TDC1011_15 Datasheet, PDF (14/49 Pages) Texas Instruments – TDC1011 Single Channel Ultrasonic Sensing Analog Front End (AFE) for Level Sensing,Concentration Sensing Applications

TDC1011
SNAS662 – JULY 2015
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Feature Description (continued)
8.3.4 Programmable Gain Amplifier (PGA)
The PGA, shown in Figure 18, is an inverting amplifier with an input resistance of R_IN = 500 Ω and a
programmable feedback resistor R_FB that can be programmed to set a 0-dB to 21-dB gain in 3-dB steps. This
can be done by programming the PGA_GAIN field in the TOF_1 register. The bandwidth of the PGA is scaled
based on its programmed gain. The typical bandwidth of the PGA with a 100-kΩ load to VCM and a 10-pF
capacitor to ground are listed in Table 1.
PGA_GAIN (Hex)
0h
1h
2h
3h
4h
5h
6h
7h
Table 1. Typical PGA Bandwidth
Gain (dB)
0
3
6
9
12
15
18
21
Bandwidth (MHz)
19.0
16.8
14.4
12.3
10.0
8.2
6.6
5.0
The PGA can be bypassed and disabled by writing a 1 to the PGA_CTRL bit in the TOF_1 register. The output of
the PGA should not be loaded directly with capacitances greater than 10 pF.
R_FB
PGA_IN ^{RIN 500 :}
PGA_OUT
PGA
VCOM
Figure 18. TDC1011 Programmable Gain Amplifier
8.3.5 Receiver Filters
It is recommended to place two filters in the RX path to minimize the receive path noise and obtain maximum
timing accuracy. As shown in Figure 19, one filter is placed between the LNAOUT and the PGAIN pins, and
another filter is placed between the PGAOUT and the COMPIN pins.
With an in-band gain of 10, the LNA has a bandwidth of 5 MHz. For most applications, a low-pass filter between
the LNAOUT and PGAIN pins is sufficient.
As shown in Figure 19, the second filter stage can use a cascade of a low-pass filter (R_F1 and C_F3) followed by a
high-pass filter (C_F2 and R_F2) referenced to VCOM. Design of the filter is straightforward. The R_F1 and C_F2 can be
chosen first. A reasonable set of values for R_F1 and C_F2 could be: R_F1 = 1 kΩ ± 10% and C_F2 = 50pF ± 10%.
Given the center frequency of interest to be ƒ_C and the filter bandwidth to be ƒ_B, the value of C_F3 can be
calculated as:
C_F3
1
2SR _F1 f_C  f_B 
(2)
R_F2 and C_F2 determine the high-pass corner of the filter. R_F2 should be referenced to VCOM to maintain the DC
bias level at the comparator input during the echo receive time. For values of R_F2 larger than R_F1, there will be
limited loading effect from the high-pass filter to the low-pass filter resulting in more accurate corner frequencies.
The chosen values shown in the figure below result in a high-pass corner frequency of about 600 kHz and a low-
pass corner frequency of about 3 MHz.
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