Datasheet

Data Sheet ADA4896-2/ADA4897-1/ADA4897-2
Rev. | Page 23 of 28
ADA4896-2/ADA4897-1/ADA4897-2 in the Ultrasound System
LNA
LNA
AD7982
18-BIT ADC
2.5V
2.5V
4nF
50
50
I
CHANNEL H
CHANNEL
A
LO
GENERATION
4
RESET
4LO+
4LO–
CWI+
CWI–
AD9279
C
FILT
C
FILT
R
FILT
1.5V
1.5V
R
FILT
R
A
R
A
R
A
R
A
AD7982
18-BIT ADC
2.5V
2.5V
4nF
50
50
Q
CWQ+
CWQ–
C
FILT
C
FILT
R
FILT
1.5V
1.5V
R
FILT




09447-032
ADA4896-2/
ADA4897-1/
ADA4897-2
ADA4896-2/
ADA4897-1/
ADA4897-2
ADA4896-2/
ADA4897-1/
ADA4897-2
ADA4896-2/
ADA4897-1/
ADA4897-2
Figure 55. Using the ADA4896-2/ADA4897-1/ADA4897-2 as Filters, I-to-V Converters, Current Summers, and ADC Drivers After the I/Q Outputs of the AD9279
The ADA4896-2/ADA4897-1/ADA4897-2 are used in the CW
Doppler path in the ultrasound application after the I/Q demod-
ulators of the AD9279. Doppler signals can be typically between
100 Hz to 100 kHz. The low noise floor and high dynamic range
of the ADA4896-2/ADA4897-1/ADA4897-2 make them excellent
choices for processing weak Doppler signals.
The rail-to-rail output and the high output current drive of
the ADA4896-2/ADA4897-1/ADA4897-2 make them suitable
candidates for the I-to-V converter, current summer, and ADC
driver.
Figure 55 shows an interconnection block diagram of all
eight channels of the AD9279. Two stages of the ADA4896-2
amplifiers are used. The first stage performs an I-to-V conver-
sion and filters the high frequency content that results from
the demodulation process. The second stage of the ADA4896-2
amplifiers is used to sum the output currents of multiple AD9279
devices, to provide gain, and to drive the AD7982 device, an
18-bit SAR ADC.
The output-referred noise of the CW signal path depends on
the LNA gain, the selection of the first stage summing amplifier,
and the value of R
FILT
. To determine the output-referred noise, it
is important to know the active low-pass filter (LPF) values R
A
,
R
FILT
, and C
FILT
, as shown as Figure 55. Typical filter values for
all eight channels of a single AD9279 are 100 Ω for R
A
, 500 Ω
for R
FILT
, and 2.0 nF for C
FILT
; these values implement a 100 kHz,
single-pole LPF.
The gain of the I-to-V converter can be increased by increasing
the filter resistor, R
FILT
. To keep the corner frequency unchanged,
decrease the filter capacitor, C
FILT
, by the same factor. The factor
limiting the magnitude of the gain is the output swing and drive
capability of the op amp selected for the I-to-V converter, in this
example, the ADA4896-2/ADA4897-1/ADA4897-2. Because
any amplifier has limited drive capability, a finite number of
channels can be summed.
B