參數(shù)資料
型號(hào)： AD9279-65EBZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 29/44頁(yè)
文件大?。?/td> 0K
描述： BOARD EVALUATION FOR AD9279
標(biāo)準(zhǔn)包裝： 1
ADC 的數(shù)量： 8
位數(shù)： 12
采樣率（每秒）： 80M
數(shù)據(jù)接口： 串行
工作溫度： -40°C ~ 85°C
已用 IC / 零件： AD9279
已供物品： 板
AD9279
Rev. 0 | Page 35 of 44
09
42
3-
0
45
LNA
AD7982
18-BIT ADC
ADA4841
2.5V
4nF
50
50
I
CHANNEL H
CHANNEL A
LO
GENERATION
4
RE
SE
T
4L
O
+
4L
O
–
CWI+
CWI–
AD9279
CFILT
RFILT
1.5V
ADA4841
1.5V
RFILT
RA
AD7982
18-BIT ADC
ADA4841
2.5V
4nF
50
50
Q
CWQ+
CWQ–
CFILT
RFILT
1.5V
ADA4841
1.5V
RFILT
Φ
Figure 66. Typical Connection Interface for I/Q Outputs in CW Mode
Phase Compensation and Analog Beamforming
Beamforming, as applied to medical ultrasound, is defined as
the phase alignment and summation of signals generated from a
common source but received at different times by a multielement
ultrasound transducer. Beamforming has two functions: it imparts
directivity to the transducer, enhancing its gain, and it defines a
focal point within the body from which the location of the return-
ing echo is derived. The primary application for the AD9279 I/Q
demodulators is in analog beamforming circuits for ultrasound
CW Doppler.
Modern ultrasound machines used for medical applications
employ an array of receivers for beamforming, with typical CW
Doppler array sizes of up to 64 receiver channels that are phase
shifted and summed together to extract coherent information.
When used in multiples, the desired signals from each of the
channels can be summed to yield a larger signal (increased by a
factor N, where N is the number of channels), whereas the noise
is increased by the square root of the number of channels. This
technique enhances the signal-to-noise performance of the
machine. The critical elements in a beamformer design are the
means to align the incoming signals in the time domain and the
means to sum the individual signals into a composite whole.
In traditional analog beamformers incorporating Doppler, a
V-to-I converter per channel and a crosspoint switch precede
passive delay lines used as a combined phase shifter and
summing circuit. The system operates at the carrier frequency
(RF) through the delay line, which also sums the signals from
the various channels, and then the combined signal is down-
converted by an I/Q demodulator. The dynamic range of the
demodulator can limit the achievable dynamic range.
The resultant I and Q signals are filtered and then sampled by
two high resolution analog-to-digital converters. The sampled
signals are processed to extract the relevant Doppler information.
Alternatively, the RF signal can be processed by downconversion
on each channel individually, phase shifting the downconverted
signal, and then combining all channels. Because the dynamic
range expansion from beamforming occurs after demodulation,
the demodulator dynamic range has little effect on the output
dynamic range. The AD9279 implements this architecture. The
downconversion is done by an I/Q demodulator on each channel,
and the summed current output is the same as in the delay line
approach. The subsequent filters after the I-to-V conversion
and the ADCs are similar.
For CW Doppler operation, the AD9279 integrates the LNA,
phase shifter, frequency conversion, and I/Q demodulation
into a single package and directly yields the baseband signal.
Figure 67 is a simplified diagram showing the concept for four
channels. The ultrasound wave (US wave) is received by four
transducer elements, TE1 through TE4, in an ultrasound probe
and generates signals E1 through E4. In this example, the phase
at TE1 leads the phase at TE2 by 45°.
In a real application, the phase difference depends on the
element spacing, wavelength (λ), speed of sound, angle of
incidence, and other factors. In Figure 67, the signals E1
through E4 are amplified by the low noise amplifiers. For
optimum signal-to-noise performance, the output of the LNA
is applied directly to the input of the demodulators. To sum the
signals E1 through E4, E2 is shifted 45° relative to E1 by setting
the phase code in Channel 2 to 0010, E3 is shifted 90° (0100), and
E4 is shifted 135° (0110). The phase aligned current signals at
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