AFE5808A datasheet(36/66 Pages) TI1

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SPI IN

LNA

0 to -40dB

SPI Logic

16X CLK

LNA IN

PGA

24, 30dB

rd LP Filter

10, 15, 20,

30 MHz

16 Phases

Generator

CW Mixer

Summing

Amplifier

1X CLK

CW I/Q Vout

AFE5808A (1 of 8 Channels)

14Bit

ADC

LVDS

Reference

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 CBYPASS 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 CBYPASS. 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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