TDC-GP2

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.

TDC-GP2 features

`2 stop channels referring to one start channel
`Each of typ. 50 ps RMS resolution
`LSB width typ. 65 ps
`15 ns pulse pair resolution
`4-fold multihit capability for each stop channel
`Measurement range 2.0 ns to 1,8 s
`Selectable rising/falling edge sensitivity for each channel
`Enable pins for powerful windowing functionality
`The possibility to arbitrarily measure all events against each other

Digital TDCs TDC-GP2 use internal propagation delays of signals through gates to measure time intervals with very high precision. Figure 5 clarifies the principal structure of such an absolute-time TDC. Intelligent circuit structures, redundant circuitry and special methods of layout on the chip make it possible to reconstruct the exact number of gates passed by the signal. The maximum possible resolution strongly depends on the maximum possible gate propagation delay on the chip.

The measuring unit TDC-GP2 is actuated by a START signal and stopped by a STOP signal. Based on the position of the ring oscillator and the coarse counter the time interval between START and STOP is calculated with a 20 Bit measurement range.

The TDC-GP2 BIN size (LSB) is typically 65 ps at 3.3 V and 25 °C ambient temperature. The RMS noise is about 50 ps (0.7 LSB). The gate propagation delay times strongly depend on temperature and voltage. Usually this is solved doing a calibration. During such a calibration the TDC measures 1 and 2 periods of the reference clock.The measurement range is limited by size of the counter

The TDC-GP2 fire-pulse generator generates a sequence of pulses which is highly programmable in frequency,phase and number of pulses. The high-speed oscillator frequency divided by the factor selected for ClkHSDiv is used as the basic frequency. This frequency is internally doubled and can freely be divided by a factor of 2 to 15.It is possible to generate 1 to 15 pulses. For each pulse the phase can be adjusted per register configuration.

The TDC-GP2 fire-pulse generator is activated by sending opcode Start_Cycle.The fire-pulse generator provides 2 outputs, Fire1 and Fire2. The driver strength of each output is 48mA @5V.These 2 outputs can be paralleled to increase the driver strength up to 96 mA. Furthermore each output signal can be inverted to double the signal amplitude. The outputs can be set individually high-Z.