AFE5808A datasheet(33/83 Pages) TI1

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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

AFE5808A

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SLOS729D – OCTOBER 2011 – REVISED NOVEMBER 2015

8.3.4 Analog-to-Digital Converter

The analog-to-digital converter (ADC) of the AFE5808A device employs a pipelined converter architecture that

consists of a combination of multi-bit and single-bit internal stages. Each stage feeds its data into the digital error

correction logic, ensuring excellent differential linearity and no missing codes at the 14-bit level. The 14 bits given

out by each channel are serialized and sent out on a single pair of pins in LVDS format. All eight channels of the

AFE5808A device operate from a common input clock (CLKP/M). The sampling clocks for each of the eight

channels are generated from the input clock using a carefully matched clock buffer tree. The 14x clock required

for the 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 AFE5808A device 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 AFE5808A device.

8.3.5 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. The

passive mixer 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 in

Figure 63 and Figure 64 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. 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 63. Simplified Block Diagram of CW Path

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