Circuit functions and advantages

The circuit in Figure 1 is a completely self-contained, microprocessor-controlled, high-precision conductivity measurement system suitable 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 provides better than 0.3% accuracy over the 0.1 μS to 10 S (10 MΩ to 0.1 Ω) conductivity range without calibration.

Provides automatic detection for 100 Ω or 1000 Ω platinum (Pt) resistance temperature detectors (RTDs), allowing conductivity measurements to be referenced to room temperature.

The system supports two-wire or four-wire conductivity cells and two-, three- or four-wire RTDs for increased accuracy and flexibility.

This circuit generates precise AC excitation voltages with minimal DC offset, thereby avoiding damage to the polarization voltage on the conductivity electrode. The amplitude and frequency of AC stimulation are user programmable.

Innovative synchronous sampling technology converts the peak-to-peak amplitude of the excitation voltage and current into DC values, which not only improves accuracy but also simplifies signal processing by the dual-channel 24-bit Σ-Δ ADC built into the precision analog microcontroller. .

Intuitive user interface using LCD display and encoder buttons. The circuit communicates with a PC using an RS-485 interface on demand and operates from a single 4 V to 7 V power supply.

Circuit description

The excitation square wave for the conductivity cell is generated by switching the ADG1419 between +VEXC and −VEXC voltages using the PWM output of the ADuCM360 microcontroller . The square wave must have an exact 50% duty cycle and very low DC offset. Even small DC imbalances can damage the conductivity cell over time.

The +VEXC and −VEXC voltages are generated by the ADA4077-2 operational amplifiers (U9A and U9B), and the amplitude of these two voltages is controlled by the DAC output of the ADuCM360, as shown in Figure 2.

The ADA4077-2 has a typical offset voltage of 15 μV (Grade A), a bias current of 0.4 nA, an offset current of 0.1 nA, an output current up to ±10 mA, and a dropout voltage below 1.2 V. The U9A op amp has a closed-loop gain of 8.33 and converts the ADuCM360 internal DAC output (0 V to 1.2 V) to a +VEXC voltage in the 0 V to 10 V range. The U9B op amp inverts +VEXC, producing the −VEXC voltage. Choose R22 such that R22 = R24||R27 to eliminate the first-order bias current. The error caused by U9A's 15 μV offset voltage is approximately (2 × 15 μV) ÷ 10 V = 3 ppm. Therefore, the main error produced by the inverting stage is the resistor matching error between R24 and R27.

The ADG1419 is a 2.1 Ω on-resistance SPDT analog switch with an on-resistance flatness of 50 mΩ over a ±10 V range, making it ideal for generating symmetrical square wave signals from ±VEXC voltages. Resistor R23 limits the maximum current through the sensor to 10 V/1 kΩ = 10 mA. The symmetry error caused by the ADG1419 is typically 50 mΩ ÷1 kΩ = 50 ppm.

The voltage V1 applied to the conductivity cell is measured using the AD8253 instrumentation amplifier (U15). U15 positive input is buffered by ADA4000-1 (U14). The ADA4000-1 was chosen because it has a low bias current of 5 pA, which minimizes low current measurement errors associated with low conductivity. The negative input of the AD8253 does not require buffering.

The synchronous sampling stage can eliminate the offset voltage of U14 and U15 without affecting the measurement accuracy.

U15 and U18 use the AD825310 MHz, 20 V/μs, programmable gain (G = 1, 10, 100, 1000) instrumentation amplifier with a gain error of less than 0.04%. The AD8253 has a slew rate of 20 V/μs and a 0.001% settling time of 1.8 μs (G = 1000). Its common-mode rejection is typically 120 dB.

Stage U19 ( ADA4627-1 ) is a precision current-to-voltage converter that converts the current flowing through the sensor into a voltage. The device's low bias current and low offset voltage performance make it an ideal choice for this stage. A 120 μV offset error creates a symmetrical error of only 120 μV/10 V = 12 ppm. The ADA4627-1 has an offset voltage of 120 μV (typ, Class A), a bias current of 1 pA (typ), a slew rate of 40 V/μs, and a 0.01% settling time of 550 ns.

The U22A and U22B ( AD8542 ) buffers provide the 1.65 V reference voltage for the U18 and U15 instrumentation amplifiers, respectively.

The remaining devices on the voltage channel signal path (U17A, U17B, U10, U13, U12A, and U12B) are described below. The current channels (U17C, U17D, U16, U21, U20A, and U20B) work the same way.

ADuCM360 can generate the PWM0 square wave switching signal for use by the ADG1419 switch, and can also generate PWM1 and PWM2 synchronization signals for use by the synchronous sampling stage. The voltage and three timing waveforms of the conductivity cell are shown in Figure 3.

The output of the AD8253 instrumentation amplifier (U15) drives two parallel sample-and-hold circuits; these two circuits consist of the ADG1211 switch (U17A/U17B), series resistor (R34/R36), holding capacitor (C50/C73), and unity gain buffer ( U10/U13) composition.

The ADG1211 is a low charge injection, four-channel SPST analog switch that operates from a ±15 V supply voltage and accepts input signals up to ±10 V. The maximum charge injection due to switching is 4 pC, resulting in a voltage error of only 4 pC ÷4.7 μF = 0.9 μV.

The PWM1 signal enables the U10 sample-and-hold buffer to sample on the negative cycle of the sensor voltage and then hold it until the next sample period. Therefore, the U10 sample-and-hold buffer output is equal to the DC level corresponding to the negative amplitude of the sensor voltage square wave.

Similarly, the PWM2 signal enables the U13 sample-and-hold buffer to sample during the positive period of the sensor voltage and then hold it until the next sample period. Therefore, the U13 sample-and-hold buffer output is equal to the DC level corresponding to the positive amplitude of the sensor voltage square wave.

The sample-and-hold buffer ( ADA4638-1 ) has a typical bias current of 45 pA, while the ADG1211 switch has a typical leakage current of 20 pA. Therefore, the worst-case leakage current of the 4.7 μF holding capacitor is 65 pA. For an excitation frequency of 100 Hz, the period is 10 ms. The voltage drop over half a cycle (5 ms) due to 65 pA leakage current is (65 pA × 5 ms) ÷ 4.7 μF = 0.07 μV.

The ADA4638-1 zero-drift amplifier has a typical offset voltage of only 0.5 μV, which contributes negligible error.

The last stage in the signal chain before the ADC is the ADA4528-2 inverting attenuator (U12A and U12B), which has a gain of −0.16 and a common-mode output voltage of +1.65 V. The ADA4528-2 offset voltage is typically 0.3 μV, so the error contribution is negligible.

The attenuator stage reduces a ±10 V maximum signal to ±1.6 V, with a common-mode voltage of +1.65 V. This range is equivalent to the ADuCM360ADC input range, which is 0 V to 3.3 V (1.65 V ± 1.65 V) using a 3.3 V AVDD supply.

The attenuator stage also provides noise filtering, with a −3 dB frequency of approximately 198 kHz.

电压通道VOUT1的差分输出施加到ADuCM360的AIN2和AIN3输入端。电流通道VOUT2的差分输出施加到ADuCM360的AIN0和AIN1输入端。

等式7显示电导率测量取决于G1、G2和R47，以及VOUT2和VOUT1的比值。因此，ADuCM360内置的ADC无需使用精密基准电压源。

AD8253增益误差(G1和G2)最大值为0.04%，并且R47选择0.1%容差的电阻。

从该点开始，VOUT1和VOUT2信号链的电阻便决定了总系统精度

软件将每个AD8253的增益按如下所述进行设置：

• 如果ADC代码超过满量程的93.2%，则AD8253增益在下一个样本减少10倍。

• 如果ADC代码低于满量程的9.13%，则AD8253增益在下一个样本增加10倍。

系统精度测量

下列4个电阻影响VOUT1电压通道的精度：R19、R20、R29和R31

下列5个电阻影响VOUT2电流通道的精度：R47、R37、R38、R48和R52。

假设所有9个电阻均为0.1%容差并包括AD8253的0.04%增益误差，则最差情况下的误差分析表明误差约为0.6%。分析内容在 CN-0359设计支持包中。

在实际应用中，电阻更有可能采取RSS方式进行组合，且正或负信号链上的电阻容差导致的RSS误差为√5 × 0.1% = 0.22%。

使用1 Ω至1 MΩ(1 S至1 ΩS)精密电阻进行精度测量，以仿真电导池。图4显示了结果，最大误差不到0.1%。

RTD测量

电导率测量系统精度只有经过温度补偿才能达到最佳。由于常见溶液温度系数在1%/°C至3%/°C或更高值之间变化，因此必须使用带有可调温度补偿的测量仪器。溶液温度系数在某种程度上是非线性的，通常还随着实际电导率变化。因此，在实际测量温度下进行校准可以达到最佳精度。

ADuCM360内置两个匹配的软件可配置激励电流源。它们可单独配置，提供10 A至1 mA电流输出，匹配优于0.5%。电流源允许ADuCM360针对Pt100或Pt1000 RTD轻松执行双线式、三线式或四线式测量。在设置过程中，软件还能自动检测RTD是否为Pt100或Pt1000。

下文给出了不同RTD配置如何工作的简化原理图。所有模式切换均通过软件实现，无需改变外部跳线设置。

图5显示了四线式RTD配置。

每个连接远程RTD的引脚寄生电阻以RP表示。激励电流(IEXC)流过1.5 kΩ精密电阻和RTD。片上ADC测量电阻(V7−V8)两端的电压。

选择R13电阻和IEXC激励电流值，使得AIN7上的ADuCM360最大输入电压不超过AVDD − 1.1 V很重要；否则，IEXC电流源会工作异常。RTD电压可以使用两个连接AIN6和AIN5的检测引脚进行精确测量。输入阻抗约为2 M(无缓冲模式，PGA增益= 1)，并且流过检测引脚电阻的电流引起的误差极小。然后，ADC测量RTD电压(V6 − V5)

随后便可如下所示计算RTD电阻：

测量值是一个比例值，且与精确的外部基准电压无关，而仅与1.5 kΩ电阻容差有关。此外，四线式配置可消除引脚电阻相关的误差。

ADuCM360提供带缓冲与不带缓冲的输入选项。如果激活内部缓冲器，则输入电压必须大于100 mV。1 k/36 电阻分压器能为RTD提供115 mV偏置电压，允许以缓冲方式工作。在无缓冲模式下，J3端点4可以接地，并连接接地屏蔽，以减少噪声。

三线式连接是另一种使用广泛的RTD配置，可消除引脚电阻误差，如图6所示。

第二个匹配的IEXC电流源(AIN5/IEXC)在引脚电阻上形成一个电压，并与端点3串联，消除与端点1串联的引脚电阻上的压降。因此，测得的V8 − V5电压不存在引脚电阻误差。

图7显示了双线式RTD配置，无引脚电阻补偿。

双线式配置是成本最低的电路，适用于非关键型应用、短路RTD连接以及较高电阻RTD(比如Pt1000)等。

电导率理论

材料或液体的电阻率ρ定义为：当立方体形状的材料反面完全导电接触时，该材料的电阻。其他形状材料的电阻R可按以下方式计算：

其中
L是接触距离
A是接触面积。

电阻率的测量单位为Ωcm。当接触1 cm × 1 cm × 1 cm立方体的反面时，1 Ωcm材料的电阻为1 Ω。

电导是电阻的倒数，电导率是电阻率的倒数。电导的测量单位为西门子(S)，电导率的测量单位为S/cm、mS/cm或μS/cm。

所有水溶液都在一定程度上导电。向纯水中添加电解质，例如盐、酸或碱，可以提高电导率并降低电阻率。

在此电路笔记中，Y为电导率的通用符号，测量单位为S/cm、mS/cm或μS/cm。但在很多情况下，为了方便起见，我们会省略距离项，电导率仅表示为S、mS或μS。

电导率系统通过电子元件连接到沉浸在溶液中的传感器(称为电导池)来测量电导率，如图8所示。

电子电路对传感器施加交流电压，并测量产生的电流大小，电流与电导率相关。由于电导率具有很大温度系数(最高达到4%/°C)，因此电路中集成了必需的温度传感器，用于将读数调整为标准温度，通常为25°C (77°F)。对溶液进行测量时，必须考虑水本身的电导率的温度系数。为了精确地补偿温度，必须使用第二个温度传感器和补偿网络。

接触型传感器通常包括相互绝缘的两个电极。电极通常为316型不锈钢、钛钯合金或石墨，具有特定的大小和间距，以提供已知的电导池常数。从理论上说，1.0/cm的电导池常数表示两个电极，每个电极面积为1 cm2，间距为1 cm。对于特定的工作范围，电导池常数必须与测量系统相匹配。例如，如果在电导率为1 μS/cm的纯水中使用电导池常数为1.0/cm的传感器，则电导池的电阻为1 MΩ。相反，相同电导池在海水中的电阻为30 Ω，由于电阻比过大，普通仪器很难在仅有一个电导池常数情况下精确测量此类极端情况。

对1 μS/cm溶液进行测量时，电导池配置了大面积电极，相距很小的间距。例如，对于电导池常数为0.01/cm的电导池，其电导池电阻测量值约为10,000 Ω，而非1 MΩ。精确测量10,000 Ω(而非1 MΩ)比较容易；因此，对于超纯水和高电导率海水，使用具有不同电导池常数的电导池，测量仪表可在相同的电导池电阻范围内工作。

电导池常数K定义为电极之间距离L与电极面积A的比值

有两类电导池：一类采用两个电极，另一类采用四个电极，如图9所示。电极通常称为极点。

双极点传感器比较适合低电导率测量时使用，比如纯净水和各种生物与医药液体。四极点传感器更适合高传导率测量，比如废水和海水分析。

双极点电导池的电导池常数范围大致是从0.1/cm到1/cm，而四极点电导池的电导池常数范围是从1/cm到10/cm。

四极点电导池可以消除电极极化和场效应引起的误差；这些误差可能会干扰测量

电极的实际配置可以是平行环、同轴导体等，而不会是如图8所示的简单平行板。

无论电导池为何种类型，都不可在电极上施加直流电压，因为液体中的离子会在电极表面聚集，从而导致极化效应并产生测量误差，更有可能损坏电极。

若采用同轴传感器，则应当密切关注传感器屏蔽。屏蔽必须连接与盛放液体的金属容器相同的电位。如果容器接地，则屏蔽必须连接电路板的接地端(J5的引脚5)。

The CN-0359 circuit allows programmable excitation voltages ranging from 100 mV to 10 V, and the R23 (1 kΩ) series resistor limits the maximum conductivity cell current to 10 mA. The most fundamental precaution is not to exceed the conductivity cell excitation voltage or excitation current rating.

power circuit

To simplify system requirements, all required voltages (±15 V and +3.3 V) are generated from a single 4 V to 7 V supply, as shown in Figure 10.

The ADP2300 buck regulator generates the 3.3 V supply voltage required by the board. The design is based on the downloadable ADP230x Buck Regulator Design Tool .

The ADP1613 boost regulator generates a +15 V regulated supply voltage and a −15 V unregulated supply voltage. The −15 V supply voltage is generated using a charge pump. The design is based on the ADP161x boost regulator design tool .

See www.analog.com/ADIsimPower for details on power supply selection and design .

Use proper layout and grounding techniques to avoid switching regulator noise coupling into analog circuitry. For more details, please refer to the Linear Circuit Design Handbook , Data Conversion Handbook , Tutorial MT-031 , and Tutorial MT-101 .

Figure 11 shows the LCD backlight driver circuit.

Each half of the AD8592 op amp is used as a 60 mA current source to supply the LCD backlight current. The AD8592 sources and sinks 250 mA maximum current and has built-in 100 nF capacitors to ensure soft start.

Software operation and user interface

EVAL-CN0359-EB1Z comes preloaded with the code required to make conductivity measurements. The code is in the CN0359-SourceCode.zip file of the CN-0359 Design Support Package .

CN-0359 has an intuitive and easy-to-use user interface. All user input is from a dual function push button/rotary encoder knob. The encoder knob can be rotated clockwise or counterclockwise (without mechanical stop) and can also be used as a push button.

Figure 12 is a photo of the EVAL-CN0359-EB1Z board showing the LCD display and encoder knob locations.

After wiring, the conductivity cell and RTD on the board are powered on. The LCD screen is shown in Figure 12.

Press the encoder knob to enter the setting menu, and then enter EXC Voltage (EXC voltage), EXC Frequency (EXC frequency), TEMP Coecient (temperature coefficient) and Cell Constant (conductivity cell constant), as shown in Figure 13.

Rotate the knob to move the cursor up and down to select various parameters.

Position the cursor on EXC Voltage and press the knob until it clicks to open the menu. Rotate the knob and position the cursor to the first digit of the number to be set. Press the button and the cursor flashes. Turn the knob to change the number and press the knob when the desired number appears. When you have finished setting all the digits, position the cursor on Save and press the button to save the settings.

Continue these steps to set the EXC Frequency , TEMP Coeffcient , and Cell Constant .

After setting all constants, select RETURN TO HOME and press the knob. At this point, the system is ready for measurement.

If a number outside the allowed range is entered, the buzzer will sound.

If the conductivity cell is not connected correctly, the screen displays Sensor Incorrect .

If the RTD is not connected correctly, the screen displays RTD Incorrect use 25°C (RTD is incorrect, use 25°C) ; at this time, the system can still perform measurements without connecting the RTD, but uses 25°C as the compensation temperature.