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AD9272

Rev. C | Page 22 of 44

LNA

LI-x

LG-x

LO-x

DOUTx+

DOUTx–

T/R

SWITCH

CLG

CFB R

FB2

CSH

GAIN

INTERPOLATOR

LOSW-x

RFB1

SWITCH

ARRAY

CWD[7:0]+

CWD[7:0]–

ATTENUATOR

–42dB TO 0dB

PIPELINE

ADC

SERIAL

LVDS

POSTAMP

AD9272

FILTER

21dB

24dB,

27dB,

30dB

15.6dB,

17.9dB,

21.3dB

Figure 37. Simplified Block Diagram of a Single Channel

CHANNEL OVERVIEW

Each channel contains both a TGC signal path and a CW Doppler

signal path. Common to both signal paths, the LNA provides user-

adjustable input impedance termination. The CW Doppler path

includes a transconductance amplifier and a crosspoint switch.

The TGC path includes a differential X-AMP® VGA, an antialiasing

filter, and an ADC. Figure 37 shows a simplified block diagram

with external components.

The signal path is fully differential throughout to maximize signal

swing and reduce even-order distortion; however, the LNA is

designed to be driven from a single-ended signal source.

Low Noise Amplifier (LNA)

Good noise performance relies on a proprietary ultralow noise

LNA at the beginning of the signal chain, which minimizes the

noise contribution in the following VGA. Active impedance

control optimizes noise performance for applications that benefit

from input impedance matching.

A simplified schematic of the LNA is shown in Figure 38. LI-x is

capacitively coupled to the source. An on-chip bias generator

establishes dc input bias voltages of around 0.9 V and centers

the output common-mode levels at 1.5 V (AVDD2 divided by

2). A capacitor, CLG, of the same value as the input coupling

capacitor, CS, is connected from the LG-x pin to ground.

LI-x

CLG

CFB

CSH

LG-x

LO-x

LOSW-x

VCM

VO+

VO–

RFB1

RFB2

T/R

SWITCH

Figure 38. Simplified LNA Schematic

The LNA supports differential output voltages as high as 4.4 V p-p

with positive and negative excursions of ±1.1 V from a common-

mode voltage of 1.5 V. The LNA differential gain sets the maximum

input signal before saturation. One of three gains is set through

the SPI. The corresponding full-scale input for the gain settings

of 6, 8, and 12 is 733 mV p-p, 550 mV p-p, and 367 mV p-p,

respectively. Overload protection ensures quick recovery time

from large input voltages. Because the inputs are capacitively

coupled to a bias voltage near midsupply, very large inputs can

be handled without interacting with the ESD protection.

Low value feedback resistors and the current-driving capability

of the output stage allow the LNA to achieve a low input-referred

noise voltage of 0.75 nV/√Hz (at a gain of 21.3 dB). This is

achieved with a current consumption of only 27 mA per channel

(80 mW). On-chip resistor matching results in precise single-

ended gains, which are critical for accurate impedance control.

The use of a fully differential topology and negative feedback

minimizes distortion. Low second-order harmonic distortion is

particularly important in second harmonic ultrasound imaging

applications. Differential signaling enables smaller swings at each

output, further reducing third-order distortion.

Recommendation

It is highly recommended that the LG-x pins form a Kelvin type

connection to the input or probe connection ground. Simply

connecting the LG pin to ground near the device can allow

differences in potential to be amplified through the LNA. This

generally shows up as a dc offset voltage that can vary from

channel to channel and part to part given the application and

layout of the PCB (see Figure 38).

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