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
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 (R
S
) and
amplifier is critical to achieve a certain overall eight identical shunt FETs placed in parallel and
performance level. Using a 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 of 20dB (10V/V). The ultralow
VCM – V
T
(FET nearly OFF), where VCM is the
input-referred noise of only 0.75nV/√Hz, along with
common source voltage and V
T
is the threshold
the linear input range of 280mV
PP
, 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 as 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 of 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 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 98 shows the
simplified schematic of this VCA stage.
Figure 98. Voltage-Controlled Attenuator Simplified Schematic
48 Copyright © 2008–2011, Texas Instruments Incorporated