2018年10月24日 (三) 10:38的最后版本

[编辑] 1 概述

工业和医学应用中，如果测量的温度范围很宽，比如 -200°C ~ +800°C ，且对精度、可重复性要求比较高，最好的选择是铂电阻温度 (RT) 检测器，即 PRTD。

PRTD 中的铂元素非常稳定，且不容易腐蚀或氧化。PRTD 的电阻与温度呈线性关系，温度越高电阻越大。

常见的 PRTD 包括 PT100、PT500 和 PT1000。在 0°C 下分别呈现 100Ω、500Ω 和 1000Ω 的阻值，其阻值会随温度变化 (PT1000 Table)。也有成本稍高的大阻值传感器，例如 PT10000

[编辑] 2 PT1000 Table

PT1000 Table

[编辑] 3 测量原理

PRTD 传统测量方法是采用电流源激励：

对于较高阻值的 PRTD，应采用高精度电压源激励 PRTD，可以利用 ADC 的电压基准为 PRTD 提供偏压。PRTD 可直接连接到 ADC，ADC 基准通过一个高精度电阻提供 PRTD 偏置电流。ADC即可以高精度比例测量温度：

[编辑] 4 ADC

[编辑] 4.1 ESP8266 ADC

硬件连接：

 VCC33 ---> PT1000 <-- (A0) --> 300R 1% ---> GND

void setup()
{
  Serial.begin(115200);
}

void loop() {
  int vd = analogRead(A0);
  Serial.print("vd = ");
  Serial.println(vd);

  uint32_t rtd = (1024 - vd) * 300 / vd;
  Serial.print("rtd = ");
  Serial.println(rtd);

  delay(1000);
}

[编辑] 4.2 MCP3421

快速测试：

#include <Wire.h>
#define MCP3421_ADDRESS 0X68

void setup()
{
  Serial.begin(115200);  
  Wire.begin();
}

void loop()
{
  Wire.requestFrom(MCP3421_ADDRESS, 4);
  if (Wire.available() != 4) {
    Serial.println("Wire.available failed");
    while(1);
  }
  int16_t v = (Wire.read() << 8);
  v |= Wire.read();
  // read but ignore status
  uint8_t s = Wire.read();
  //print voltage from channel one in millivolts
  Serial.print(v);Serial.println(" mv");
  delay(1500);
}

改进：

#include <Wire.h>

// I2C address for MCP3422 - base address for MCP3424 = 0x68
#define MCP342X_ADDRESS 0X68

// fields in configuration register
#define MCP342X_GAIN_FIELD 0X03 // PGA field
#define MCP342X_GAIN_X1    0X00 // PGA gain X1, 12 bits, 1 mV
#define MCP342X_GAIN_X2    0X01 // PGA gain X2, 14 bits, 250 uV
#define MCP342X_GAIN_X4    0X02 // PGA gain X4, 16 bits, 62.5 uV
#define MCP342X_GAIN_X8    0X03 // PGA gain X8, 18 bits, 15.625 uV

#define MCP342X_RES_FIELD  0X0C // resolution/rate field
#define MCP342X_RES_SHIFT  0X02 // shift to low bits
#define MCP342X_12_BIT     0X00 // 12-bit 240 SPS
#define MCP342X_14_BIT     0X04 // 14-bit 60 SPS
#define MCP342X_16_BIT     0X08 // 16-bit 15 SPS
#define MCP342X_18_BIT     0X0C // 18-bit 3.75 SPS

#define MCP342X_CONTINUOUS 0X10 // 1 = continuous, 0 = one-shot

#define MCP342X_CHAN_FIELD 0X60 // channel field
#define MCP342X_CHANNEL_1  0X00 // select MUX channel 1
#define MCP342X_CHANNEL_2  0X20 // select MUX channel 2
#define MCP342X_CHANNEL_3  0X40 // select MUX channel 3
#define MCP342X_CHANNEL_4  0X60 // select MUX channel 4

#define MCP342X_START      0X80 // write: start a conversion
#define MCP342X_BUSY       0X80 // read: output not ready

uint8_t chan = 0XFF, gain = 0XFF, res = 0XFF;

//------------------------------------------------------------------------
// default adc configuration register - resolution and gain added in setup()
uint8_t adcConfig = MCP342X_START | MCP342X_CHANNEL_1 | MCP342X_CONTINUOUS;

// divisor to convert ADC reading to milivolts
uint32_t mvDivisor;

void halt(void)
{
   Serial.println("Halted - check address and wiring");
   while(1);
}

// read mcp342x data - updated 10mar11/wbp
uint8_t mcp342xRead(int32_t &data)
{
   // pointer used to form int32 data
   uint8_t *p = (uint8_t *)&data;
   // timeout - not really needed?
   uint32_t start = millis();
   if ((adcConfig & MCP342X_RES_FIELD) == MCP342X_18_BIT)  // in 18 bit mode?
   {
      do {   // 18-bit mode
         Wire.requestFrom(MCP342X_ADDRESS, 4);
         if (Wire.available() != 4) {
            Serial.println("read failed");
            return false;
         }
         for (int8_t i = 2; i >= 0; i--) {
            p[i] = Wire.read();
         }
         // extend sign bits
         p[3] = p[2] & 0X80 ? 0XFF : 0;
         // read config/status byte
         uint8_t s = Wire.read();
         if ((s & MCP342X_BUSY) == 0) return true;  // escape here
      } while (millis() - start < 500);   // allows rollover of millis()
   } else {
      do {  // 12-bit to 16-bit mode
         Wire.requestFrom(MCP342X_ADDRESS, 3);
         if (Wire.available() != 3) {
            Serial.println("read failed");
            return false;
         }
         p[1] = Wire.read();
         p[0] = Wire.read();
         // extend sign bits
         p[2] = p[1] & 0X80 ? 0XFF : 0;
         p[3] = p[2];
         // read config/status byte
         uint8_t s = Wire.read();
         if ((s & MCP342X_BUSY) == 0) return true;  // or escape here
      } while (millis() - start < 500);   // allows rollover of millis()
   }
   Serial.println("read timeout");      // dang it
   return false;
}

// write mcp342x configuration byte
uint8_t mcp342xWrite(uint8_t config)
{
   Wire.beginTransmission(MCP342X_ADDRESS);
   Wire.write(config);
   Wire.endTransmission();
}

void setup()
{
   Serial.begin(115200); 
   pinMode(D4, OUTPUT);
   digitalWrite(D4, LOW);
   Wire.begin();

   do {
      Serial.println();
      Serial.flush();
      Serial.print("Enter gain (1, 2, 4, or 8): ");
      while(Serial.available() < 1);
      switch (Serial.read()) {
      case '1': // ACSCII number received
         gain = MCP342X_GAIN_X1; 
         break;
      case '2': 
         gain = MCP342X_GAIN_X2; 
         break;
      case '4': 
         gain = MCP342X_GAIN_X4; 
         break;
      case '8': 
         gain = MCP342X_GAIN_X8; 
         break;
      } 
   } 
   while (gain > 3);
   Serial.println(1 << gain, DEC);

   do {
      Serial.flush();
      Serial.println();
      Serial.print("Enter resolution (12, 14, 16, or 18): ");
      while (Serial.available() < 2);
      if (Serial.read() != '1') continue;
      switch (Serial.read()) {
      case '2': 
         res = 0; 
         break;
      case '4': 
         res = 1; 
         break;
      case '6': 
         res = 2; 
         break;
      case '8': 
         res = 3; 
         break;
      }
   } 
   while (res > 3);
   Serial.println(12 + 2*res, DEC);

   adcConfig |= res << 2 | gain | MCP342X_START;
   
   // divisor to convert ADC reading to millivolts
   mvDivisor = 1 << (11 + 2*res);
   Serial.print("md = ");
   Serial.println(mvDivisor);
}

void loop()
{         
   int32_t data;

   mcp342xWrite(adcConfig);

   if (!mcp342xRead(data)) halt();

   Serial.print("data = ");
   Serial.println(data);
      
   // voltage in millivolts
   double mv = (double)data * 2048 / mvDivisor;
   // uncomment line below to convert reading to microvolts
   uint32_t uv = mv * 1000;
   
   Serial.print("V = ");
   Serial.print(mv);
   Serial.print(" mV");
   Serial.print(" , ");
   Serial.print(uv);
   Serial.println(" uV");
   
   delay(2000);                          // 5 seconds + conversion time x 4
}

[编辑] 5 专用芯片

选型: Temperature Sensor Tutorial

• 高精度温度测量带动铂电阻温度检测器(PRTD)和Σ-Δ ADC应用
• RTD Measurement System Design Essentials
• PT100温度传感器的正温度系数补偿

[编辑] 5.1 MAX31865

• SPI Interface
• Handles 100 Ω to 1000 Ω (at 0°C) Platinum RTDs (PT100 to PT1000)
• Compatible with 2-, 3-, and 4-Wire Sensor Connections
• 15-Bit ADC Resolution; Nominal Temperature Resolution 0.03125NC (Varies Due to RTD Nonlinearity)
• Total Accuracy Over All Operating Conditions: 0.5NC (0.05% of Full Scale) max
• Fully Differential VREF Inputs
• 21ms (max) Conversion Time
• 20-Pin TQFN and SSOP Packages

• MAX31865 Datasheet

[编辑] 5.2 MAX6603

• https://datasheets.maximintegrated.com/en/ds/MAX6603.pdf

[编辑] 6 电压基准

对于传感器测量应用，基准输出用作ADC或AFE输入，也用作变送器激励。如果噪声或其它原因造成基准电压波动，传感器输入和ADC都将发生相同的波动，从而降低总误差

压基基准噪声源降低 ADC 的噪声性能，所以应选择性能优于 ADC 的基准。对于高精度系统，基准的初始误差以及温漂和时漂是增益误差的最重要因素之一。对于经过校准的系统，温漂和时漂是最关键的参数

选择基准时的关键参数包括：负载驱动、初始精度、噪声、温漂和稳定度

[编辑] 6.1 TL431

• http://www.ti.com/lit/ds/symlink/tl431.pdf

[编辑] 6.2 MAX

• https://www.maximintegrated.com/cn/app-notes/index.mvp/id/2879

Maxim电压基准的精简列表
Part Number	Output Voltage (V)	Supply Voltage Range (V)	Temp. Drift (ppm/°C max)	Initial Accuracy TA = +25°C (% F.S. max)	Quiescent Current (mA max)	0.1Hz to 10Hz Noise (µVP-P), max (typ)	Package Options	Temp. Ranges*
MAX6160	Adj.(1.23 to 12.4)	2.7 to 12.6	100	1	100µA	(15)	SOT143, SO	E
MAX6120	1.2	2.4 to 11	100	1	70µA	(10)	SOT23, SO	E
MAX6520	1.2	2.4 to 12.6	50	1	70µA	(10)	SOT23, SO	E
MAX6001	1.25	2.5 to 12.6	100	1	45µA	25	SOT23	E
MAX6012	1.25	2.5 to 12.6	20 to 30	0.3 to 0.5	35µA	25	SOT23	E
MAX6190	1.25	2.5 to 12.6	5 to 25	0.16 to 0.48	35µA	25	SO	E
MAX6021	2.048	2.5 to 12.6	20 to 30	0.2 to 0.4	35µA	40	SOT23	E
MAX6191	2.048	2.5 to 12.6	5 to 25	0.1 to 0.5	35µA	40	SO	E
MAX873	2.5	4.5 to 18	7 to 20	0.06 to 0.1	28µA	(16)	DIP, SO	C, E
MAX6002	2.5	2.7 to 12.6	100	1	45µA	60	SOT23	E
MAX6025	2.5	2.7 to 12.6	20 to 30	0.2 to 0.4	35µA	60	SOT23	E
MAX6125	2.5	2.7 to 12.6	50	1	100µA	(15)	SOT23, SO	E
MAX6192	2.5	2.7 to 12.6	5 to 25	0.1 to 0.4	35µA	60	SO	E
MAX6225	2.5	8 to 36	2 to 5	0.04 to 0.1	2.7	(1.5)	DIP, SO	C, E
MAX6325	2.5	8 to 36	1 to 2.5	0.04	2.7	(1.5)	DIP, SO	C, E
MAX6003	3	3.2 to 12.6	100	1	45µA	75	SOT23	E
MAX6030	3	3.2 to 12.6	20 to 30	0.2 to 0.4	35µA	75	SOT23	E
MAX6193	3	3.2 to 12.6	5 to 25	0.07 to 0.33	35µA	75	SO	E
MAX6004	4.096	4.3 to 12.6	100	1	45µA	100	SOT23	E
MAX6041	4.096	4.3 to 12.6	20 to 30	0.2 to 0.4	35µA	100	SOT23	E
MAX6141	4.096	4.3 to 12.6	50	1	105µA	(25)	SOT23, SO	E
MAX6198	4.096	4.3 to 12.6	5 to 25	0.05 to 0.24	35µA	100	SO	E
MAX6241	4.096	8 to 36	2 to 5	0.025 to 0.1	2.9	(2.4)	DIP, SO	C, E
MAX6341	4.096	8 to 36	1 to 2.5	0.025	2.9	(1.5)	DIP, SO	C, E
MAX6045	4.5	4.7 to 12.6	20 to 30	0.2 to 0.4	35µA	110	SOT23	E
MAX6145	4.5	4.7 to 12.6	50	1	105µA	(30)	SOT23, SO	E
MAX6194	4.5	4.7 to 12.6	5 to 25	0.04 to 0.22	35µA	110	SO	E
MAX675	5	8 to 33	12 to 20	0.15	1.4	15	TO-99, DIP, SO	C, E
MAX875	5	7 to 18	7 to 20	0.06 to 0.1	0.28	(32)	DIP, SO	C, E
MAX6005	5	5.2 to 12.6	100	1	45µA	120	SOT23	E
MAX6050	5	5.2 to 12.6	20 to 30	0.2 to 0.4	35µA	120	SOT23	E
MAX6150	5	5.2 to 12.6	50	1	110µA	(35)	SOT23, SO	E
MAX6195	5	5.2 to 12.6	5 to 25	0.04 to 0.2	35µA	120	SO	E
MAX6250	5	8 to 36	2 to 5	0.02 to 0.1	3	(3)	DIP, SO	C, E
MAX6350	5	8 to 36	1 to 2.5	0.02	3	(1.5)	DIP, SO	C, E
REF02	5	8 to 33	8.5 to 250	0.3 to 2	1.4	15	TO-99, DIP, SO	C
*温度范围：C = 0°C至+70°C，E = -40°C至85°C

文献
The Art of Electronics, by Paul Horowitz & Winfield Hill, Chapter 6
Micro-Electronic Circuits, by Adel S. Sedra & Kenneth C. Smith

[编辑] 7 资源

• http://martin.engineer/wp/?p=453
• PT1000 table: http://www.hayashidenko.co.jp/en/info14.html

• http://openenergymonitor.org/emon/buildingblocks/rtd-temperature-sensing
• http://forum.arduino.cc/index.php?topic=16731.0
• http://stackoverflow.com/questions/21644642/how-to-connect-pt-1000
• http://41j.com/blog/2015/01/esp8266-analogue-input/