Datasheet

AD9279
Rev. 0 | Page 20 of 44
CHANNEL OVERVIEW
POST
AMP
LNA
GAIN–
GAIN+
SERIAL
LVDS
AAF
ATTENUATOR
–45dB TO 0dB
GAIN
INTERPOLATOR
PIPELINE
ADC
LOSW-x
LO-x
LI-x
LG-x
RESET
4LO+
4LO–
R
FB2
LO
GENERATION
15.6dB,
17.9dB,
21.3dB
21dB,
24dB,
27dB,
30dB
CWI+
CWI–
CWQ+
CWQ–
DOUTx+
DOUTx–
R
FB1
C
LG
C
SH
TRANSDUCE
R
C
S
T/R
SWITCH
09423-038
Figure 41. Simplified Block Diagram of a Single Channel
Each channel contains both a TGC signal path and a CW Doppler
signal path. Common to both signal paths, the LNA provides four
user-adjustable input impedance termination options for
matching different probe impedances. The CW Doppler path
includes an I/Q demodulator with programmable phase rotation
needed for analog beamforming. The TGC path includes a
differential X-AMP® VGA, an antialiasing filter, and an ADC.
Figure 41 shows a simplified block diagram with external
components.
TGC OPERATION
The TGC signal path is fully differential throughout to
maximize signal swing and reduce even-order distortion;
however, the LNAs are designed to be driven from a single-
ended signal source. Gain values are referenced from the single-
ended LNA input to the differential ADC input. A simple
exercise in understanding the maximum and minimum gain
requirements is shown in Figure 42.
The maximum gain required is determined by
(ADC Noise Floor/LNA Input Noise Floor) + Margin =
20 log(224/3.1) + 11 dB = 46 dB
The minimum gain required is determined by
(ADC Input FS/LNA Input FS) + Margin =
20 log(2/0.45) − 10 dB = 3 dB
Therefore, 45 dB of gain range for a 12-bit, 40 MSPS ADC with
15 MHz of bandwidth should suffice in achieving the dynamic
range required for most of today’s ultrasound systems.
The system gain is distributed as listed in Table 7.
Table 7. Channel Gain Distribution
Section Nominal Gain (dB)
LNA 15.6/17.9/21.3 (LNA
GAIN
)
Attenuator 0 to −45 (VGA
AT T
)
VGA Amplifier 21/24/27/30 (PGA
GAIN
)
Filter 0
ADC 0
Each LNA output is dc-coupled to a VGA input. The VGA
consists of an attenuator with a range of −45 dB to 0 dB followed
by an amplifier with 21 dB/24 dB/27 dB/30 dB of gain. The
X-AMP gain interpolation technique results in low gain error
and uniform bandwidth, and differential signal paths minimize
distortion.
The linear-in-dB gain (law conformance) range of the TGC path
is 45 dB. The slope of the gain control interface is 28 dB/V, and
the gain control range is −0.8 V to +0.8 V. Equation 3 is the
expression for the differential voltage, V
GAIN
, at the gain control
interface. Equation 4 is the expression for the VGA attenuation,
VGA
ATT
as a function of V
GAIN
.
V
GAIN
(V) = (GAIN+) − (GAIN−) (3)
)8.0(
V
dB
28)dB(
GAIN
ATT
VVGA −−=
(4)
The total channel gain can then be calculated as in Equation 5.
GAIN
ATT
GAIN
PGAVGALNAnChannelGai ++
=
)dB(
(5)