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TDC1011_15 Datasheet, PDF (17/49 Pages) Texas Instruments – TDC1011 Single Channel Ultrasonic Sensing Analog Front End (AFE) for Level Sensing,Concentration Sensing Applications
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TDC1011
SNAS662 – JULY 2015
8.3.7 Common-mode Buffer (VCOM)
The output of the internal common-mode buffer is present at the VCOM pin. This pin should be bypassed to
ground with a low-leakage 10-nF capacitor and it should not be loaded with more than 20 µA. The common-
mode buffer can be disabled with the VCOM_SEL bit in the CONFIG_2 register. If disabled, an external
reference voltage must be applied to the VCOM pin.
During a time-of-flight measurement, the common-mode reference will take approximately 16 µs to settle if
starting from zero initial conditions. Using a larger capacitor will increase the settling time of the internal common-
mode reference. The implications of a larger VCOM capacitor are further explored in the Common-mode
Reference Settling Time section.
8.3.8 Temperature Sensor
TDC1011
MEAS_MODE
tof_start
MUX
START
MEAS_MODE
tof_stop
MUX
STOP
Temp
Sensor
RTD1
RTD1
(external)
RTD2
RTD2
(external)
Figure 23. Temperature Sensor Measurement
Accurate measurements of level, and concentration may require compensation for the temperature dependency
of the speed of sound in the medium. The TDC1011 provides two temperature sensor connections, allowing to
measure up to two locations with RTDs, as shown in Figure 23.
The temperature sensor block supports PT1000 or PT500 sensors. The type of RTD used must be selected in
the TEMP_RTD_SEL bit of the CONFIG_3 register. The system requires a temperature-stable external reference
resistor (RREF). If the RTD type is PT500, then RREF should be 500 Ω. If the RTD type is PT1000, then RREF
should be 1 kΩ. The reference resistor needs to have either a low temperature coefficient or be calibrated for
temperature shift.
The logic timing in a temperature measurement is controlled by the TEMP_CLK_DIV bit in the CONFIG_3
register. As shown in Figure 24, the external clock can be divided by 8 or by the value resulting from the
TX_FREQ_DIV field configuration in the CONFIG_0 register. It is recommended to operate the temperature
measurement block at frequencies of 1 MHz or less.
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