TDC1011_15 Datasheet, PDF (15/49 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
More complex filters can be used; external gain is acceptable if the signal amplitude is too low. If the pass-band
of the filter is wider than an octave, it is recommended to use a filter design which has linear group delay.
LNAOUT
C_F1 = 1 nF
PGAIN PGAOUT
R_F1 = 1 NŸ
C_F2 = 50 pF
COMPIN
C_F3 = 53 pF
R_F2 = 5 NŸ
Figure 19. Filter for a 1-MHz Operation
GND
VCOM
8.3.6 Comparators for STOP Pulse Generation
The STOP pulse generation block of the TDC1011 contains two auto-zeroed comparators (a zero-cross detect
and a threshold-detect comparator), a threshold setting DAC, and an event manager.
Comparator auto-zero periods occur at the beginning of every TOF receive cycle. During these periods, the
comparator’s input offset is stored in an internal 2.5-pF capacitor, and it is subtracted from the input signal during
the echo processing phase. The duration of auto-zero period is configured with the AUTOZERO_PERIOD field
located in the CLOCK_RATE register.
VCOM
COMPIN
Zero Cross Detect
±
+
ECHO_QUAL_THLD
+
DAC
±
VTHLD
Threshold
Detect
±
+
RECEIVE_MODE
NUM_RX
Event Manager
STOP
Figure 20. STOP Pulse Generation Circuit
8.3.6.1 Threshold Detector and DAC
The threshold detect comparator in Figure 20 compares the echo amplitude with a programmable threshold level
(V_THLD) controlled by a DAC. The DAC voltage is set by the ECHO_QUAL_THLD field in register CONFIG_3 and
provides 8 programmable threshold levels, V_THLD. The typical levels are summarized in Table 2:
ECHO_QUAL_THLD
Typical V_THLD (mV)
Table 2. Echo Qualification Threshold Levels
0h
1h
2h
3h
4h
5h
–35
–50
–75
–125
–220
–410
6h
–775
7h
–1500
8.3.6.2 Zero-cross Detect Comparator
The zero-cross detect comparator compares the amplified echo signal at COMPIN with its reference voltage,
which is V_COM. As shown in Figure 21, the comparator produces a low-to-high transition when the amplitude of
the echo signal rises above V_COM. The comparator produces a high-to-low transition when the echo amplitude
falls below V_COM – V_HYST. The built-in negative-sided hysteresis of 10 mV in reference to V_COM ensures accurate
zero-cross time instances associated with the rising edges of the echo signal and immunity of the comparator
output to noise.
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