AFE5807 datasheet(63/75 Pages) TI1

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AFE5807

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SLOS703C – SEPTEMBER 2010 – REVISED MAY 2013

In the 16/8/4×fcw operations modes, low phase noise clock is required for 16/8/4×ƒcw clocks (i.e. CLKP_16X/

CLKM_16X pins) in order to maintain good CW phase noise performance. The 1×ƒcw clock (i.e. CLKP_1X/

CLKM_1X pins) is only used to synchronize the multiple AFE5807 chips and is not used for demodulation. Thus

1×fcw clock’s phase noise is not a concern. However, in the 1×fcw operation mode, low phase noise clocks are

required for both CLKP_16X/ CLKM_16X and CLKP_1X/ CLKM_1X pins since both of them are used for mixer

demodulation. In general, higher slew rate clock has lower phase noise; thus clocks with high amplitude and fast

slew rate are preferred in CW operation. In the CMOS clock mode, 5V CMOS clock can achieve the highest slew

rate.

Clock phase noise can be improved by a divider as long as the divider’s phase noise is lower than the target

phase noise. The phase noise of a divided clock can be improved approximately by a factor of 20logN dB where

N is the dividing factor of 16, 8, or 4. If the target phase noise of mixer LO clock 1×fcw is 160dBc/Hz at 1KHz off

carrier, the 16×fcw clock phase noise should be better than 160-20log16=136dBc/Hz. TI’s jitter cleaners

LMK048X/CDCM7005/CDCE72010 exceed this requirement and can be selected for the AFE5807. In the 4X/1X

modes, higher quality input clocks are expected to achieve the same performance since N is smaller. Thus the

16X mode is a preferred mode since it reduces the phase noise requirement for system clock design. In addition,

the phase delay accuracy is specified by the internal clock divider and distribution circuit.

NOTE

In the 16X operation mode, the CW operation range is limited to 8 MHz due to the 16X

CLK. The maximum clock frequency for the 16X CLK is 128 MHz. In the 8X, 4X, and 1X

modes, higher CW signal frequencies up to 15 MHz can be supported with small

degradation in performance., e.g. the phase noise is degraded by 9 dB at 15MHz,

compared to 2MHz.

As the channel number in a system increases, clock distribution becomes more complex. It is not preferred to

use one clock driver output to drive multiple AFEs since the clock buffer’s load capacitance increases by a factor

of N. As a result, the falling and rising time of a clock signal is degraded. A typical clock arrangement for multiple

AFE5807s is illustrated in Figure 90. Each clock buffer output drives one AFE5807 in order to achieve the best

signal integrity and fastest slew rate, i.e. better phase noise performance. When clock phase noise is not a

concern, e.g. the 1×fcw clock in the 16/8/4×fcw operation modes, one clock driver output may excite more than

one AFE5807s. Nevertheless, special considerations should be applied in such a clock distribution network

design. In typical ultrasound systems, it is preferred that all clocks are generated from a same clock source, such

as 16×fcw , 1×fcw clocks, audio ADC clocks, RF ADC clock, pulse repetition frequency signal, frame clock and

etc. By doing this, interference due to clock asynchronization can be minimized

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