化学分析和分析仪器

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.

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 circuit shown in Figure 1 uses a photometric front end and a network of 860 nm infrared (IR) emitters and silicon PIN photodiodes to achieve a water turbidity measurement system. Turbidity is an important water quality indicator for the presence of dispersed or suspended solids, which affects potable water and environmental conditions. Turbidity is a qualitative characteristic imparted by how these suspended solids obstruct the transmittance of light. Turbidity is not a direct measure of suspended particles in water but rather a measure of the scattering effect that such particles have on light.

The system can measure low to high water turbidity levels ranging from 0 FTU to 1000 FTU. The IR LED and photodiode network is arranged in such a way that it can support two of the most recognized turbidity measurement standards: ISO7027 (both ratio and nonratio) and the GLI method. With three-point calibration, the typical accuracy that the system can achieve is ±0.50 FTU or ±5% of the reading, whichever is greater. This accuracy combined with the 0.05 FTU noise level makes the measurements obtained using this system very reliable.

The ADPD105 ambient light rejection feature makes this circuit ideal for applications where accurate, robust, and noncontact turbidity measurements are critical. Applications include chemical analysis and environmental monitoring of natural bodies of water (such as wastewater and drinking water).

The printed circuit board (PCB) is designed in an Arduino shield-compatible form factor and directly interfaces to the EVAL-ADICUP360 Arduino form factor-compatible platform board for rapid prototyping.

图1. 3线RTD测量配置

The system functional diagram in Figure 1 is a precision analog front end for measurement of current down to the femtoampere range. This industry-leading solution is ideal for chemical analyzers and laboratory grade instrument where an ultrahigh sensitivity analog front end is required for signal conditioning current output sensors such as photodiodes, photomultiplier tubes, and Faraday cups. Applications that can use this solution include mass spectrometry, chromatography, and coulometry.

The EVAL-CN0407-SDPZ provides a reference design for real-world application by partitioning the system into a low-leakage mezzanine board and a data acquisition board. The input signal conditioning is implemented with the ADA4530-1 on the mezzanine board. The ADA4530-1 is an electrometer-grade amplifier with ultralow input bias current of 20 fA maximum at 85°C. A guard buffer is integrated on the chip to isolate the input pins from leakage to the printed circuit board (PCB). The default amplifier configuration is in the transimpedance mode with a 10 GΩ glass resistor and a metal shield that prevents leakage current from entering any of the high impedance paths on the board. In addition, the mezzanine board includes unpopulated resistor and capacitor pads to allow prototyping with surfacemount feedback resistors as well as other input configurations.

The data acquisition board uses an AD7172-2 24-bit Σ-Δ analog-to-digital-converter (ADC) and is powered from a single 9 V dc supply. The on-board supply generates all necessary voltages required to power both boards. The board connects to a PC via the SDP-S board (EVAL-SDP-CS1Z) and uses digital isolation to prevent noise from the USB bus or ground loops from degrading low current measurements.

经过仔细选择的精密信号调理元件组合可在 0.1 μS 至 10 S (10 MΩ 至 0.1 Ω) 电导率范围内提供优于 0.3% 的精度，且无 需校准。

针对 100 Ω 或 1000 Ω 铂 (Pt) 电阻温度检测器 (RTD) 提供自动 检测功能，允许以室温为参考测量电导率。

系统支持双线式或四线式电导池以及双线式、三线式或四 线式 RTD ，以提高精度和灵活性。

该电路能以极小的直流失调产生精确交流激励电压，从而 避免电导率电极上的极化电压受损。交流激励的幅度和频 率为用户可编程。

创新的同步采样技术可将激励电压和电流的峰峰值幅度转 化为直流值，这样不仅提升了精度，同时简化了内置于精 密模拟微控制器的双通道 24 位 Σ-Δ 型 ADC 对于信号的处理。

采用 LCD 显示器和编码器按钮实现直观的用户界面。该电 路可以按需使用 RS-485 接口实现与 PC 的通信，并采用 4 V 至 7 V 单电源供电。

图 1. 高性能电导率测量系统 ( 原理示意图：未显示所有连接和去耦 )

该电路可为0至14范围内的pH值提供精度为0.5%的读数，无噪声代码分辨率大于14位，适用于多种工业应用，如化工、食品加工、水处理、污水分析等。

该电路支持众多内部电阻超高(范围从1 MΩ至数GΩ) 的pH传感器，其数字信号和电源隔离设计使其免受恶劣工业环境中常见的噪声和瞬变电压的影响。

图1所示电路是使用电化学传感器的单电源、低功耗、电池供电、便携式气体探测器。本示例中使用Alphasense CO-AX一氧化碳传感器。

对于检测或测量多种有毒气体浓度的仪器，电化学传感器能够提供多项优势。大多数传感器都是针对特定气体而设计，可用分辨率小于气体浓度的百万分之一(ppm)，所需工作电流极小，非常适合便携式电池供电的仪器。

图1所示电路使用双通道微功耗放大器ADA4505-2，该器件在室温下的最大输入偏置电流为2 pA，每个放大器的功耗仅为10 μA。此外，ADR291 精密、低噪声、微功耗基准电压源的功耗仅为12 μA，可建立2.5 V共模伪地基准电压。

图1. 低功耗气体探测器电路 

ADP2503 高效率、降压/升压调节器支持两节AAA电池的单电源供电，在节能模式下的功耗仅为38 μA。

图1所示电路（不包括 AD7798 ADC）的总功耗在正常条件下（未探测到气体）约为110 μA，在最差条件下（探测到2000 ppm CO）约为460 μA。AD7798工作时的功耗约为180 μA（G = 1，缓冲模式），节能模式下仅为1 μA。

由于电路功耗极低，两节AAA电池便可提供合适的电源。当连接到ADC和微控制器或者内置ADC的微控制器时，电池寿命可从6个月以上到一年以上不等。

图1所示电路是一个双通道色度计，集成调制光源发射器 和同步检波器接收器。电路以三种不同的波长测量样本与 参考容器的吸收光线之比。

该电路针对许多化学分析和环境监控仪器提供有效的解决 方案，这些仪器仪表用于通过吸收光谱测量浓度和表征材 料。

光电二极管接收器调理路径包括可编程增益跨阻放大器， 用于将二极管电流转换为电压，并允许分析光吸收情况大 不相同的不同液体。16位Σ-Δ型模数转换器(ADC)提供额外 的动态范围，确保宽范围光电二极管输出电流具有足够的 分辨率。

使用调制光源和同步检波器而非恒流(直流)源可消除环境 光线和低频噪声产生的测量误差，并提供更高的精度。

图1所示电路是一个双通道、库隔离、宽带宽数据采集 (DAQ) 系统，采用同步采样架构，每通道使用一个模数转换器 (ADC)。该系统实现了高通道密度以及库和数字背板之间的隔离，而且性能优异。设计还将ADC配置为菊花链模式，并使用具有微调延迟时钟特性的隔离器产品，从而高效使用隔离通道。集成脉宽调制 (PWM) 控制器和变压器驱动器的隔离器在隔离栅上执行DC/DC转换，使电源生成也得到了简化。该系统还具有典型DAQ信号链的许多常见特性，比如输入电路保护、可编程增益通道、高精度和高性能。

同步采样消除了多路复用DAQ信号链固有的多通道采样速率限制。模拟前端 (AFE) 设计也比多路复用方案简单，因为系统的建立时间性能要求有所降低。各通道同步采样，而顺序采样系统的通道之间会有延迟。

数字库隔离DAQ设计为数字后端电路提供了保护，减少了接地环路和库间共模干扰。每个接地层支持多个DAQ信号链，所需的数字隔离器件比通道间隔离系统要少。

ADI公司的ADA4945-1CP-EBZ评估板允许用户评估ADA4945-1全差分放大器的性能。ADA4945-1CP-EBZ评估板可配置为接受单端或差分输入信号。

ADA4945-1CP-EBZ评估板利用多个2引脚和3引脚接头控制ADA4945-1的各种特性。施加正确的跳线以便设置ADA4945-1高低输出箝位电平，设置ADA4945-1输出共模电压，选择ADA4945-1的高功率或低功率模式并设置ADA4945-1数字接地电平。

电源层和接地层经优化可确保低噪声和高速运算。器件放置和电源旁路可以最大程度地提高电路的灵活性和性能。ADA4945-1CP-EBZ评估板支持0402表面贴装技术(SMT)器件、0805旁路电容和2.54 mm接头。

输入和输出信号通过50 Ω侧边引出超小A型(SMA)连接器转至/自此电路板。

ADA4945-1数据手册提供ADA4945-1的完整规格。使用ADA4945-1CP-EBZ评估板时，应同时参阅数据手册和本用户指南。

片内 ADuCM355 系统具备偏置和测量各种不同电化学传感器的功能。EVAL-ADuCM355QSPZ 使用户能够在实施一系列不同的电化学技术，评估 ADuCM355 的性能，包括计时电流法、伏安法和电化学阻抗谱（EIS）。

UG-1201 介绍了用于 ADA4625-1 低噪声、快速稳定单电源轨到轨输出 (RRO) 结型场效应晶体管 (JFET) 运算放大器的评估板，此评估板采用具有裸焊盘的 8 引脚小外形集成电路 (SOIC) 封装。此评估板的设计注重简易性和易用性。此评估板是一款双层板，可在输入和输出端容纳边缘安装型微型 A (SMA) 连接器。使用 SMA 连接器可以高效连接到测试设备或其它电路。

已针对最佳电路灵活性和性能优化了评估板的接地平面、组件布置和电源旁路。ADA4625-1 的裸焊盘连接到评估板上的接地平面，以增强热性能和噪声性能。该评估板组合使用 0603 和 0805 表面安装技术 (SMT) 组件外壳规格，但旁路电容器 C3 和 C5 除外，其最大标准规格为 1206。该评估板还采用各种各样的未填充电阻器和电容器焊盘，为用户提供了多种选择和更大灵活性，适用于不同应用电路和配置，例如有源环路滤波器、互阻抗放大器 (TIA) 和电荷放大器。

ADA4625-1 数据手册介绍了器件工作和应用电路配置的规格和详细信息以及指南。请参阅数据手册以及 UG-1201，更好地了解器件操作，尤其是在首次为评估板加电时。

EVAL-ADAQ7980SDZ评估板设计用于演示低功耗产品ADAQ7980的性能，以利于了解各种系统应用的接口。ADAQ7980是一款16位模数转换器(ADC)子系统，系统中集成四个常见信号处理和调理模块，采用支持各种应用的系统级封装(SiP)设计。

尽管装有ADAQ7980，EVAL-ADAQ7980SDZ还可以评估ADAQ7988。要模拟ADAQ7988性能评估，在ADAQ798x评估软件中将ADAQ7980的最大采样速率限制到500 kSPS。

该评估板非常适合与ADI公司的系统演示平台(SDP)板EVAL-SDP-CB1Z一起使用。EVAL-ADAQ7980SDZ通过一个120引脚连接器与SDP板接口。采用SMA连接器的P1、P2、P3和P4来连接低噪声模拟信号源。

ADAQ798x评估软件可执行文件通过USB连接的EVAL-SDP-CB1Z来控制评估板。

ADAQ7980的完整说明和技术规格参见ADAQ7980/ADAQ7988数据手册。使用评估板时，应同时参阅数据手册和本用户指南。有关EVAL-SDP-CB1Z的全部详细信息，请访问SDP-B产品页面。