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參數(shù)資料
| 型號： | AD8320-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Serial Digital Controlled Variable Gain Line Driver |
| 中文描述： | 串行數(shù)字控制可變增益線路驅(qū)動器 |
| 文件頁數(shù)： | 13/20頁 |
| 文件大小： | 583K |
| 代理商： | AD8320-EB |
AD8320
–13–
REV. 0
E nhanced Dynamic Range Application
T he AD8320 can be combined with the AD603 to give addi-
tional dynamic range as shown in Figure 47. T he AD603 is a
voltage controlled variable gain amplifier. T he gain of the AD603 is
determined by the difference in voltage between the GPOS and
GNEG pins. T his differential voltage has a range of
±
0.5 V. In
this example, the voltage on GNEG is tied to +0.5 V. As the
voltage on GPOS is varied from 0 V to 1 V, the gain of the AD603
changes from –10 dB to +30 dB with a slope of 25 mV/dB (i.e.,
linear in dB). T he gain control voltage is supplied by the AD7801
DAC. T he output voltage of the DAC (0 V to +2.5 V) is divided
down to fit the 0 V to 1 V range of the AD603 using a resistor
attenuator network.
In order that the same gain control word can be used for both
the AD603 and the AD8320, the serial data stream is converted
to the parallel format of the AD7801 DAC using a serial-to-
parallel shift register. T he rising edge of the enable pulse simul-
taneously updates both amplifiers.
As the control word is varied from 00Dec to 255Dec, the gain of
the signal chain varies from –26 dB to +50 dB (there is 6 dB of
attenuation between AD603 and AD8320). In practice, this
circuit is not usable at the lower end of the gain range due to the
small input signal (11 dBmV or about 10 mV p-p). Figure 48
shows the spectrum of the output signal at a frequency of
42 MHz and an output level of 61 dBmV (3.1 V p-p, max gain).
T he gain vs. code transfer function of the two amplifiers along
with the overall gain is shown in Figure 49. T he overall gain
transfer function combines a linear in dB transfer function with
a linear in Volts/Volt transfer function. It is clear from Figure 49
that the overall gain transfer function can be considered to be
approximately linear in dB over the top 50 dB of its range.
Figure 48. Output Spectrum of Enhanced Dynamic Range
Circuit (Output Level = 61 dBmV, Frequency = 42 MHz)
0
GAIN CONTROL WORD – Decimal
10
0
161
21
G
41
61
81 101 121 141
50
30
20
60
40
181 201 221 241
–10
–20
–30
AD8320/AD603
AD8320
AD603
Figure 49. Gain Transfer Function of Enhanced Dynamic
Range Circuit
75
V
TO MODEM
RECEIVE
CHANNEL
AD7801*
DIPLEXER
+12V
VIN
220
V
220
V
75
V
41dBmV
61dBmV
58dBmV
VIN
COMM
1.5
V
1k
V
+5V
–5V
+0.5V
11dBmV
DATA
CLK
ENABLE
PD
D0–D7
CLK
100
V
GPOS
GNEG
VNEG
FBDK
VOUT
VPOS
AD8320*
CS
D0–D7
WR LDAC
AD603*
74HCT164*
A B
8
20pF
0.1
m
F
*ADDITIONAL PINS AND
DECOUPLING CAPACITORS
OMITTED FOR CLARITY
PD
CLR
VDD
REFIN
1
5V
AGND DGND
Figure 47. Enhanced Dynamic Range Circuit
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