VCA8500_0803 Datasheet, PDF (23/40 Pages) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

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