Datasheet

R
S
Attenuator
Input
Attenuator
Output
A1-A8AttenuatorStages
Control
Input
Q
1
V
B
Q
2
Q
3
C
1
V1
Q
4
Q
S
C -C ClippingAmplifiers
1 8
Q
5
Q
6
Q
7
Q
8
C
2
V2
C
3
V3
C
4
V4
C
5
V5
C
6
V6
C
7
V7
C8
V8
A1 A2 A3 A4 A5 A6 A7 A8
VCNTL
AFE5805
SBOS421D –MARCH 2008–REVISED MARCH 2010 www.ti.com
LOW-NOISE AMPLIFIER (LNA) The attenuator is essentially a variable voltage divider
that consists of the series input resistor (R
S
) and
As with many high-gain systems, the front-end
eight identical shunt FETs placed in parallel and
amplifier is critical to achieve a certain overall
controlled by sequentially activated clipping amplifiers
performance level. Using a new proprietary
(A1 through A8). Each clipping amplifier can be
architecture, the LNA of the AFE5805 delivers
understood as a specialized voltage comparator with
exceptional low-noise performance, while operating
a soft transfer characteristic and well-controlled
on a very low quiescent current compared to
output limit voltage. Reference voltages V1 through
CMOS-based architectures with similar noise
V8 are equally spaced over the 0V to 1.2V control
performances.
voltage range. As the control voltage rises through
the input range of each clipping amplifier, the
The LNA performs a single-ended input to differential
amplifier output rises from 0V (FET completely ON) to
output voltage conversion and is configured for a
VCM – V
T
(FET nearly OFF), where VCM is the
fixed gain of 20dB (10V/V). The ultralow
common source voltage and V
T
is the threshold
input-referred noise of only 0.7nV/√Hz, along with the
voltage of the FET. As each FET approaches its off
linear input range of 250mV
PP
, results in a wide
state and the control voltage continues to rise, the
dynamic range that supports the high demands of
next clipping amplifier/FET combination takes over for
PW and CW ultrasound imaging modes. Larger input
the next portion of the piecewise-linear attenuation
signals can be accepted by the LNA, but distortion
characteristic.
performance degrades as input signal levels
increase. The LNA input is internally biased to
Thus, low control voltages have most of the FETs
approximately +2.4V; the signal source should be
turned on, producing maximum signal attenuation.
ac-coupled to the LNA input by an adequately-sized
Similarly, high control voltages turn the FETs off,
capacitor. Internally, the LNA directly drives the VCA,
leading to minimal signal attenuation. Therefore, each
avoiding the typical drawbacks of ac-coupled
FET acts to decrease the shunt resistance of the
architectures, such as slow overload recovery.
voltage divider formed by R
S
and the parallel FET
network.
VOLTAGE-CONTROLLED ATTENUATOR
(VCA)
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 47 shows the
simplified schematic of this VCA stage.
Figure 47. Voltage-Controlled Attenuator Simplified Schematic
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