TDC1000: Question for TDC1000

Annie,

Both the minimum and maximum distance able to be measured by the TDC1000 depend on the specific application. The maximum distance is determined by the speed of sound in the medium it is traveling through, and to a lesser extent the frequency of the sound produced by the transducer (higher frequencies attenuate faster, and will be harder to detect at long distances). The maximum listen period for the TDC1000 is set at 8ms. To determine the maximum distance under ideal conditions use the following formula: 

Max distance = speed of sound in medium (m/s) * 0.008 seconds / 2

This formula accounts for the round trip time in the case of a single transducer being used as both transmitter and receiver. As an example, the speed of sound in water is ~1480m/s, so the maximum distance measurable by the TDC1000 in water is ~5.9m one way.

The minimum distance measurable is dependent on the transducer (its frequency and ringdown characteristics) and the number of TX pulses selected by the user. The frequency of the transducer will determine the period of the TX pulses output by the TDC1000, and therefore the length of time that the TDC1000 is transmitting. Since ultrasonic transducers are resonant devices, the transducer will continue to output sound for a short time after the TX pulses have stopped (called ringdown). While the transducer is ringing, it will be mask any incoming echo signals and the TDC1000 will not be able to register a return echo to generate a STOP signal. The theoretical minimum distance can be calculated with the following formula:

Min time (theoretical) = (number of TX pulses (at least 3)) * (1 / transducer frequency) + ringdown time

In practice, the minimum time is further limited by the blanking time of the TDC1000. For the smallest possible time of flight measurement, the TDC1000must be in Short TOF mode. The TDC1000 will not start receiving until the Short TOF Blanking Period has ended. The minimum blanking period determines the minimum time of flight, and is set using the following formula:

Min blanking period = 8 * (1 / system clock frequency)

The maximum system clock frequency is 16Mhz, which equates to a minimum blanking time of 500ns.

The effective minimum time that can be measured will be the higher of the two times calculated above. Finally the minimum distance can be calculated using the following formula along with the min time determined above:

Min distance = speed of sound in medium (m/s) * min time / 2.