Datasheet

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THEORY OF OPERATION
C
IN
LNA
V/I
CWSwitchMatrix
Attenuator
(VCA)
PGA
Clamp
LPF
CW/I
OUT
OUT
OUT
T/R
Switch
V
CNTL
VCA8500
VCA8500
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
While the LNA is designed to be driven from a
Built on TI ’ s bipolar-complementary (BiCOM)
single-ended source, the internal TGC signal path is
process, the VCA8500 is a third-generation, octal
designed to be fully differential to maximize dynamic
variable gain amplifier that implements a number of
range while also optimizing for low, even-order
proprietary circuit design techniques to specifically
harmonic distortion.
address the performance demands of medical
ultrasound systems. CW doppler signal processing is facilitated by routing
the differential LNA outputs to V/I amplifier stages.
The VCA8500 is an 8-channel VGA that is ideally
The resulting signal currents of each channel then
suited for portable ultrasound applications. It offers
connect to an 8 × 10 switch matrix that is controlled
unparalleled low-noise and low-power performance at
through the serial interface and a corresponding
a high level of integration. For the TGC signal path,
register. The CW outputs are typically routed to a
each channel consists of a 20dB fixed-gain low-noise
passive delay line that allows coherent summing
amplifier (LNA), a linear-in-dB voltage-controlled
(beam forming) of the active channels and additional
attenuator (VCA), and a programmable gain amplifier
off-chip signal processing, as shown in Figure 60 .
(PGA), as well as a clamping and low-pass filter
stage. Digitally controlled through the logic interface, Applications that do not utilize the CW path can
the PGA gain can be set to four different settings: simply operate the VCA8500 in TGC mode. In this
20dB, 25dB, 27dB, and 30dB. At its highest setting, mode, the CW blocks (V/I amplifiers and switch
the total available gain of the VCA8500 is therefore matrix) remain powered down, and the CW outputs
50dB, sufficient for 10-bit systems. To facilitate the can be unconnected.
logarithmic time-gain compensation required for
ultrasound systems, the VCA is designed to provide a
46dB attenuation range. Here, all channels are
simultaneously controlled by an externally-applied
control voltage (V
CNTL
) in the range of 0V to 1.2V.
Figure 60. Functional Block Diagram
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