VCA8500_0803 Datasheet, PDF(24/40 Page) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

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VCA8500_0803 Datasheet, PDF (24/40 Pages) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

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VCA8500

SBOS390A â JANUARY 2008 â REVISED MARCH 2008

LOW-NOISE AMPLIFIER (LNA)

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

amplifier is critical to achieve a certain overall

performance level. Using a proprietary new

architecture, the LNA of the VCA8500 delivers

exceptional low-noise performance, while operating

on a very low quiescent current of only 8.3mA per

channel. This current consumption is significantly

lower compared to CMOS-based architectures with

similar noise performances.

The LNA performs a single-ended input to differential

output voltage conversion and is configured for a

fixed gain of 20dB (10V/V). The ultralow

input-referred noise of only 0.7nV/âHz, along with the

linear input range of 250mVPP, results in a wide

dynamic range that supports the high demands of

PW and CW ultrasound imaging modes. Larger input

signals can be accepted by the LNA, but distortion

performance degrades as input signals levels

increase. The LNA input is internally biased to

approximately 2.4V; the signal source should be

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

capacitor. Internally, the LNA directly drives the VCA,

avoiding the typical drawbacks of ac-coupled

architectures, such as slow overload recovery.

VOLTAGE-CONTROLLED ATTENUATOR

(VCA)

The amplified differential signal swing that comes

from the LNA is reduced by the subsequent VCA

stage. The VCA is designed to have a 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 61 shows the

simplified schematic of this VCA stage.

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The attenuator is essentially a variable voltage divider

that consists of the series input resistor (RS) and

eight identical shunt FETs placed in parallel and

controlled by sequentially activated clipping amplifiers

(A1 through A8). Each clipping amplifier can be

understood as a specialized voltage comparator with

a soft transfer characteristic and well-controlled

output limit voltage. Reference voltages V1 through

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

voltage range. As the control voltage rises through

the input range of each clipping amplifier, the

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

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

common source voltage and VT is the threshold

voltage of the FET. As each FET approaches its off

state and the control voltage continues to rise, the

next clipping amplifier/FET combination takes over for

the next portion of the piecewise-linear attenuation

characteristic.

Thus, low control voltages have most of the FETs

turned on, producing maximum signal attenuation.

Similarly, high control voltages turn the FETs off,

leading to minimal signal attenuation. Therefore, each

FET acts to decrease the shunt resistance of the

voltage divider formed by RS and the parallel FET

network.

Attenuator

Input

VCNTRL

A1-A8 Attenuator Stages

Attenuator

Output

Control

Input

C1-C8 Clipping Amplifiers

Figure 61. Voltage-Controlled Attenuator Simplified Schematic

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