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MAX2035

Ultrasound Variable-Gain Amplifier

Maxim Integrated 

Ultrasound Variable-Gain Amplifier

Applications Information

External Compensation

External compensation is required for bypassing inter-

nal biasing circuitry. Connect, as close as possible,

individual 4.7µF capacitors from each pin EXT_C1,

EXT_C2, and EXT_C3 (pin 13, 14, 15) to ground.

External Bias Resistor

An external resistor at EXT_RES is required to set the

bias for the internal biasing circuitry. Connect, as close

as possible, a 7.5kΩ resistor from EXT_RES (pin 38) to

ground.

Analog Input and Output Coupling

In typical applications, the MAX2035 is being driven

from a low-noise amplifier (such as the MAX2034) and

is typically driving a discrete differential anti-alias filter

into an ADC (such as the MAX1434 octal ADC). The

differential input impedance of the MAX2035 is typically

200Ω. The differential outputs are capable of driving a

differential load resistance of 1kΩ. The output imped-

ance is 100Ω differential. The differential outputs have

a common-mode bias of approximately 3V. AC-couple

these differential outputs if the next stage has a differ-

ent common-mode input range.

Ultrasound-Specific IMD3 Specification

Unlike typical communications specs, the two input

tones are not equal in magnitude for the ultrasound-

specific IMD3 two-tone specification. In this measure-

ment, f1 represents reflections from tissue and f2

represents reflections from blood. The latter reflections

are typically 25dB lower in magnitude, and hence the

measurement is defined with one input tone 25dB lower

than the other. The IMD3 product of interest (f1 - (f2 - f1))

presents itself as an undesired Doppler error signal in

ultrasound applications. See Figure 1.

PCB Layout

The pin configuration of the MAX2035 is optimized to

facilitate a very compact physical layout of the device

and its associated discrete components. A typical

application for this device might incorporate several

devices in close proximity to handle multiple channels

of signal processing.

The exposed pad (EP) of the MAX2035’s TQFP-EP

package provides a low thermal-resistance path to the

die. It is important that the PCB on which the MAX2035

is mounted be designed to conduct heat from the EP.

In addition, provide the EP with a low-inductance path

to electrical ground. The EP MUST be soldered to a

ground plane on the PCB, either directly or through an

array of plated via holes.

ULTRASOUND

IMD3

-25dB

f1 - (f2 - f1)

f1

f2

f2 + (f2 - f1)

Figure 1. Ultrasound IMD3 Measurement Technique

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