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鍨嬭櫉(h脿o)锛� AD8368ACPZ-WP
寤犲晢锛� Analog Devices Inc
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AD8368
Rev. B | Page 16 of 20
The choice of CDETO is a compromise of averaging time constant,
response time, and carrier leakage. If CDETO is selected to be too
small to speed up the response time, the AGC loop could start
tracking and leveling any amplitude envelope and corrupt the
constellation. Figure 38 illustrates a 16 QAM, 100 ksymbols per
second constellation with a degraded error vector magnitude
(EVM) of 5%. By increasing CDETO to 0.01 渭F, the EVM is
improved to 1.1%.
STABILITY AND LAYOUT CONSIDERATIONS
In some applications, the printed circuit board (PCB) parasitic, in
combination with the source impedance presented by the driving
stage, can present some troublesome impedance at high frequency
and can potentially unstablize the amplifier under certain extreme
conditions, such as high gain and high temperature. To avoid such
scenarios, it is recommended to include a simple parallel RL
snubbing network directly at the input terminal of the AD8368.
Figure 40 depicts an example of this network. The RL network
formed by R3 and L1 is used to minimize the negative impact
due to reflective source conditions at high RF frequencies and
ensures the amplifier operates unconditionally stable and
maintains the typical device performance.
REF 鈥�4.9dBm
CF 100MHz
SR 10kHz
CONST DIAG
MEAS SIGNAL
16 QAM
1U
鈥�1U
鈥�1.31289U
262.578mU/
1.31289U
05
90
7-
0
40
On the underside of the chip scale package, there is an exposed
compressed paddle. This paddle is internally connected to the
ground of the chip. Solder the paddle to the low impedance ground
plane on the PCB to ensure specified electrical performance and to
provide thermal relief. It is also recommended that the ground
planes on all layers under the paddle be stitched together with
vias to reduce thermal impedance.
Figure 38. Degraded Error Vector Magnitude Performance for 16 QAM at
100 ksymbols per second (CDETO Too Small)
Figure 39 illustrates the measured EVM performance for a 16 QAM
modulation at 10 Msymbols per second using CDETO = 1 nF.
10
0
1
2
3
4
5
6
7
8
9
鈥�40
鈥�30
鈥�20
鈥�10
0
10
20
EV
M
(
%
)
POWER IN (dBm)
05
90
7-
0
41
Figure 39. Error Vector Magnitude Performance for
16 QAM 10 Msymbols per second
鐩搁棞(gu膩n)PDF璩囨枡
PDF鎻忚堪
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鍙冩暩(sh霉)鎻忚堪
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AD8369 鍒堕€犲晢:AD 鍒堕€犲晢鍏ㄧū:Analog Devices 鍔熻兘鎻忚堪:45 dB Digitally Controlled VGA LF to 600 MHz
AD8369ARM 鍒堕€犲晢:Analog Devices 鍔熻兘鎻忚堪:500 MHZ DIGITAL VGA - Bulk
AD8369ARU 鍒堕€犲晢:Analog Devices 鍔熻兘鎻忚堪:SP Amp Variable Gain Amp Single 5.5V 16-Pin TSSOP Tube 鍒堕€犲晢:Rochester Electronics LLC 鍔熻兘鎻忚堪:TSSOP IF DIGITAL CONTROL VGA - Bulk 鍒堕€犲晢:Analog Devices 鍔熻兘鎻忚堪:IC SEMICONDUCTOR ((NS))
AD8369ARU-REEL 鍒堕€犲晢:Rochester Electronics LLC 鍔熻兘鎻忚堪:- Tape and Reel
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