TDC1011_15 Datasheet, PDF (13/49 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
Feature Description (continued)
8.3.3 Low Noise Amplifier (LNA)
The LNA in the TDC1011’s front-end limits the input-referred noise and ensures timing accuracy for the
generated STOP pulses. The LNA is an inverting amplifier designed for a closed-loop gain of 20 dB with the aid
of an external input capacitor or resistor, and it can be programmed for two feedback configurations. The band-
pass configuration, referred to as capacitive feedback mode, must be combined with an input capacitor. The low-
pass configuration, referred to as resistive feedback mode, must be combined with an input resistor. The
recommended values for the input components are 300 pF and 900 Ω respectively.
The LNA can be configured in capacitive feedback mode for transducers with resonant frequencies in the order
of a couple of MHz. This is done by clearing the LNA_FB bit in the TOF_1 register to 0. As shown in Figure 16,
the external capacitor, C_IN, should be placed between the transducer and the input pin. This provides an in-band
gain of C_IN/C_F, where C_F is the on-chip 30-pF feedback capacitor. Provided that C_IN = 300 pF, the in-band gain of
the LNA circuit is:
Gain_{in band}

C_IN
C_F

300 pF
30 pF
 10
(1)
CIN
300 pF _RX
CF 30 pF
RF
9 NŸ
VCOM
±
LNA
+
LNAOUT
Figure 16. LNA Capacitive Feedback Configuration
The capacitive feedback configuration of the LNA has a band-pass frequency response. The high-pass corner
frequency is set by the internal feedback components R_F (9 kΩ) and C_F (30 pF), and is approximately 590 kHz.
The in-band gain is set by the capacitor ratio and the LNA’s 50-MHz gain-bandwidth product sets the low-pass
corner of the frequency response. For example, an in-band gain of 10 results in a bandpass response between
590 kHz and 5 MHz.
The LNA can be configured in resistive feedback mode for transducers with resonant frequencies in the order of
a couple of hundreds of kHz. This is done by setting the LNA_FB bit in the TOF_1 register to 1. In this
configuration, the internal feedback capacitor C_F is disconnected (see Figure 17), and the DC gain of the LNA
circuit is determined by the ratio between the internal feedback resistor R_F (9 kΩ) and an external resistor R_IN.
For R_IN = 900 Ω, the gain of the circuit is 10.
R_IN
Transducer _{900 Ÿ} RX
R_F
9 NŸ
±
LNA
+
LNAOUT
VCOM
Figure 17. LNA Resistive Feedback Configuration
The LNA can be bypassed and disabled by writing a 1 to the LNA_CTRL bit in the TOF_1 register.
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