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

W resistor should be used between

each input and ground at R1 and R2. This option will also

result in a 75

W differential input termination to the AD8321.

If a 50

W single-ended input source is applied to the VIN+/

DIFF IN port, the R1 and R2 values should be 26.7

W.

Differential Input Option 3: A differential input may be

applied 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 differen­

tial 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

W

or 50

W, 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

AD8321

R1

R2

AD8321

R3

OPTION 2 DIFFERENTIAL INPUT TERMINATION

T1

OPTION 1 DIFFERENTIAL INPUT TERMINATION

DIFF IN

T1

VIN+

VIN–

R1

R2

AD8321

OPTION 3 DIFFERENTIAL INPUT TERMINATION

Figure 27. Differential Input Termination Options

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”

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

W 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

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. A