The RTD or the Pt100 is one of the high accuracy temperature sensor for laboratory. Using the high resolution delta-sigma converter, enables designer to use a simple voltage divider circuit for measuring the resistance of the RTD without the need of any DC amplifications. This instrument shows how to use the LTC2420, 20-bit delta-sigma converter and the LM385 reference voltage for measuring four Pt100 sensors and displays on the text LCD.
The circuit is simple voltage divider. VREF and R1 are fixed values. R2 is the Pt100 sensor. Its resistance is changed with temperature. We can measure the sensor’s resistance easily by detecting the voltage dropped across R2. The DC signal is equal to R2*(VREF/(R1+R2)). This signal can tie to the delta-sigma ADC directly.
The MCU is 40-pin DIP package Microchip PIC18F4620 running with internal oscillator. The low power 32768Hz oscillator is for 1s time base. PORTB, PB0-PB7 are for LCD 4-bit interface. The display is 16×2 text LCD. Reference voltage, +1.2V is produced by D2, LM385. This reference voltage is tied to U2 , LTC2420 and U1A, LM358. U3, CD4051 is 8-channel voltage multiplexer. This project uses only 4-channel, X0, X1, X2 and X3. The signal from the sensor for each channel is measured by the dropped voltage across each Pt100 sensor.
The sample sensor is a cheap two wires Pt100 sensor, WZP.
The firmware provides the mode for adjusting value for each channel, +/-5C with 0.1C step. When power up the instrument, keep pressing left key will enter the Calibrate Mode. We can select channel to be adjusted by pressing the left key. The center key is for increasing +0.1C and the right key is for decreasing -0.1C. The adjusting value will be saved to internal EEPROM. Recycle the power will exit this mode and will return to normal operation. Note that the firmware for converting the Pt100’s resistance to temperature is prepared for precision sensor. Thus for high accuracy Pt100, we can set all adjusting values to 0.0C.