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Preliminary Technical Data

AD9271

Rev. PrA | Page 23 of 58

100

RS (Ω)

INCLUDES NOISE OF VGA

RESISTIVE TERMINATION

(RS = RIN)

ACTIVE IMPEDANCE MATCH

UNTERMINATED

SIMULATION

Figure 37. Noise Figure vs. RS for Resistive,

Active Matched and Unterminated Inputs, Gain = 1 V

100

RS (Ω)

INCLUDES NOISE OF VGA

RIN = 50Ω

RIN = 75Ω

RIN = 100Ω

RIN = 200Ω

RFB = ∞

SIMULATION

Figure 38. Noise Figure vs. RS for Various Fixed Values of RIN,

Actively Matched, Gain = 1 V.

The primary purpose of input impedance matching is to

improve the system transient response. With resistive termination,

the input noise increases due to the thermal noise of the

matching resistor and the increased contribution of the LNA’s

input voltage noise generator. With active impedance matching,

however, the contributions of both are smaller than they would

be for resistive termination by a factor of 1/(1 + LNA Gain).

Figure 37 shows the relative noise figure (NF) performance. In

this graph, the input impedance was swept with RS to preserve

the match at each point. The noise figures for a source impedance

of 50 Ω are 7.1 dB, 4.1 dB, and 2.5 dB for the resistive, active,

and unterminated configurations, respectively. The noise

figures for 200 Ω are 4.6 dB, 2.0 dB, and 1.0 dB, respectively.

Figure 38 shows the NF vs. RS for various values of RIN, which is

helpful for design purposes. The plateau in the NF for actively

matched inputs mitigates source impedance variations. For

comparison purposes, a preamp with a gain of 15.6 dB and

noise spectral density of 1.2 nV/√Hz, combined with a VGA

with 4 nV/√Hz, yields a noise figure degradation of

approximately 1.5 dB (for most input impedances), which is

significantly worse than the AD9271 performance.

INPUT OVERDRIVE

Excellent overload behavior is of primary importance in ultra-

sound. Both the LNA and VGA have built-in overdrive

protection and quickly recover after an overload event.

Input Overload Protection

As with any amplifier, voltage clamping prior to the inputs is

highly recommended if the application is subject to high

transient voltages.

A block diagram of a simplified ultrasound transducer interface

is shown in Figure 39. A common transducer element serves the

dual functions of transmitting and receiving ultrasound energy.

During the transmitting phase, high voltage pulses are applied

to the ceramic elements. A typical transmit/receive (T/R) switch

may consist of four high voltage diodes in a bridge configuration.

Although the diodes ideally block transmit pulses from the

sensitive receiver input, diode characteristics are not ideal, and

resulting leakage transients imposed on the LI-x inputs can be

problematic.

Because ultrasound is a pulse system and time-of-flight is used

to determine depth, quick recovery from input overloads is

essential. Overload can occur in the preamp and the VGA.

Immediately following a transmit pulse, the typical VGA gains

are low, and the LNA is subject to overload from T/R switch

leakage. With increasing gain, the VGA can become overloaded

due to strong echoes that occur near field echoes and

acoustically dense materials, such as bone.

Figure 39 illustrates an external overload protection scheme. A

pair of back-to-back Schottky diodes is installed prior to

installing the ac-coupling capacitors. Although the BAS40

diodes are shown, any diode is prone to exhibiting some amount

of shot noise. Many types of diodes are available for achieving the

desired noise performance. The configuration shown in Figure

39 tends to add 2 nV√Hz of input-referred noise. Decreasing the

5 kΩ resistor and increasing the 2 kΩ resistor may improve noise

contribution, depending on the application. With the diodes

shown in Figure 39, clamping levels of ±0.5 V or less

significantly enhances the system overload performance.

TRANSDUCER

10nF

2kΩ

5kΩ

AD9271

DRIVER

BAS40-04

+5V

–5V

LNA

Figure 39. Input Overload Protection

CW DOPPLER OPERATION

Modern ultrasound machines used for medical applications

employ a 2n binary array of receivers for beam forming, with

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