參數(shù)資料
型號(hào)： AD9279-80KITZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 28/44頁(yè)
文件大?。?/td> 0K
描述： KIT 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 34 of 44
I/Q Demodulator and Phase Shifter
The I/Q demodulators consist of double-balanced passive mixers.
The RF input signals are converted into currents by transconduc-
tance stages that have a maximum differential input signal
capability matching the LNA output full scale. These currents
are then presented to the mixers, which convert them to base-
band (RF LO) and twice RF (RF + LO). The signals are phase
shifted according to the codes programmed into the SPI latch
(see Table 16). The phase shift function is an integral part of the
overall circuit. The phase shift listed in Column 1 of Table 16 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 AD9279, the baseband outputs are in phase for matching
phase codes. However, if the phase code for Channel 1 is 0000
and that of Channel 2 is 0001, then Channel 2 leads Channel 1
by 22.5°.
Table 16. Phase Select Code for Channel-to-Channel Phase Shift
Φ Shift
I/Q Demodulator Phase
(SPI Register 0x2D[3:0])
0°
0000
22.5°
0001
45°
0010
67.5°
0011
90°
0100
112.5°
0101
135°
0110
157.5°
0111
180°
1000
202.5°
1001
225°
1010
247.5°
1011
270°
1100
292.5°
1101
315°
1110
337.5°
1111
Dynamic Range and Noise
Figure 66 is an interconnection block diagram of all eight
channels of the AD9279. Two stages of ADA4841 amplifiers are
used. The first stage does an I-to-V conversion and filters the
high frequency content that results from the demodulation
process. In beamforming applications, the I and Q outputs of a
number of receiver channels are summed. In the AD9279, the
summation of eight channels is the input to the first stage of the
ADA4841s. The second stage of ADA4841 amplifiers is used to
do the summation of additional AD9279 to ADA4841 outputs,
provide gain, and drive the AD7982, 18-bit SAR ADC. The
dynamic range of the system increases by the factor 10log10(N),
where N is the total number of channels (assuming random
uncorrelated noise). The noise in the 8-channel example of
Figure 66 is increased by 9 dB while the signal quadruples (18 dB),
yielding an aggregate SNR improvement of (18 9) = 9 dB.
The output-referred noise of the CW signal path depends on
the LNA gain and the selection of the first stage summing
amplifier and the value of RFILT. To determine the output
referred noise, it is important to know the active low-pass filter
(LPF) values RA, RFILT, and CFILT, shown in Figure 66. Typical
filter values for all eight channels of a single AD9279 are 100 Ω
for RA, 500 Ω for RFILT, and 2.0 nF for CFILT; these values
implement a 100 kHz single-pole LPF.
If the RF and LO are offset by 10 kHz, the demodulated signal is
10 kHz and is passed by the LPF. The single-channel mixing gain
from the RF input to the ADA4841 output (for example, I1,
Q1) is approximately the LNA gain for RFILT and CFILT of 500 Ω
and 2.0 nF.
This gain can be increased by increasing the filter resistor while
maintaining the corner frequency. The factor limiting the mag-
nitude of the gain is the output swing and drive capability of the
op amp selected for the I-to-V converter, in this example, the
ADA4841. Because any amplifier has limited drive capability,
there is a finite number of channels that can be summed.
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