Data Sheet

AD8432

Rev. D | 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.

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

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

section. The IND1/IND2 clamping diodes can be connected to

IN1/IN2 to protect the LNA input from being overdriven.

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

27

3 4

6

+

–

Q1OP

AD8021

VPOS

VNEG 4LOP

15Ω

G = 12dB

G = –1.3dB

LPF

4LO

15Ω

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

27

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