AFE5804ZCF datasheet(48/63 Pages) TI1

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Attenuator

Input

Attenuator

Output

A1-A8AttenuatorStages

Control

Input

C -C ClippingAmplifiers

VCNTL

AFE5804

SBOS442C

– JUNE 2008 – REVISED OCTOBER 2011

www.ti.com

LOW-NOISE AMPLIFIER (LNA)

The attenuator is essentially a variable voltage divider

As with many high-gain systems, the front-end

that consists of the series input resistor (RS) and

amplifier is critical to achieve a certain overall

eight identical shunt FETs placed in parallel and

performance

level.

Using

proprietary

new

controlled by sequentially activated clipping amplifiers

architecture, the LNA of the AFE5804 delivers

(A1 through A8). Each clipping amplifier can be

exceptional low-noise performance, while operating

understood as a specialized voltage comparator with

on a very low quiescent current compared to

a soft transfer characteristic and well-controlled

CMOS-based

architectures

with

similar

noise

output limit voltage. Reference voltages V1 through

performances.

V8 are equally spaced over the 0V to 1.2V control

voltage range. As the control voltage rises through

The LNA performs a single-ended input to differential

the input range of each clipping amplifier, the

output voltage conversion and is configured for a

amplifier output rises from 0V (FET completely ON) to

fixed

gain

20dB

(10V/V).

The

ultralow

VCM

– VT (FET nearly OFF), where VCM is the

input-referred noise of only 0.75nV/

√Hz, along with

common source voltage and VT is the threshold

the linear input range of 280mVPP, results in a wide

voltage of the FET. As each FET approaches its off

dynamic range that supports the high demands of

state and the control voltage continues to rise, the

PW and CW ultrasound imaging modes. Larger input

next clipping amplifier/FET combination takes over for

signals can be accepted by the LNA, but distortion

the next portion of the piecewise-linear attenuation

performance

degrades

input

signals

levels

characteristic.

increase. The LNA input is internally biased to

approximately 2.4V; the signal source should be

Thus, low control voltages have most of the FETs

ac-coupled to the LNA input by an adequately-sized

turned on, producing maximum signal attenuation.

capacitor. Internally, the LNA directly drives the VCA,

Similarly, high control voltages turn the FETs off,

avoiding

the

typical

drawbacks

ac-coupled

leading to minimal signal attenuation. Therefore, each

architectures, such as slow overload recovery.

FET acts to decrease the shunt resistance of the

voltage divider formed by RS and the parallel FET

network.

VOLTAGE-CONTROLLED ATTENUATOR

(VCA)

The

VCA

designed

have

linear-in-dB

attenuation characteristic; that is, the average gain

loss in dB is constant for each equal increment of the

control voltage (VCNTL). Figure 98 shows the

simplified schematic of this VCA stage.

Figure 98. Voltage-Controlled Attenuator Simplified Schematic

AFE5804ZCF Datasheet(PDF) 48 Page - Texas Instruments

AFE5804ZCF Datasheet(HTML) 48 Page - Texas Instruments

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