Datasheet

S0498-01
CLAMP
AFE
LNAx
DC Offset
Correction
ACTx
C
ACT
C
IN
C
BYPASS
INPx
INMx
Input
Optional
Diodes
AFE5808A
SLOS729B –OCTOBER 2011–REVISED APRIL 2012 www.ti.com
A typical application circuit diagram is listed above. The configuration for each block is discussed below.
LNA CONFIGURATION
LNA Input Coupling and Decoupling
The LNA closed-loop architecture is internally compensated for maximum stability without the need of external
compensation components. The LNA inputs are biased at 2.4 V and AC coupling is required. A typical input
configuration is shown in Figure 83. C
IN
is the input AC coupling capacitor. C
ACT
is a part of the active termination
feedback path. Even if the active termination is not used, the C
ACT
is required for the clamp functionality.
Recommended values for C
ACT
= 1 µF and C
IN
are ≥ 0.1 µF. A pair of clamping diodes is commonly placed
between the T/R switch and the LNA input. Schottky diodes with suitable forward drop voltage (e.g. the
BAT754/54 series, the BAS40 series, the MMBD7000 series, or similar) can be considered depending on the
transducer echo amplitude.
Figure 83. LNA Input Configurations
This architecture minimizes any loading of the signal source that may otherwise lead to a frequency-dependent
voltage divider. The closed-loop design yields low offsets and offset drift. C
BYPASS
(≥0.015 µF) is used to set the
high-pass filter cut-off frequency and decouple the complimentary input. Its cut-off frequency is inversely
proportional to the C
BYPASS
value, The HPF cut-off frequency can be adjusted through the register 59[3:2] a
Table 11 lists. Low frequency signals at T/R switch output, such as signals with slow ringing, can be filtered out.
In addition, the HPF can minimize system noise from DC-DC converters, pulse repetition frequency (PRF)
trigger, and frame clock. Most ultrasound systems’ signal processing unit includes digital high-pass filters or
band-pass filters (BPFs) in FPGAs or ASICs. Further noise suppression can be achieved in these blocks. In
addition, a digital HPF is available in the AFE5808A ADC. If low frequency signal detection is desired in some
applications, the LNA HPF can be disabled.
Table 11. LNA HPF Settings (C
BYPASS
= 15 nF)
Reg59[3:2] (0x3B[3:2]) Frequency
00 100 KHz
01 50 KHz
10 200 KHz
11 150 KHz
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