AD8320ARP datasheet(14/20 Pages) AD

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AD8320

–14–

REV. 0

Varying Gain by Varying Load Impedance

As already mentioned, the AD8320 has a dynamic output im-

pedance of 75

Ω. The specified gain range assumes that the

output is terminated with a 75

Ω load impedance. Varying the

load impedance allows the gain to be varied, up to a maximum

of twice the specified gain (for RL =

). The variation in gain

with load resistance is shown in Figure 50 for the case of a gain

control word of 255Dec (i.e., max gain).

RLOAD –

100

1000

10000

Figure 50. Gain vs. RLOAD (Gain Control Word = 255Dec)

The gain can be described by the following equation:

AV = 20 log10

2 RLOAD

RLOAD + 75









0.316

+ 0.077 × Code

()













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

0.1 F

DATEN

SDATA

GND GND GND GND GND

POWER-

DOWN

SWITCH

INTER

VOUT

VOCM

C10

0.1 F

ATTENUATOR CORE

DATA LATCH

DATA SHIFT REGISTER

CLK

0.1 F

10 F

0.1 F

C11

0.1 F

VREF

VIN

115

0.1 F

INPUT

0.1 F

REFERENCE

VCC

GND

BYP

VREF

C12

0.1 F

TP2

VCC

TP4

OUTPUT

36-PIN CENTRONICS CONNECTOR

16, 19-30, 33

AD8320

TP3

VOCM

OPTIONAL

TP1

Figure 51. Evaluation Board Schematic

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

®-based soft-

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

µF capacitor, located

fairly close to the device, provides good decoupling for lower

frequency signals. In addition, and more importantly, five

0.1

µF 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.

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