Applications for time-to-digital converters TDC - Cookbook
Acam USA Newsletter
GP2 - Application for Ultrasonic Heat Meter
The TDC-GP2 is perfectly suited for low-cost ultrasonic heatmeter designs. Thanks to the implemented functionality, including precision temperature measurement, fire pulse generation, windowing and clock calibration it is sufficient to add a simple microprocessor (without A/D converter) and a transducer dependant driver and receiver. The extremely low current consumption guarantees the necessary long battery lifetime in such applications.
The measurement is fairly automated by the TDC-GP2. The microprocessor just sends a start command. The TDC then fires the transducers and measures the time of flight. It calibrates the results and provides them to the microprocessor.
a. Time measurement
32.768kHz: are provided by the µP: < 0.5 µA
4 MHz: 0 µA in power down, 300 µA when active
TDC: < 50 nA if not active, 15 mA during active time of the high-speed unit
With 2 measurements per second (forth and back flow) the total consumption is < 2 µA .
b. Temperature measurement
A full measurement over all four ports takes less than 2 µAs current. Typically the temperature is measured once in 30 seconds. The average current then is about 0.085 µA . This is about 50 times less than with solutions without a TDC.
c. Quiescent current
Thanks to the current optimized 0.35µ technology the quiescent current is less than 50 nA.
d. Total system current
The complete current for the measuring unit (TDC, analog part, Transducer) will depend on the analog circuit part. It will be in the range of 3 to 5 µA .
With a low-power µP (e.g. MSP430 series from TI) the average current consumption of the total device might be in the range of 10 to 15 µA .
It is possible to operate the system from a lithium-thionylchloride ½ AA cell for 10 years without changing the battery. At 6 years runtime it maybe even possible to work with a low-cost 3V CR2450 coin cell battery.
