AD9675KBCZ datasheet(40/61 Pages) AD

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Data Sheet

AD9675

Rev. A | Page 39 of 60

CW DOPPLER OPERATION

Each channel of the AD9675 includes an I/Q demodulator.

Each demodulator has an individual programmable phase shifter.

The I/Q demodulator is ideal for phased array beamforming

applications in medical ultrasound. Each channel can be

programmed for 16 phase settings/360° (or 22.5°/step), selectable

via the SPI port. The device has a RESET± input that is used to

synchronize the LO dividers of each channel. If multiple AD9675

devices are used, a common reset across the array ensures a

synchronized phase for all channels. If the RESET± input is not

used, tie each input pin to ground. Internal to the AD9675, the

individual Channel I and Channel Q outputs are current summed.

If multiple AD9675 devices are used, current sum and convert the

I and Q outputs from each AD9675 to a voltage using an external

transimpedance amplifier.

Quadrature Generation

The internal 0° and 90° LO phases are digitally generated by a

divide-by-M logic circuit, where M is 4, 8, or 16. The internal

divider is selected via Address 0x02E, Bits[2:0] (see Table 31). The

divider is dc-coupled and inherently broadband; the maximum

LO frequency is limited only by its switching speed. Ensure that

the duty cycle of the quadrature LO signals is as near 50% as

possible for the 4LO and 8LO modes. The 16LO mode does not

require a 50% duty cycle. Furthermore, the divider is implemented

such that the multiple local oscillator (MLO) signal reclocks the

final flip-flops that generate the internal LO signals and thereby

minimizes noise introduced by the divide circuitry.

For optimum performance, the MLO signal input is driven

differentially, as on the AD9675 evaluation board. The common-

mode voltage on each pin is approximately 1.2 V with the

nominal 3 V supply. It is important to ensure that the MLO source

have very low phase noise (jitter), a fast slew rate, and an

adequate input level to obtain optimum performance of the CW

signal chain.

Beamforming applications require a precise channel-to-channel

phase relationship for coherence among multiple channels. The

RESET± input is provided to synchronize the LO divider

circuits in different AD9675 devices when they are used in

arrays. The RESET± input is a synchronous edge-triggered

input that resets the dividers to a known state after power is

applied to multiple AD9675 devices. The RESET± signal can be

either a continuous signal or a single pulse, and it can be either

synchronized with the MLO± clock edge (recommended) or it

can be asynchronous. If a continuous signal is used for the

RESET±, it must be at the LO rate. For synchronous RESET±,

the device can be configured to sample the RESET± signal with

either the falling or rising edge of the MLO± clock, which

makes it easier to align the RESET± signal with the opposite

MLO± clock edge. Register 0x02E is used to configure the

RESET signal behavior. Synchronize the RESET± input to the

MLO signal input. Achieve accurate channel-to-channel phase

matching via a common clock on the RESET± input when using

more than one AD9675.

I/Q Demodulator and Phase Shifter

The I/Q demodulators consist of double-balanced, harmonic

rejection, passive mixers. The RF input signals are converted

into currents by transconductance stages that have a maximum

differential input signal capability of matching the LNA output

full scale. These currents are then presented to the mixers that

convert them to baseband (RF − LO) and 2× RF (RF + LO).

The signals are phase shifted according to the codes that are

programmed into the SPI latch (see Table 26). The phase shift

function is an integral part of the overall circuit. The phase shift

listed in Table 26 is defined as being between the baseband I or

Q channel outputs. As an example, for a common signal applied

to a pair of RF inputs to an AD9675, the baseband outputs are

in phase for matching phase codes. However, if the phase code

for Channel 1 is 0000 and the phase code for Channel 2 is 0001,

Channel 2 leads Channel 1 by 22.5°.

Table 26. Phase Select Code for Channel-to-Channel Phase Shift

Phase Shift

I/Q Demodulator Phase (Address 0x02D[3:0])

0°

0000

22.5°

0001 (not valid in 4LO mode)

45°

0010

67.5°

0011 (not valid in 4LO mode)

90°

0100

112.5°

0101 (not valid in 4LO mode)

135°

0110

157.5°

0111 (not valid in 4LO mode)

180°

1000

202.5°

1001 (not valid in 4LO mode)

225°

1010

247.5°

1011 (not valid in 4LO mode)

270°

1100

292.5°

1101 (not valid in 4LO mode)

315°

1110

337.5°

1111 (not valid in 4LO mode)

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