Datasheet

SPI IN
LNA
0 to -40dB
SPI Logic
16X CLK
LNA IN
PGA
24, 30dB
3
rd
LP Filter
10, 15, 20,
30 MHz
16 Phases
Generator
CW Mixer
Summing
Amplifier
1X CLK
1X CLK
CW I/Q Vout
AFE5808A (1 of 8 Channels)
14Bit
ADC
LVDS
ReferenceReference
Differential
TGC Vcntl
EXT/INT
REFs
16X8
Crosspoint SW
SPI OUT
VCAT
AFE5808A
SLOS729B –OCTOBER 2011–REVISED APRIL 2012 www.ti.com
THEORY OF OPERATION
AFE5808A OVERVIEW
The AFE5808A is a highly integrated Analog Front-End (AFE) solution specifically designed for ultrasound
systems in which high performance and small size are required. The AFE5808A integrates a complete time-gain-
control (TGC) imaging path and a continuous wave Doppler (CWD) path. It also enables users to select one of
various power/noise combinations to optimize system performance. The AFE5808A contains eight channels;
each channels includes a Low-Noise Amplifier (LNA), a Voltage Controlled Attenuator (VCAT), a Programmable
Gain Amplifier (PGA), a Low-pass Filter (LPF), a 14-bit Analog-to-Digital Converter (ADC), and a CW mixer.
In addition, multiple features in the AFE5808A are suitable for ultrasound applications, such as active
termination, individual channel control, fast power up/down response, programmable clamp voltage control, fast
and consistent overload recovery, etc. Therefore the AFE5808A brings premium image quality to ultra– portable,
handheld systems all the way up to high-end ultrasound systems. Its simplified function block diagram is listed in
Figure 64.
Figure 64. Functional Block Diagram
LOW-NOISE AMPLIFIER (LNA)
In many high-gain systems, a low noise amplifier is critical to achieve overall performance. Using a new
proprietary architecture, the LNA in the AFE5808A delivers exceptional low-noise performance, while operating
on a low quiescent current compared to CMOS-based architectures with similar noise performance. The LNA
performs single-ended input to differential output voltage conversion. It is configurable for a programmable gain
of 24/18/12dB and its input-referred noise is only 0.63/0.70/0.9nV/√Hz respectively. Programmable gain settings
result in a flexible linear input range up to 1Vpp, realizing high signal handling capability demanded by new
transducer technologies. Larger input signal can be accepted by the LNA; however the signal can be distorted
since it exceeds the LNA’s linear operation region. Combining the low noise and high input range, a wide input
dynamic range is achieved consequently for supporting the high demands from various ultrasound imaging
modes.
The LNA input is internally biased at approximately +2.4V; the signal source should be ac-coupled to the LNA
input by an adequately-sized capacitor, e.g. ≥0.1uF. To achieve low DC offset drift, the AFE5808A incorporates a
DC offset correction circuit for each amplifier stage. To improve the overload recovery, an integrator circuit is
used to extract the DC component of the LNA output and then fed back to the LNA’s complementary input for DC
offset correction. This DC offset correction circuit has a high-pass response and can be treated as a high-pass
filter. The effective corner frequency is determined by the capacitor C
BYPASS
connected at INM. With larger
capacitors, the corner frequency is lower. For stable operation at the highest HP filer cut-off frequency, a ≥15nF
capacitor can be selected. This corner frequency scales almost linearly with the value of the C
BYPASS
. For
example, 15nF gives a corner frequency of approximately 100 kHz, while 47nF can give an effective corner
frequency of 33 KHz. The DC offset correction circuit can also be disabled/enabled through register 52[12].
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