Time To Digital Converters
The unit has 4 resistor ports with the following function:
The temperature sensor should have a minimum resistance of 500 Ω (PT500/1000);. The TDC-GP2 measures the discharge times of the RC-networks made of each resistor and the capacitor.
The precision of the temperature measurement is about 0.004°C which is 5 times better than needed. When measuring every 30 seconds the current consumption is 0.08 µA only.
The temperature measurement is fully automated. It is triggered by the µC sending the opcode "Start_Temp". The TDC-GP2 controls the 4 measurements by itself. After the 4 measurements have finished the interrupt flag is set. The four data are found in registers 0 to 3.
From Res_2/RES_1 and RES_3/RES_4 the microcontroller can calculate the ratio Rtemp/Rref. By means of a look-up table it can calculate the temperature for the special type of sensor in use.
It is not possible with TDC-GP2 to use 4-wire temperature sensors.
By means of the PICOSTRAIN technology the temperature measurement needs an extremely low current, much less than an A/D converter does.
A full temperature measurement with 2 sensors, 2 references, including all calculations takes less than 2.5 µAs. With one temperature measurement in 30 seconds (typical for heatmeters) the average current consumption is 0.08 µA only. This is about 50 times less than other solutions.