ad8320arpz-reel Analog Devices, Inc., ad8320arpz-reel Datasheet - Page 14

• Manufacturer Part Number: ad8320arpz-reel
• Description: Serial Digital Controlled Variable Gain Line Driver
• Manufacturer: Analog Devices, Inc.

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AD8320
Varying Gain by Varying Load Impedance
As already mentioned, the AD8320 has a dynamic output im-
pedance of 75 . The specified gain range assumes that the
output is terminated with a 75
load impedance allows the gain to be varied, up to a maximum
of twice the specified gain (for R
with load resistance is shown in Figure 50 for the case of a gain
control word of 255Dec (i.e., max gain).
The gain can be described by the following equation:
where Code is the decimal equivalent of the 8-bit word.
Evaluation Board
A two layer evaluation board for the AD8320 is available (part
number AD8320-EB). This board has been carefully laid out
Figure 50. Gain vs. R
A
32
30
28
26
24
22
20
V
0
20 log
INPUT
VREF
10
115
R
TP2
2 R
R1
VCC
LOAD
100
LOAD
TP1
LOAD
0.1 F
C12
0.1 F
10 F
C1
(Gain Control Word = 255Dec)
R
C7
LOAD
75
OPTIONAL
L
load impedance. Varying the
=
–
0.316 0.077 Code
0.1 F
VREF
VIN
C9
). The variation in gain
C6
1000
CLK
VCC
Figure 51. Evaluation Board Schematic
0.1 F
REFERENCE
R4
0
C5
36-PIN CENTRONICS CONNECTOR
VCC
SDATA
6
10000
0.1 F
5
C4
DATA SHIFT REGISTER
ATTENUATOR CORE
2
AD8320
VCC
DATA LATCH
–14–
16, 19-30, 33
0.1 F
C2
and tested to demonstrate the specified high speed performance
of the device. Figure 51 shows the schematic of the evaluation
board. The silkscreen for the component side layer is shown in
Figure 52. The layout of the board is shown in Figure 53 and
Figure 54.
The evaluation board package includes a fully populated board
with BNC-type connectors along with Windows
ware for controlling the board from a PC’s printer port via a
standard printer cable.
A prototyping area is provided to allow for additional circuitry
on the board. The single supply and ground to the board are
brought over to this area and are available on two strips. There
are also two extra strips available on the prototyping area which
can be used for additional power supplies.
The board should be powered with a good quality (i.e., low
noise) single supply of between +5 V and +12 V. Extensive
decoupling is provided on the board. A 10 F capacitor, located
fairly close to the device, provides good decoupling for lower
frequency signals. In addition, and more importantly, five
0.1 F decoupling capacitors are located close to each of the
five power supply pins (7, 8, 9, 17 and 20).
Controlling the Evaluation Board from a PC
The evaluation board ships with Windows-based control soft-
ware. A standard printer cable can be used to connect the
evaluation board to a PC’s printer port (also called parallel
port). The cable length should be kept to less than about 5 feet.
The wiring of a standard printer cable, with respect to the sig-
nal lines that are used in this application, is shown in Figure 55.
Although the software controls the evaluation board via the
PC’s parallel port, the AD8320 digital interface is serial. Three
of the parallel port’s eight bits (and one digital ground line) are
used to implement this serial interface. A fourth bit is used to
control the PD pin.
VCC
DATEN
0.1 F
3
C11
GND GND GND GND GND
VCC
GND
POWER-
SWITCH
DOWN
INTER
0.1 F
/
C3
VOCM
BYP
VOUT
PD
0.1 F
C10
C8
0.1 F
TP4
TP3
OUTPUT
VOCM
®
-based soft-
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