AFE5809ZCF datasheet(58/90 Pages) TI

58 / 90 page

Sum Amp

with LPF

LNA1

Voltage to

Current

Converter

Sum Amp

with LPF

I-CH

Q-CH

I-CLK

Q-CLK

LNA8

Voltage to

Current

Converter

I-CH

Q-CH

I-CLK

Q-CLK

I-CH

Q-CH

Clock

Distribution

Circuits

1×

fcw CLK

N×

fcw CLK

AFE5809

SLOS738C – SEPTEMBER 2012 – REVISED JANUARY 2013

www.ti.com

serializer is generated internally from the CLKP/M pins. A 7x and a 1x clock are also given out in LVDS format,

along with the data, to enable easy data capture. The AFE5809 operates from internally-generated reference

voltages that are trimmed to improve the gain matching across devices. The nominal values of REFP and REFM

are 1.5 V and 0.5 V, respectively. Alternately, the device also supports an external reference mode that can be

enabled using the serial interface.

Using serialized LVDS transmission has multiple advantages, such as a reduced number of output pins (saving

routing space on the board), reduced power consumption, and reduced effects of digital noise coupling to the

analog circuit inside the AFE5809.

CONTINUOUS-WAVE (CW) BEAMFORMER

Continuous-wave Doppler is a key function in mid-end to high-end ultrasound systems. Compared to the TGC

mode, the CW path needs to handle high dynamic range along with strict phase noise performance. CW

beamforming is often implemented in analog domain due to the mentioned strict requirements. Multiple

beamforming methods are being implemented in ultrasound systems, including passive delay line, active mixer,

and passive mixer. Among all of them, the passive mixer approach achieves optimized power and noise. It

satisfies the CW processing requirements, such as wide dynamic range, low phase noise, accurate gain and

phase matching.

A simplified CW path block diagram and an In-phase or Quadrature (I/Q) channel block diagram are illustrated

below respectively. Each CW channel includes a LNA, a voltage-to-current converter, a switch-based mixer, a

shared summing amplifier with a low-pass filter, and clocking circuits.

NOTE

The local oscillator inputs of the passive mixer are cos(

ωt) for I-CH and sin(ωt) for Q-CH

respectively. Depending on users' CW Doppler complex FFT processing, swapping I/Q

channels in FPGA or DSP may be needed in order to get correct blood flow directions.

All blocks include well-matched in-phase and quadrature channels to achieve good image frequency rejection as

well as beamforming accuracy. As a result, the image rejection ratio from an I/Q channel is better than -46 dBc

which is desired in ultrasound systems.

Figure 76. Simplified Block Diagram of CW Path

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