AD8432

Rev. 0 | Page 23 of 28

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

20

Ω

787

Ω

787

Ω

2.2nF

2.2nF

RF1N

RF1P

I1OP

Q1OP

4LOP

20

Ω

0.1µF

0.1µF

0.1µF

AD8432

AD8339

Q1

I1

Figure 65. Block Diagram of AD8432 + 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 65 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. The 4LOP and 4LON pins of the AD8339 are driven by

a differential clock signal, which 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

being summed. The external RC feedback network of the

AD8021 is a 100 kHz low-pass filter as shown in Figure 65.

Refer to 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 test. A detailed reference

schematic of the setup is shown in Figure 66. The AD8432 is

shown in this configuration with a gain of 12.04 dB, with

unterminated inputs. If active termination is preferred, use an

Clamping diodes IND1/IND2 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

20

Ω

20

Ω

20

Ω

20

Ω

RF2P

RF2N

Q1 + Q2

787

Ω

0

Ω

27

3

4

6

+

–

Q1OP

AD8021

VPOS

VNEG

4LOP

15

Ω

G = 12dB

G = –1.3dB

LPF

4LO

15

Ω

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

787

Ω

0

Ω

27

3

4

6

+

–

AD8021

−5V

0.1µF

0.1µF

+5V

2.2nF

Figure 66. Schematic of AD8432 (G = 12.04 dB) + AD8339 Application for Ultrasound Beamforming