AD8321
Differential Input Option 2:
Install the Mini-Circuits T1-6T-
KK81 1:1 transformer in the T1 location of the evaluation
board. Jumpers J1, J2 and J3 should be applied pointing in the
direction of the transformer. Apply an open circuit in the R3
position while J4 is applied connecting the center-tap of the
secondary to ground. A 41
Ω
resistor should be used between
each input and ground at R1 and R2. This option will also
result in a 75
Ω
differential input termination to the AD8321.
If a 50
Ω
single-ended input source is applied to the VIN+/
DIFF IN port, the R1 and R2 values should be 26.7
Ω.
Differential Input Option 3:
A differential input may be ap-
plied to both VIN– and VIN+ inputs of the evaluation board. In
this example, no transformer is employed. Jumpers J1, J2 and J3
are installed in line with the input signals. Select the differential
input termination configuration of either
Option 1
or
Option 2.
Apply
Option 1
resistor value to R3 for a true differential input
or apply Option 2 values to R1 and R2 to produce ground refer-
enced inputs that are separated in phase by 180°. If the differen-
tial input signal source impedance is anything other than 75
Ω
or 50
Ω,
calculate the appropriate value according to the equa-
tions below:
For
Option 1
Configurations:
Desired Input Impedance
=
R3
900
For
Option 2
and
3
(R1 = R2 = R):
Desired Input Impedance
= 2
×
(R 450)
DIFF IN
T1
R3
Controlling the Evaluation Board from a PC
The AD8321-EVAL package comes with the circuit described
by Figure 28 and includes a –2 dB attenuator (reference Rc, Rd
and Re) and the control software allowing the user to program
the gain/attenuation of the AD8321 via a standard printer cable
connected to the parallel port of a PC.
Install Software
To install the “CABDRIVE” software that controls the AD8321-
EVAL evaluation circuit, close all Windows applications and
select the “SETUP” file located on Disk 1 of the AD8321-
EVAL software. Follow the on screen instructions (see Figure
29) and insert Disk 2 when prompted to do so. Enter the path
of the directory into which the control software will be installed.
Select the button in the upper left corner to begin the installa-
tion of “CABDRIVE” software into the specified directory.
Running the Software
To invoke the control software, select the “Ad8321” icon from
the directory containing the installed software. After invoking
the control software, choose the appropriate printer port from
the display portrayed in Figure 30.
Controlling the Gain/Attenuation of AD8321
The AD8321 control panel has four different functions. The
slide bar controls the gain/attenuation of the AD8321. Adjust
the slider to the gain/attenuation displayed in units of dB. The
additional displays show the selection in units of Volts (output)/
Volts (input), and the corresponding control codes in decimal,
binary and hexadecimal. (See Figure 31.)
“POWER UP” and “POWER DOWN”
AD8321
OPTION 1 DIFFERENTIAL INPUT TERMINATION
R2
DIFF IN
T1
AD8321
R1
OPTION 2 DIFFERENTIAL INPUT TERMINATION
R2
VIN+
The buttons marked “Power Up” and “Power Down” select
the mode of operation of the AD8321. The “Power Up” button
puts the AD8321 in forward transmit mode feeding the condi-
tioned signal to the VOUT port on the evaluation board. Con-
versely, the “Power Down” button selects the reverse mode
where the forward signal transmission is disabled and the low
noise reverse amplifier actively maintains a 75
Ω
back termina-
tion. These features may be selected asynchronously (at any
time). (See the section on Between Burst Transient Reduction
for more specific details.)
Enable Output Switch
AD8321
VIN–
R1
OPTION 3 DIFFERENTIAL INPUT TERMINATION
Figure 27. Differential Input Termination Options
An Alpha Industries AS128-73 GaAs 2W Hi Linearity switch is
installed on a standard AD8321-EVAL circuit and is controlled
by the check box on the control panel portrayed in Figure 31.
This feature is intended to remove the output of the AD8321
from the VOUT port prior to using the “Power Up” and
“Power Down” feature described above. This application circuit
may be used to reduce any transients created between bursts to
DOCSIS compliant levels. (See the section on Between Burst
Transient Reduction for more specific details.)
–12–
REV. 0
AD [ ANALOG DEVICES ]
