Datasheet
Data Sheet AD8432
Rev. C | Page 23 of 32
I/Q DEMODULATION FRONT END
The AD8432 low noise amplifiers can be used to drive the
differential RF inputs of the dual AD8333 or the quad AD8339
I/Q demodulators. The primary application for the AD8339 is
phased array beamforming in medical ultrasound, specifically
in CW Doppler processing. Other applications include phased
array radar and smart antennas for mobile communications.
08341-041
AD8021
AD8021
20Ω
787Ω
787Ω
2.2nF
2.2nF
RF1N
RF1P
I1OP
Q1OP
4LO
20Ω
0.1µF
0.1µF
0.1µF
AD8432
AD8339
Q1
I1
Figure 70. Block Diagram of AD8432 and AD8339 Application for
Ultrasound Beamforming
Because of its low output noise and low distortion, the AD8432
ensures minimal degradation in dynamic range while amplifying
the RF input signal. At the lowest gain of 12.04 dB, the AD8432
contributes only 3.4 nV/√Hz output voltage noise.
Figure 70 shows a simplified block diagram of one channel of
the AD8432 driving the AD8339. The AD8432 outputs can be
connected directly to the AD8339 RF inputs through 20 Ω
resistors. A differential clock signal, 4LO, which is applied to the
4LOP and 4LON pins of the AD8339, has a frequency 4× that of
the RF inputs. The AD8339 downconverts the RF signals,
generates quadrature, and phase-shifts the resultant I and Q
signals.
The I and Q outputs of the AD8339 are current outputs. A
transimpedance amplifier, such as the AD8021, processes the
outputs and performs several functions, including the following:
• Current-to-voltage conversion
• Summation amplifier for multiple channels
• Active low-pass filter
In beamforming applications, the I and Q outputs of a number
of receiver channels are summed, which increases the system
dynamic range by 10 log
10
(N), where N is the number of
channels being summed. The external RC feedback network of
the AD8021 is a 100 kHz low-pass filter as shown in Figure 70.
See the AD8333 and AD8339 datasheets for more details on
implementing I/Q demodulators.
Evaluation boards are available for the AD8432 and the AD8339
to facilitate system level design and testing. A detailed reference
schematic of the setup is shown in Figure 71. The AD8432 is
shown in this configuration with a gain of 12.04 dB, with
unterminated inputs. If active termination is preferred, use an
R
FB
and C
FB
network as discussed in the Theory of Operation
section. The IND1/IND2 clamping diodes can be connected to
IN1/IN2 to protect the LNA input from being overdriven.
08341-043
BIAS
AD8432
INL2
IND2
INH2
INL1
IND1
INH1
ENB VPS1 VPS2 COMM
OPH2
LNA1
LNA2
IN2
IN1
0.1µF
AD8339
RF1P
RF1N
20Ω
20Ω
20Ω
20Ω
RF2P
RF2N
Q1 + Q2
787Ω
0Ω
2 7
3
4
6
+
–
Q1OP
AD8021
VPOS VNEG
4LOP
R
SH1
15Ω
G = 12dB
G = –1.3dB
LPF
f
C
= 100kHz
4LO
C
SH1
47
pF
R
SH2
15Ω
C
SH2
47
pF
0.1µF
0.1µF
0.1µF
0.1µF 0.1µF
0.1µF
0.1µF
OPL2
OPH1
OPL1
GMH2
GOH2
GOL2
GML2
GMH1
GOH1
GOL1
GML1
−5V
−5V
+5V
0.1µF
0.1µF
+5V
I1OP
Q2OP
I2OP
2.2nF
I1 + I2
787Ω
0Ω
2 7
3
4
6
+
–
AD8021
−5V
0.1µF
0.1µF
+5V
2.2nF
Figure 71. Schematic of the AD8432 (G = 12.04 dB) and AD8339 Application for Ultrasound Beamforming