SSP1220 Dreileiter-RTD-Messung

Ⅰ、 Dreileiter-PT100-Temperaturmessprinzip

Der Hauptzweck der Drei-Draht-Verbindungsmethode besteht darin, den Einfluss des Leitungswiderstands auf die Messgenauigkeit zu eliminieren. Der Widerstandswert des PT100 ist sehr klein (100Ω @ 0℃), und der Widerstand der Anschlussdrähte (einige Zehntel Ohm bis einige Ohm, geschrieben als R6, R7, R15, siehe Dreidraht-RTD-Prüfschema) kann nicht zu vernachlässigende Fehler verursachen. Das Dreileitersystem löst dieses Problem durch ein cleveres Schaltungsdesign, bei dem alle drei Leitungen eines Dreileiter-RTDs in der Regel gleich lang sind. Unter der Annahme, dass der Widerstand der drei Leitungen gleich ist (RL1 = RL2 = RL3, d.h. R6=R7=R15 im Schaltplan), wird die interne Doppelstromquelle (IDAC) des SSP1220 verwendet, um die Auswirkungen dieser Leitungswiderstände auszugleichen.

Ausführliche Erläuterung des Messprinzips:

1. Bei Verwendung zweier passender programmierbarer Stromquellen (IDAC1 und IDAC2) innerhalb des SSP1220, die denselben Strom ausgeben, wird empfohlen, die Stromquelle mit weniger als 1 mA auszuwählen; der Auswahlwert für diesen Test beträgt 500uA.
2. Es wird angenommen, dass die drei Leitungswiderstände des PT100 gleich sind: R6 = R7 = R15 = Rl
3. Der SSP1220 misst die Spannung am PT100 über ein differentielles Eingangspaar (AIN0, AIN1): V_IN = V_AIN1 - V_AIN0

in Erwägung nachstehender Gründe V_AIN1 = I_IDAC1 x (R_L1+R_PT100) + (I_IDAC1 + I_IDAC2) x R_L3, V_AIN0 = I_IDAC2 x R_L2 + (I_IDAC1 + I_IDAC2) x R_L3

Seit: I_IDAC1 = I_IDAC2 = I_IDAC UND R_L1 = R_L2 = R_L3 = R_L

zu ersetzen: VIN = [I_IDAC x (R_L + R_PT100) + 2I_IDAC x R_L] - [IIDAC x RL + 2IIDAC x RL] = I_IDAC x R_L + I_IDAC x R_PT100 + 2I_IDAC x R_L - 2I_IDAC x R_L = I_IDAC x R_PT100

Durch eine geschickte Schaltungskonfiguration wird der Einfluss des Leitungswiderstandes vollständig aus der differentiellen Eingangsspannung VIN eliminiert, und nur der Spannungsabfall über dem PT100-Widerstand wird berücksichtigt.

4. Die SSP1220-Referenzspannung Vref wird durch die Zusammenführung von zwei IDAC-Strömen über einen hochpräzisen externen Referenzwiderstand, Rref (R5), erzeugt, d.h. Vref = (Iidac1 + Iidac2) * R
5. Bei Verhältnismessungen ist der endgültige ADC-Ausgangscode proportional zu (Rpt100) / (Rref), unabhängig vom Absolutwert, der Genauigkeit und der Drift des IDAC-Stroms, wobei auch die Wirkung der Leitungswiderstände Rl1 und Rl2 ausgeglichen wird:

Für den 24-Bit SSP1220 lautet der numerische Ausgangscode:

Code = (2²³ - 1) x (V_IN/V_REF) = (2²³ - 1) x [R_PT100/(2 x R_REF) ]

Invertierung des PT100-Widerstandswertes durch ADC-Code:

R_PT100 = [Code/(2²³-1)] x 2 x R_REF

Schließlich wird Rpt100 entsprechend der Widerstands-Temperatur-Charakteristik von PT100 (in der Regel unter Verwendung der Callendar-Van Dusen-Gleichung oder der Tabellen-Nachschlagemethode) in einen Temperaturwert umgerechnet: T = f (Rpt100). Für den PT100 beträgt der Widerstandstemperaturkoeffizient bei 0℃, R0 = 100,00Ω ungefähr α≈ 0,00385 Ω/Ω/℃.

Ⅱ、Hardware-Schaltungsentwurf

Gemäß der typischen Anwendung im Datenblatt sieht eine typische Dreileiter-PT100-Anschlussschaltung wie folgt aus:

1. Anweisungen zum Anschluss des Stromkreises

• PT100-Anschluss: Der PT100 (Dreileitersystem) wird wie im Schaltplan dargestellt angeschlossen.
• Erzeugung einer Spannungsreferenz: Der IDAC1-Ausgang ist mit AIN2 verbunden (interne Softwarekonfiguration erforderlich), der IDAC2-Ausgang ist mit AIN3 verbunden (interne Softwarekonfiguration erforderlich), und die beiden IDAC-Ströme laufen am Knoten zusammen und fließen gemeinsam durch den externen Referenzwiderstand Rref(R5). Das andere Ende von REF ist mit der analogen Masse AVSS verbunden. Der positive Referenzeingang des SSP1220, REFP0, ist mit dem oberen Ende von RREF (R5) (dem IDAC-Zusammenführungspunkt) verbunden. Der negative Referenzeingang des SSP1220, REFN0, ist mit AVSS verbunden. Daher ist die Referenzspannung, VREF= (IIDAC1 + IIDAC2) * RREF.
• Signalmessung: AIN1 für SSP1220 ist als positiver Differenzeingang AINP konfiguriert und AIN0 des SSP1220 ist als negativer Differenzeingang AINN konfiguriert, so dass die gemessene Spannung die Potentialdifferenz zwischen AIN1 und AIN0 ist.
• Filterung der Schaltung: RC-Tiefpassfilter müssen sowohl an den Analogeingängen (AIN0, AIN1, AIN2) als auch an den Referenzeingängen (REFP0) zur Antialiasing- und Rauschunterdrückung hinzugefügt werden. Eingangsfilter: bestehend aus R1, R2, C1 und C6, C5. Referenzfilter: Besteht aus R3, R4, C2 und C3, C4. Um die Genauigkeit der Skalenmessungen zu erhalten, sollte die Grenzfrequenz des Referenzfilters mit der des Eingangsfilters übereinstimmen.

Ⅲ、Geräteauswahl und Parameterberechnung

The hypothetical design objectives are as follows: PT100 type: three-wire; Temperature measurement range: -200°C ~ +850°C; Supply voltage AVDD: 3.3V (AVSS = 0V); DAC Current: 500μA (per channel); Data rate: 20 SPS (for optimal noise performance).

1. Reference resistance (Rref) selection and calculation

Rref is at the heart of the accuracy of the entire system. Function: Generate the reference voltage V ref of the ADC, and its accuracy and stability directly determine the measurement results.

Resistance Calculation:

To maximize the range of the ADC and meet the common mode voltage requirements of the PGA, the Vref is typically set at about half the supply voltage. In this design, AVDD = 3.3V and the target VREF is about 1.65V.

I_IDAC = I_IDAC1 + I_IDAC2 = 500uA + 500uA = 1mA

R_REF = V_REF /(I_IDAC1 + I_IDAC2) = 1.65V/1mA = 1.65kΩ

A resistor with a nominal value of 1.65 kΩ can be selected. If not found, 1.62kΩ or 1.69kΩ is also an acceptable approximation.

Selection requirements:

Accuracy: At least ±0.1%, recommended ±0.05% or higher for high-precision applications.

Temperature Bleating: Must be very low, with precision film resistance of ±5 ppm/°C or ±10 ppm/°C recommended.

Long-term stability: high.

Never use a normal 1%, 100ppm/°C chip resistor.

2. IDAC current and PGA gain options

IDAC Current: 500μA selected. This value strikes a good balance between power consumption, self-heating effect, and signal amplitude. If the current is too small, the signal is weak and easily affected by noise; Too much current may cause the PT100 to self-heat or exceed IDAC compliant voltages.

PGA Gain Selection: The PT100 has a smaller voltage (e.g. 500μA × 100Ω = 50mV), but uses a ratio measurement (the reference voltage is also from IDAC), so there is no need to amplify to avoid saturation, and the gain selection is 1X.

3. Filter circuit component selection

Filter Resistors (R1, R2, R3, R4): 1kΩ is usually selected. This value is large enough to effectively filter and small enough to avoid significant offset voltages at the input (due to input bias current). They also act as current-limiting protection.

Differential filter capacitors (C1, C2): Set the cut-off frequency together with the resistor. For example, for a data rate of 20SPS, the cut-off frequency can be set in the tens of Hz. fc = 1 / (2π * (R1+R2) * C1)。 If R1+R2=2kΩ and expects fc ≈ 16Hz, C1 ≈ 1 / (2* 2000 * 16) ≈ 4.7μF. In real-world applications, 100nF (0.1μF) is often used to obtain a wider noise rejection bandwidth. Type: C0G (NPO) ceramic capacitors are recommended for their stable dielectric constant, low voltage coefficient, and low microacoustic effect.

Common-mode filtered capacitors (C5, C6, C3, C4): Typically chosen an order of magnitude smaller than differential capacitors, such as 10nF, to ensure that mismatches of differential capacitors do not result in excessive common-mode noise being converted into differential noise.

Ⅳ、Software configuration

1. Master Logic:

float SSP1x20_read_temperature(void)

{

uint32_t ADC_data;

uint32_t ADC_temp1;

//SSP1x20_read_register(SSP1x20_REG0, 4, &Read_REGTab[0]);

Write_REGTab[0] = SSP1x20_MUX_AIN0_AIN1 | SSP1x20_GAIN_1 | SSP1x20_PGA_BYPASS_ON;

Write_REGTab[1]=SSP1x20_DR_20SPS|SSP1x20_MODE_NORMAL|SSP1x20_SC|SSP1x20_TS_ON| SSP1x20_BCS_OFF;

Write_REGTab[2]=SSP1x20_VREF_2048|SSP1x20_REJECT_OFF|SSP1x20_PSW_OFF | SSP1x20_IDAC_1000uA;

Write_REGTab[3] = SSP1x20_IDAC1_AIN2 | SSP1x20_IDAC2_AIN3 | SSP1x20_DRDYM_DRDY;

SSP1x20_WriteRegister(SSP1x20_REG0, 4, &Write_REGTab[0]);

SSP1x20_SendCommand(SSP1x20_CMD_START);

SPI_ADC_CS_LOW();

while (ADC_DRDY_GAIN == 1);//SSP1x20_DRDYM_DRDY

2. The main configuration and description of the program

• Configure register 0: MUX and gain

Write_REGTab[0] = SSP1x20_MUX_AIN0_AIN1 | SSP1x20_GAIN_1 | SSP1x20_PGA_BYPASS_ON;

• Konfigurationsregister 1: Abtastrate und Modus

Write_REGTab[1] = SSP1x20_DR_20SPS | SSP1x20_MODE_NORMAL | SSP1x20_SC | SSP1x20_TS_OFF | SSP1x20_BCS_OFF;

• Register 2 konfigurieren: Referenzspannung mit IDAC

Write_REGTab[2] = SSP1x20_VREF_2048 | SSP1x20_REJECT_OFF | SSP1x20_PSW_OFF | SSP1x20_IDAC_500uA;

(4) Register 3 konfigurieren: IDAC-Routenkanal mit DRDY

3. Dreidraht-PT100-Kern:

Aktueller Pfad:

• IDAC1 → AIN2 → PT100 → AIN1
• IDAC2 → AIN3 → AIN1(Rückgabe)
• Two currents are equal → offset the voltage drop on the R_LEAD2R_LEAD2

uint32_t raw_u24 = SSP1x20_read_data_drdy();

SSP1220 outputs 24-bit data, but MCUs typically read in 32-bit (SPI reads 4 bytes at a time).

if (raw < 0) raw = -raw;

The PT100 voltage is always positive (current flows from AIN0 to AIN1).

If raw < 0, the AIN0 and AIN1 software configurations are reversed.

printf(“Raw: %ld, R=%.3f Ω, Temp=%.2f °C\r\n”, raw, R_pt100, temperature);

Print the original code value, calculate the resistance, and the final temperature for easy debugging

If Raw is negative→ the configuration is reversed

If R > 1400Ω → indicates that the IDAC or Rref is set incorrectly

If Temp = -999 → indicates that the R-value is outside the reasonable range

Ⅴ、Measurement procedure and results

1. PT100 voltage measurement program at both ends:

void SSP1x20_ADC_MeasurePt100(void)

{

float V_ref = 2.048;              // Internal reference voltage 2.048V

//printf(“\r\n Multi-point single voltage measurement \r\n”);

Write_REGTab[0] = SSP1x20_MUX_AIN1_AIN0 | SSP1x20_GAIN_1 | SSP1x20_PGA_BYPASS_OFF;

Write_REGTab[1] = SSP1x20_DR_20SPS | SSP1x20_MODE_NORMAL | SSP1x20_SC | SSP1x20_TS_OFF | SSP1x20_BCS_OFF;

Write_REGTab[2] = SSP1x20_VREF_REF0 | SSP1x20_REJECT_OFF | SSP1x20_PSW_OFF | SSP1x20_IDAC_500uA;

Write_REGTab[3] = SSP1x20_IDAC1_AIN2 | SSP1x20_IDAC2_AIN3 | SSP1x20_DRDYM_DRDY;

SSP1x20_WriteRegister(SSP1x20_REG0, 4, &Write_REGTab[0]);

printf(“Write_REGTab[0]=%x\r\n”, Write_REGTab[0]);

printf(“Write_REGTab[1]=%x\r\n”, Write_REGTab[1]);

printf(“Write_REGTab[2]=%x\r\n”, Write_REGTab[2]);

printf(“Write_REGTab[3]=%x\r\n”, Write_REGTab[3]);

while (1)

{

SSP1x20_SendCommand(SSP1x20_CMD_START); // When continuous measurement is enabled, this command is sent only once

HAL_Delay(100);

SPI_ADC_CS_LOW();

}

SSP1220 test results

2. SSP1220 internal temperature measurement

Internal temperature test function

float SSP1x20_read_temperature(void)

{

uint32_t ADC_data;

uint32_t ADC_temp1;

//SSP1x20_read_register(SSP1x20_REG0, 4, &Read_REGTab[0]);

Write_REGTab[0] = SSP1x20_MUX_AIN0_AIN1 | SSP1x20_GAIN_1 | SSP1x20_PGA_BYPASS_ON;

Write_REGTab[1] = SSP1x20_DR_20SPS | SSP1x20_MODE_NORMAL | SSP1x20_SC | SSP1x20_TS_ON | SSP1x20_BCS_OFF;

Write_REGTab[2] = SSP1x20_VREF_2048 | SSP1x20_REJECT_OFF | SSP1x20_PSW_OFF | SSP1x20_IDAC_1000uA;

Write_REGTab[3] = SSP1x20_IDAC1_AIN2 | SSP1x20_IDAC2_AIN3 | SSP1x20_DRDYM_DRDY;

SSP1x20_WriteRegister(SSP1x20_REG0, 4, &Write_REGTab[0]);

SSP1x20_SendCommand(SSP1x20_CMD_START);

SPI_ADC_CS_LOW();

while (ADC_DRDY_GAIN == 1);//SSP1x20_DRDYM_DRDY

Internal temperature test configuration details:

 Write_REGTab[1] = SSP1x20_DR_20SPS | SSP1x20_MODE_NORMAL | SSP1x20_SC | SSP1x20_TS_ON | SSP1x20_BCS_OFF;

• SSP1x20_TS_ON: Enable the internal temperature sensor (critical), this configuration has the highest priority
• SSP1x20_SC: Perform self-calibration (recommended)
• 20SPS: Low speed and high accuracy, suitable for temperature measurement

Write_REGTab[2] = SSP1x20_VREF_2048 | SSP1x20_REJECT_OFF | SSP1x20_PSW_OFF | SSP1x20_IDAC_1000uA;

• SSP1x20_VREF_2048: Use an internal 2.048V reference voltage (not external REF0!) ）
  • Because the internal temperature sensor is an absolute voltage output, a fixed reference voltage must be used to convert the temperature.
• IDAC_1000uA: Although IDAC is enabled, IDAC in TS_ON mode does not affect internal temperature measurements (negligible).

Write_REGTab[3] = SSP1x20_IDAC1_AIN2 | SSP1x20_IDAC2_AIN3 | SSP1x20_DRDYM_DRDY;

• Configure the IDAC pin and DRDY, but have no effect on the internal temperature measurement (just keep the registers intact).
• 2 Start the conversion and wait for DRDY

SSP1x20_SendCommand(SSP1x20_CMD_START); SPI_ADC_CS_LOW();while (ADC_DRDY_GAIN == 1); // 等待 DRDY 变低

• Send the START command to start a continuous transition
• Wait for the DRDY pin to go low, indicating that the data is ready

The measurement of indoor room temperature is shown in the figure below:

3. External temperature measurement (method 1, simplified factor 0.385 calculation)

External temperature test related code:

uint32_t ADC_gain_value = 0; // Readout data

uint32_t ADC_value = 0;    // Measure the data value

float tmpPt100=0;

float RTD=0;

void SSP1x20_ADC_Measure(void)

{

printf(“\r\n Multi-point single voltage measurement \r\n”);

Write_REGTab[0] = SSP1x20_MUX_AIN1_AIN0 | SSP1x20_GAIN_1 | SSP1x20_PGA_BYPASS_OFF; SSP1x20_MUX_AIN1_AIN0 Interface AIN1 AIN0 should be selected based on the actual circuit diagram

Write_REGTab[1] = SSP1x20_DR_20SPS | SSP1x20_MODE_NORMAL | SSP1x20_SC | SSP1x20_TS_OFF | SSP1x20_BCS_OFF;

Write_REGTab[2] = SSP1x20_VREF_REF0 | SSP1x20_REJECT_OFF | SSP1x20_PSW_OFF | SSP1x20_IDAC_500uA;

Write_REGTab[3] = SSP1x20_IDAC1_AIN2 | SSP1x20_IDAC2_AIN3 | SSP1x20_DRDYM_DRDY;

SSP1x20_WriteRegister(SSP1x20_REG0, 4, &Write_REGTab[0]);

printf(“Write_REGTab[0]=%x\r\n”, Write_REGTab[0]);

printf(“Write_REGTab[1]=%x\r\n”, Write_REGTab[1]);

printf(“Write_REGTab[2]=%x\r\n”, Write_REGTab[2]);

printf(“Write_REGTab[3]=%x\r\n”, Write_REGTab[3]);

while (1)

{

SSP1x20_SendCommand(SSP1x20_CMD_START); When continuous measurement is enabled, this command is sent only once

HAL_Delay(100);

SPI_ADC_CS_LOW();

ADC_gain_value =0;

ADC_gain_value = SPI_ADC_ReadByte();

ADC_gain_value = (ADC_gain_value << 8) | SPI_ADC_ReadByte();

ADC_gain_value = (ADC_gain_value << 8) | SPI_ADC_ReadByte();

SPI_ADC_CS_HIGH();

RTD = 1650*( (float)ADC_gain_value /(0x3fffff));//Reference resistance 1650 ohms

tmpPt100 = (RTD-100)/0.38;

__NOP();

printf(“R=%.3f Ω, Temp=%.2f °C\r\n”,RTD, tmpPt100 );

}

The results of the three-line RTD measurement of the temperature of the ice water mixture are shown in the figure below:

External temperature measurements (method two, calculated by the Callendar-Van Dusen equation) are more accurate

Master Code:

//High accuracy RTD -> temperature

static float rtd_to_temperature_iec60751(float rtd)

{

if (rtd < 0.0f) return -999.0f; // illegal value

float t = (rtd – R0_PT100) / 0.385f; // initial guess

if (rtd <= R0_PT100) {

//T < = 0°C: Use the complete equation

for (int i = 0; i < 10; i++) {

float rt_calc = R0_PT100 * (1.0f + A_COEFF*t + B_COEFF*t*t + C_COEFF*(t – 100.0f)*t*t*t);

float dr_dt = R0_PT100 * (A_COEFF + 2.0f*B_COEFF*t + C_COEFF*(4.0f*t*t*t – 300.0f*t*t));

float error = rt_calc – rtd;

t -= error / dr_dt;

if (fabsf(error) < 0.001f) break;

}

} else {

// T >= 0°C:: Use the simplified equation

for (int i = 0; i < 10; i++) {

float rt_calc = R0_PT100 * (1.0f + A_COEFF*t + B_COEFF*t*t);

float dr_dt = R0_PT100 * (A_COEFF + 2.0f*B_COEFF*t);

float error = rt_calc – rtd;

t -= error / dr_dt;

if (fabsf(error) < 0.001f) break;

}

}

return t;

}

/**

* @brief   Analog channel ADC measurement (external temperature measurement)

* @param   None

* @retval   None

*/

uint32_t ADC_gain_value = 0; //     Readout data

uint32_t ADC_value = 0;                 // Measure the data value

float tmpPt100=0;

float RTD=0;

void SSP1x20_ADC_Measure(void)

{

printf(“\r\n  Multi-point single voltage measurement \r\n”);

Write_REGTab[0] = SSP1x20_MUX_AIN1_AIN0 | SSP1x20_GAIN_1 | SSP1x20_PGA_BYPASS_OFF;

Write_REGTab[1] = SSP1x20_DR_20SPS | SSP1x20_MODE_NORMAL | SSP1x20_SC | SSP1x20_TS_OFF | SSP1x20_BCS_OFF;

Write_REGTab[2] = SSP1x20_VREF_REF0 | SSP1x20_REJECT_OFF | SSP1x20_PSW_OFF | SSP1x20_IDAC_500uA;

Write_REGTab[3] = SSP1x20_IDAC1_AIN2 | SSP1x20_IDAC2_AIN3 | SSP1x20_DRDYM_DRDY;

//        Write_REGTab[3] = SSP1x20_IDAC1_AIN3 | SSP1x20_IDAC2_AIN2 | SSP1x20_DRDYM_DRDY;

SSP1x20_WriteRegister(SSP1x20_REG0, 4, &Write_REGTab[0]);

printf(“Write_REGTab[0]=%x\r\n”, Write_REGTab[0]);

printf(“Write_REGTab[1]=%x\r\n”, Write_REGTab[1]);

printf(“Write_REGTab[2]=%x\r\n”, Write_REGTab[2]);

printf(“Write_REGTab[3]=%x\r\n”, Write_REGTab[3]);

while (1)

{

SSP1x20_SendCommand(SSP1x20_CMD_START); When continuous measurement is enabled, this command is sent only once

HAL_Delay(100);

SPI_ADC_CS_LOW();

ADC_gain_value =0;

ADC_gain_value = SPI_ADC_ReadByte();

ADC_gain_value = (ADC_gain_value << 8) | SPI_ADC_ReadByte();

ADC_gain_value = (ADC_gain_value << 8) | SPI_ADC_ReadByte();

SPI_ADC_CS_HIGH();

#define CALIBRATED_FULL_SCALE 4210300.0f  // ccording to calibration data

RTD = 1650.0f * ((float)ADC_gain_value / CALIBRATED_FULL_SCALE);

tmpPt100 = rtd_to_temperature_iec60751(RTD);

__NOP();

printf(“R=%.3f Ω, Temp=%.2f °C\r\n”,RTD, tmpPt100 );

}

}

The test results are shown in the figure:

Hot Water Temperature Test:

Ice Water Mixture Test:

4. External temperature test configuration details:

——————————————————————————————————

Key Part Code Formula Calculation:

RTD = 1650*( (float)ADC_gain_value /(0x3fffff)); //reference resistance 1650 ohms tmpPt100 = (RTD-100)/0.385;

Reference resistance 1650 ohms,

First line code RTD = 1650 * (ADC / 0x3FFFFF)

Designed to convert the original ADC value to the resistance value of the PT100 (ratio measurement)

• VIN = I × RPT100 (voltage across PT100)
• VREF = I × RREF (voltage across reference resistor)

The same constant current source I adc is used at both ends

So:     Vin/Vref  =  Rpt100/Rref

The output of the ADC is the digitized result of this ratio

ADC_Code =  Vin/Vref  x 2²⁴

So pushed back

Rpt100​=  Rref  x  ADC_Code /2²⁴

———————————————————————————————————————

Second line code: tmpPt100 = (RTD – 100) / 0.385;

Estimate the temperature with a linear approximation formula

At 0°C, Rpt100 = 100 Ω

For every 1°C increase in temperature, the resistance increases by about 0.385 Ω

So

T ≈ (R-100)/0.385

Ⅵ、 Common Problem Debugging Guide

Ⅶ、 SSP1220 Core Register Konfigurationstabelle

Tabelle der Koeffizienten der Ⅷ、Callendar-Van Dusen-Gleichung

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