DC/DC преобразование

ADuM3224/ADuM4224 – это изолированные драйверы затворов транзисторов полумоста с выходных током 4 A, в которых для формирования изолированных, независимых выходных управляющих сигналов в цепях высокого и низкого напряжений применяется технология iCoupler® компании Analog Devices. ADuM3224 выпускается в узком 16-выводном корпусе SOIC и имеет выдерживаемое напряжение 3000 В, ср.кв., а ADuM4224 выпускается в широком 16-выводном корпусе SOIC и имеет выдерживаемое напряжение 5000 В, ср.кв. Благодаря объединению быстродействующих схем КМОП и технологии монолитных трансформаторов эти компоненты гальванической развязки обеспечивают выдающиеся характеристики, превосходящие характеристики альтернативных решений, например, комбинации импульсных трансформаторов и драйверов затворов.

Каждый из изоляторов ADuM3224/ADuM4224 реализует два независимых канала гальванической развязки. Они работают с входным напряжением питания в диапазоне от 3.0 В до 5.5 В, поддерживая совместимость с низковольтными системами. По сравнению с драйверами затворов, в которых применяются методы преобразования высоких напряжений, ADuM3224/ADuM4224 обеспечивают истинную гальваническую развязку между входом и каждым из выходов. Разница между напряжением на каждом из выходов и входным напряжением может достигать 560 В в непрерывном режиме, что позволяет поддерживать переключение в область отрицательных напряжений в цепи низкого напряжения. Пиковое дифференциальное напряжение между цепями высокого и низкого напряжений может достигать 800 В.

В результате ADuM3224/ADuM4224 обеспечивают надежное управление характеристиками коммутации IGBT/MOSFET транзисторов в широком диапазоне положительных и отрицательных напряжений.

ОБЛАСТИ ПРИМЕНЕНИЯ

• Импульсные источники питания
• Изолированные драйверы затворов IGBT/MOSFET транзисторов
• Промышленные инвертеры

ADuM120N/ADuM121N – это двухканальные цифровые изоляторы, построенные на базе технологии iCoupler® компании Analog Devices. Благодаря сочетанию технологий изготовления быстродействующих КМОП транзисторов и монолитных трансформаторов с воздушным сердечником эти компоненты гальванической развязки обеспечивают отличные характеристики, превосходящие характеристики альтернативных решений, например, на базе оптопар и других интегрированных устройств сопряжения. Максимальная задержка распространения изоляторов составляет 13 нс, а искажение длительности импульса — менее 3 нс при работе от 5 В. Компоненты обеспечивают межканальное согласование временных характеристик в пределах 3 нс.

ADuM120N/ADuM121N имеют независимые каналы данных и выпускаются в различных конфигурациях с выдерживаемым напряжением 3 кВ, ср.кв. (см. раздел Ordering Guide технического описания). Они работают с напряжением питания на любой из сторон канала в диапазоне от 1.8 В до 5 В, обеспечивая совместимость с низковольтными системами и позволяя осуществлять преобразование уровней при передаче данных через изоляционный барьер.

В отличие от других альтернатив оптопарам компоненты гарантируют корректное постоянное напряжение на выходе при статическом состоянии входного логического сигнала. Они имеют два различных варианта поддержания отказоустойчивости, в которых выходы переходят в предопределенное состояние при отсутствии напряжения питания на входной стороне или при отключенных входах.

ADuM120N0 совместим по выводам с ADuM1285, а ADuM120N1 совместим по выводам с ADuM1280 и ADuM1200. ADuM121N0 совместим по выводам с ADuM1286, а ADuM121N0 совместим по выводам с ADuM1281 и ADuM1201.

Области применения

• Универсальные многоканальные схемы гальванической развязки
• Гальваническая развязка промышленных шин

ADuM141E - это четырехканальный цифровой изолятор, построенный на базе технологии iCoupler^® компании Analog Devices. Благодаря сочетанию технологий изготовления быстродействующих КМОП транзисторов и монолитных трансформаторов с воздушным сердечником эти компоненты обеспечивают отличные характеристики, превосходящие характеристики альтернативных решений, например, на базе оптопар и других интегрированных устройств сопряжения. Максимальная задержка распространения составляет 13 нс, а искажение длительности импульса — менее 3 нс. Компоненты обеспечивают межканальное согласование временных характеристик в пределах 3 нс.

Компоненты серии ADuM141E имеют независимые каналы данных и выпускаются в конфигурациях с различным направлением каналов и выдерживаемым напряжением 3.0 кВ, ср.кв. или 3.75 кВ, ср.кв. Они работают с напряжением питания на любой из сторон канала в диапазоне от 1.8 В до 5 В, обеспечивая совместимость с низковольтными системами и позволяя осуществлять преобразование уровней при передаче данных через изоляционный барьер.

В отличие от других альтернатив оптопарам компонент гарантирует корректное постоянное напряжение на выходе при статическом состоянии входного логического сигнала. Он имеет два различных варианта поддержания отказоустойчивости, в которых выходы переходят в предопределенное состояние при отсутствии напряжения питания на входной стороне или при отключенных входах. ADuM140E1/ADuM141E1/ADuM142E1 совместим по выводам с ADuM1400/ADuM1401/ADuM1402.

Области применения

The LT8361 is a current mode DC/DC converter with a 100V, 2A switch operating from a 2.8V to 60V input. With a unique single feedback pin architecture it is capable of boost, SEPIC or inverting configurations. Burst Mode operation consumes as low as 9μA quiescent current to maintain high efficiency at very low output currents, while keeping typical output ripple below 15mV.

An external compensation pin allows optimization of loop bandwidth over a wide range of input and output voltages and programmable switching frequencies between 300kHz and 2MHz. A SYNC/MODE pin allows synchronization to an external clock. It can also be used to select between burst or pulse-skip modes of operation with or without Spread Spectrum Frequency Modulation for low EMI. For increased efficiency, a BIAS pin can accept a second input to supply the INTV_CC regulator. Additional features include frequency foldback and programmable soft-start to control inductor current during startup.

The LT8361 is available in a thermally enhanced 16-lead MSOP package with four pins removed for high voltage pin spacings.

Applications

• Industrial and Automotive
• Telecom
• Medical Diagnostic Equipment
• Portable Electronics

The low output voltage hysteresis and low long-term output voltage drift improve lifetime system accuracy.

These CMOS references are available in five output voltages, all of which are specified over the automotive temperature range of −40°C to +125°C.

Applications

• Auto battery monitors
• Portable instrumentation
• Process transmitters
• Remote sensors
• Medical Instrumentation

AD8210 выпускается в корпусе SOIC. Рабочий температурный диапазон составляет от −40°C до +125°C.

Благодаря превосходной температурной стабильности статических характеристик погрешности в контуре измерения поддерживаются компонентом на минимальном уровне. Значения дрейфа напряжения смещения и дрейфа коэффициента усиления компонента гарантированно не превышают 8 мкВ/°C и 20 ppm/°C, соответственно.

Выходное напряжение смещения можно регулировать в диапазоне от 0.05 В до 4.9 В при питании 5 В, используя выводы V_REF1 и V_REF2. При подключении V_REF1 к выводу V+, а V_REF2 - к выводу GND выходное напряжение устанавливается в середину шкалы. При подключении обоих выводов к земле выходной сигнал является однополярным и изменяется относительно уровня, примерно равного напряжению земли. При подключении обоих выводов к V+ выходной сигнал является однополярным и изменяется относительно уровня, примерно равного V+. Другие напряжения смещения можно получить, прикладывая внешнее напряжение к выводам V_REF1 и V_REF2.

Области применения

• Измерение тока в системах
  управления электрическими двигателями
  управления трансмиссией
  управления впрыском дизельного топлива
  управления двигателями
  управления подвеской
  динамической стабилизации курсовой устойчивости автомобиля
  преобразования постоянного напряжения

The AD8418A is a high voltage, high resolution current shunt amplifier. It features an initial gain of 20 V/V, with a maximum ±0.15% gain error over the entire temperature range. The buffered output voltage directly interfaces with any typical converter. The AD8418A offers excellent input common-mode rejection from −2 V to +70 V. The AD8418A performs bidirectional current measurements across a shunt resistor in a variety of automotive and industrial applications, including motor control, power management, and solenoid control.

The AD8418A offers breakthrough performance throughout the −40°C to +150°C temperature range. It features a zero drift core, which leads to a typical offset drift of 0.1 μV/°C throughout the operating temperature range and the common-mode voltage range. The AD8418A is qualified for automotive applications. The device includes EMI filters and patented circuitry to enable output accuracy with pulse-width modulation (PWM) type input common-mode voltages. The typical input offset voltage is ±100 μV. The AD8418A is offered in 8-lead MSOP and SOIC packages.

Applications

• High-side current sensing in
  
  • Motor controls
  • Solenoid controls
  • Power management
• Low-side current sensing
• Diagnostic protection

The LTC3871/LTC3871-1 is a high performance bidirectional buck or boost switching regulator controller that operates in either buck or boost mode on demand. It regulates in buck mode from V_HIGH-to-V_LOW and boost mode from V_LOW-to-V_HIGH depending on a control signal, making it ideal for 48V/12V automotive dual battery systems. An accurate current programming loop regulates the maximum current that can be delivered in either direction. The LTC3871/LTC3871-1 allows both batteries to supply energy to the load simultaneously by converting energy from one battery to the other.

Its proprietary constant-frequency current mode architecture enhances the signal-to-noise ratio enabling low noise operation and provides excellent current matching between phases. Additional features include discontinuous or continuous mode of operation, OV/UV monitors, independent loop compensation for buck and boost operation, accurate output current monitoring and overcurrent protection. The LTC3871 and LTC3871-1 have different current limit foldback characteristics.

Applications

• Automotive 48V/12V Dual Battery Systems
• Backup Power Systems

The LTC4380 low quiescent current surge stopper protects loads from high voltage transients. Overvoltage protection is provided by clamping the gate voltage of an external N-channel MOSFET to limit the output voltage to a safe value during overvoltage events such as load dump in automobiles. Fixed gate clamp voltages are selectable for 12V and 24V/28V systems. For systems of any voltage up to 72V, use the adjustable gate clamp versions. Overcurrent protection is also provided.

An internal multiplier generates a TMR pin current proportional to V_DS and I_D, so that operating time in both overcurrent and overvoltage conditions is limited in accordance with MOSFET stress.

The GATE pin can drive back-to-back MOSFETs for reverse input protection, eliminating the voltage drop and dissipation of a Schottky diode solution. A low 8µA operating current permits use in always-on and battery powered applications. An accurate ON pin comparator monitors the input supply for undervoltage (UV) conditions and also serves as a shutdown input, reducing the quiescent current to 6µA.

Applications

• Automotive/Avionic/Industrial Surge Protection
• Hot Swap, Live Insertion
• High Side Switch for Battery Powered Systems
• Automotive Load Dump Protection

The LT8316 is a micropower, high voltage flyback controller. No opto-isolator is needed for regulation. The device samples the output voltage from the isolated flyback waveform appearing across a third winding on the transformer. Quasi-resonant boundary mode operation improves load regulation, reduces transformer size, and maintains high efficiency.

At start-up, the LT8316 charges its INTV_CC capacitor via a high voltage current source. During normal operation, the current source turns off and the device draws its power from a third winding on the transformer minimizing standby power dissipation. 

The LT8316 operates from a wide range of input supply voltages and can deliver up to 100W of power. It is available in a thermally enhanced 20-pin TSSOP package with four pins removed for high-voltage spacing.

Applications

• Isolated Telecom, Automotive, Industrial, Medical Power Supplies
• Isolated Off-Line Housekeeping Power Supplies
• Electric Vehicles and Battery Stacks
• Multioutput Isolated Power Supplies for Inverter Gate Drives

The circuit shown in Figure 1 is a complete thermopile-based gas sensor using the nondispersive infrared (NDIR) principle. This circuit is optimized for CO₂ sensing, but can also accurately measure the concentration of a large number of gases by using thermopiles with different optical filters.

The printed circuit board (PCB) is designed in an Arduino shield form factor and interfaces to the EVAL-ADICUP360 Arduino-compatible platform board. The signal conditioning is implemented with the AD8629 and the ADA4528-1 low noise amplifiers and the ADuCM360 precision analog microcontroller, which contains programmable gain amplifiers, dual 24-bit Σ-Δ analog-to-digital converters (ADCs), and an ARM Cortex-M3 processor.

The circuit shown in Figure 1 is a complete high performance resolver-to-digital (RDC) circuit that accurately measures angular position and velocity in automotive, avionics, and critical industrial applications where high reliability is required over a wide temperature range.

The circuit has an innovative resolver rotor driver circuit that has two modes of operation: high performance and low power. In the high performance state, the system operates on a single 12 V supply and can supply 6.4 V rms (18 V p-p) to the resolver. In the low power state, the system operates on a single 6 V supply and can supply 3.2 V rms (9.2 V p-p) to the resolver, with less than 100 mA of current consumption. Active filtering is provided in both the driver and receiver to minimize the effects of quantization noise.

The maximum tracking rate of the RDC is 3125 rps in the 10-bit mode (resolution = 21 arc min) and 156.25 rps in the 16-bit mode (resolution = 19.8 arc sec).

The circuit shown in Figure 1 measures indoor air quality by using a metal-oxide sensor to detect gases composed of volatile organic compounds. The sensor is composed of a heating resistor and a sensing resistor. When the sense resistor is heated, its value changes as a function of the concentrations of different gases.

The circuit uses a 12-bit, current output digital-to-analog converter (DAC) for precision control of the heater current, and the flexible software allows the heater to operate in one of the following four modes: constant current, constant voltage, constant resistance, and constant temperature.

The circuit is able measure a wide range of sense resistance values by using a software-selectable, five range resistor divider. The board also includes a temperature and humidity sensor that is used for compensating the gas concentration value.

The circuit shown in Figure 1 is a high performance, resolver-to-digital converter (RDC) circuit that accurately measures angular position and velocity in automotive, avionics, and critical industrial applications where high reliability is required over a wide temperature range. The AD8397 high current driver can supply 310 mA into a 32 Ω load and eliminates the requirement for discrete push-pull buffer solutions.

Common applications of RDCs are in automotive and industrial markets to provide motor shaft position and/or velocity feedback.

Figure 1. High Current Buffer Using the AD8397 for the AD2S1210 RDC Excitation Signal Output (Simplified Schematic: Decoupling and All Connections Not Shown)  

The circuit in Figure 1 is a completely isolated, robust, industrial, 4-channel data acquisition system that provides 16-bit, noise free code resolution and an automatic channel switching rate of up to 42 kSPS. The channel to channel crosstalk at 42 kSPS switching is less than 15 ppm FS (less than −90 dB) because of the unique selection of fast settling components in the multiplexed signal chain.

The circuit acquires and digitizes standard industrial signal levels of ±5 V, ±10 V, 0 V to 10 V, and 0 mA to 20 mA. The input buffers also provide overvoltage protection, thereby eliminating the leakage errors associated with conventional Schottky diode protection circuits.

Applications for the circuit include process control (PLC/DCS modules), battery testing, scientific multichannel instrumentation, and chromatography.

Figure 1. Functional Block Diagram of 4-Channel Data Acquisition System (Simplified Schematic: All Connections and Decoupling Not Shown)  

The circuit shown in Figure 1 monitors current in systems with high positive common-mode dc voltages of up to +500 V with less than 0.2% error. The load current passes through a shunt resistor, which is external to the circuit. The shunt resistor value is chosen so that the shunt voltage is approximately 500 mV at maximum load current.

The AD8212 accurately amplifies a small differential input voltage in the presence of large positive common-mode voltages greater than 500 V when used in conjunction with an external PNP transistor.

Galvanic isolation is provided by the ADuM5402 quad channel isolator. This is not only for protection but to isolate the downstream circuitry from the high common-mode voltage. In addition to isolating the output data, the ADuM5402 digital isolator can also supply isolated +3.3 V for the circuit.

The measurement result from the AD7171 is provided as a digital code utilizing a simple 2-wire, SPI-compatible serial interface.

This combination of parts provides an accurate high voltage positive rail current sense solution with a small component count, low cost, and low power.

The circuit in Figure 1 is a complete, low power signal conditioner for a bridge type sensor and includes a temperature compensation channel. This circuit is ideal for a variety of industrial pressure sensors and load cells that operate with drive voltages of between 5 V and 15 V.

The circuit can process full-scale signals from approximately 10 mV to 1 V, using the internal programmable gain amplifier (PGA) of the 24-bit, sigma-delta (Σ-Δ) ADC, making it suitable for a wide variety of pressure sensors.

The entire circuit uses only three ICs and requires only 1 mA (excluding the bridge current). A ratiometric technique ensures that the accuracy and stability of the system does not depend on a voltage reference.

Looking for true 16-bit level set performance in a small package and ultralow power? This circuit provides a low power, small footprint solution for precision 16-bit digital-to-analog conversion using the AD5542A/ AD5541A voltage output DAC with the ADR421BRZ voltage reference and the 20 μA AD8657 as the voltage reference buffer.

The reference buffer is critical to the design because the input impedance at the DAC reference input is heavily code- dependent and will lead to linearity errors if the DAC reference is not adequately buffered. With a high open-loop gain of 120 dB, the AD8657 has been proven and tested to meet the settling time, offset voltage, and low impedance drive capability required by this circuit application.

The combination of parts shown in Figure 1 minimizes PC board area, as well as power dissipation. The AD5542A is available in a 3 mm × 3 mm, 16-lead LFCSP or 16-lead TSSOP package. The AD5541A is available in 3 mm × 3 mm, 10-lead LFCSP or 10-lead MSOP.

This combination of parts provides industry-leading 16-bit integral nonlinearity (INL) of ±1 LSB and differential nonlinearity (DNL) of ±1 LSB with guaranteed monotonicity, as well as low power, small PCB area, and cost effectiveness.

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.

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.

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.

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.

High-side current monitors are likely to encounter overvoltage conditions from transients or when the monitoring circuits are connected, disconnected, or powered down. This circuit, shown in Figure 1, uses the overvoltage protected ADA4096-2 op amp connected as a difference amplifier to monitor the high-side current. The ADA4096-2 has input overvoltage protection, without phase reversal or latch-up, for voltages of 32 V higher than and lower than the supply rails.

The circuit is powered by the ADP3336 adjustable low dropout 500 mA linear regulator, which can also be used to supply power to other parts of the system, if desired. Its input voltage can range from 5.2 V to 12 V when set for a 5 V output. To save power, the current sensing circuit can be powered down by removing power to the ADP3336; however, the power source, such as a solar panel, can still operate.

This applies voltage to the inputs of the unpowered ADA4096-2; however, no latch-up or damage occurs for input voltages up to 32 V. If slower throughput rates are required, the AD7920 can also be powered down between samples. The AD7920 draws a maximum of 5 μW when powered down and 15 mW when powered up. The ADA4096-2 requires only 120 μA under operational conditions. When operating at 5 V, this is only 0.6 mW. The ADP3336 draws only 1 μA in the shutdown mode.

This circuit provides precision data conversion using the AD5542 voltage output DAC together with the ADR421BRZ voltage reference and the AD8628 auto-zero op amp as the reference buffer. The AD8628 reference buffer provide benefits previously found only in expensive auto-zeroing or chopper-stabilized amplifiers. Using Analog Devices, Inc., circuit topology, these zero-drift amplifiers combine low cost with high accuracy and low noise. No external capacitor is required, and the digital switching noise associated with most chopper-stabilized amplifiers is greatly reduced, thereby making this the optimum choice for reference buffering.

This circuit provides precision, low power, voltage output, digital-to-analog conversion. The AD5542 can be operated in either the buffered or unbuffered mode. The application and its requirements on settling time, input impedance, noise, etc., determine which mode of operation is best. The selection of the output buffer amplifier can be tailored to suit either dc precision or fast settling time. Where the DAC is required to drive a load less than 60 kΩ, an output buffer will be required. The output impedance of the DAC is constant and code independent, but to minimize gain errors the input impedance of the output amplifier should be as high as possible. The output amplifier should also have a 3 dB bandwidth of 1 MHz or greater. The output amplifier adds another time constant to the system, thereby increasing the settling time of the final output.

A higher 3 dB amplifier bandwidth results in a faster effective settling time of the combined DAC and amplifier. All devices in the circuit can be powered from a single +5 V supply. The input voltage range of the ADR421 reference is 4.5 V to 18 V.

The circuit in Figure 1 is a 4 mA-to-20 mA current loop transmitter for communication between a process control system and its actuator. Besides being cost effective, this circuit offers the industry’s low power solution. The 4 mA-to-20 mA current loop has been used extensively in programmable logic controllers (PLCs) and distributed control systems (DCS’s), with digital or analog inputs and outputs. Current loop interfaces are usually preferred because they offer the most cost effective approach to long distance noise immune data transmission. The combination of the low power AD8657 dual op amp, AD5641DAC, and ADR02 reference allows more power budget for higher power devices, such as microcontrollers and digital isolators. The circuit output is 0 mA to 20 mA of current, and it operates on a single supply from 8 V to 18 V. The 4 mA to 20 mA range is usually mapped to represent the input control range from the DAC or micro-controller, while the output current range of 0 mA to 4 mA is often used to diagnose fault conditions.

The 14-bit, 5 V AD5641 requires 75 μA typical supply current. The AD8657 is a rail-to-rail input/output dual op amp and is one of the lowest power amplifiers currently available in the industry (22 μA per amplifier over the full supply voltage and input common-mode range) with high operating voltage of up to 18 V. The ADR02 ultracompact precision 5 V voltage reference requires only 650 μA. Together, these three devices consume a typical supply current of 747 μA.

The circuit has a 12-pin Pmod™ digital interface (Digilent specification).

This circuit is a weigh scale system, which uses the AD7190, an ultralow noise, low drift, 24-bit Σ-Δ ADC with internal PGA. The AD7190 simplifies the weigh scale design because most of the system building blocks are included on the chip.

The AD7190 maintains good performance over the complete output data rate range, from 4.7 Hz to 4.8 kHz, which allows it to be used in weigh scale systems that operate at low speeds along with higher speed weigh scale systems, such as hopper scales.

Figure 1. Weigh Scale System Using the AD7190 (Simplified Schematic: All Connections Not Shown)

The circuit shown in Figure 1 is an isolated, flyback power supply that uses a linear isolated error amplifier to supply the feedback signal from the secondary side to the primary side. Unlike optocoupler-based solutions, which have a nonlinear transfer function that changes over time and temperature, the linear transfer function of the isolated amplifier is stable and minimizes offset and gain errors when transferring the feedback signal across the isolation barrier.

The entire circuit operates from 5 V to 24 V, allowing it to be used with standard industrial and automotive power supplies. The output capability of the circuit is up to 1 A with a 5 V input and 5 V output configuration.

This solution can be adapted for use in applications where higher dc input voltages are used to create lower voltage isolated supplies with good efficiency and a small form factor. Examples include 10 W to 20 W telecommunication and server power supplies, where power efficiency and printed circuit board (PCB) density are important, and −48 V supplies are common.

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.

The circuit shown in Figure 1 is a cost effective, isolated, multi- channel data acquisition system that is compatible with standard industrial signal levels. The components are specifically selected to optimize settling time between samples, providing 18-bit performance at channel switching rates up to approximately 750 kHz.

The circuit can process eight gain-independent channels and is compatible with both single-ended and differential input signals.

The analog front end includes a multiplexer, programmable gain instrumentation amplifier (PGIA); precision analog-to- digital converter (ADC) driver for performing the single-ended to differential conversion; and an 18-bit, 2.0 MSPS precision PulSAR^® ADC for sampling the signal on the active channel. Gain configurations of 0.4, 0.8, 1.6, and 3.2 are available.

The maximum sample rate of the system is 2 MSPS in turbo mode, and 1.5 MSPS in normal mode. The channel switching logic is synchronous to the ADC conversions, and the maximum channel switching rate is 1.5 MHz. A single channel can be sampled at up to 2 MSPS with 18-bit resolution in turbo mode. Channel switching rates up to 750 kHz also provide 18-bit performance.