) type output.

Nonintrusive emulation and program download are supported via the serial wire debug port (SW-DP) interface.

The ADuCM355 operates from a 2.8 V to 3.6 V supply and is specified over a temperature range of −40°C to +85°C. The chip is packaged in a 72-lead, 6 mm × 5 mm land grid array (LGA) package.

Note that, throughout this data sheet, multifunction pins, such as P0.0/SPI0_CLK, are referred to either by the entire pin name or by a single function of the pin, for example, P0.0, when only that function is relevant.

Applications

• Gas detection
• Food quality
• Environmental sensing (air, water, and soil)
• Blood glucose meters
• Life sciences and biosensing analysis
• Bioimpedance measurements
• General Amperometry, voltammetry, and impedance spectroscopy functions