AD8321(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

AD8321
(Rev.:Rev0)

Gain Programmable CATV Line Driver

Analog Devices 

AD8321

OPERATIONAL DESCRIPTION

The AD8321 is a digitally controlled variable gain power ampli-

fier that is optimized for driving a 75 Ω cable. As a multifunc-

tional bipolar device on a single silicon die, it incorporates all

the analog features necessary to accommodate reverse path

(upstream) high speed (5 MHz to 65 MHz) cable data modem

requirements. The AD8321 has an overall gain range of ap-

proximately 53 dB and is capable of greater than 100 MHz

operation at output signal levels exceeding 12 dBm. Overall,

when considering the device’s wide gain range, low distortion,

wide bandwidth and variable load drive, the device can be used

in many variable gain block applications.

VCC

GND

VIN+

VIN–

AD8321

INV

ATTENUATOR CORE

PWR

AMP

REVERSE

AMP

VOUT

DATADASTHAIFLTARTECGHISTER

POWER-

DOWN/

PD

SWITCH

DATA SHIFT REGISTER

INTER

DATEN CLK

SDATA

Figure 22. Functional Block Diagram

The digitally programmable gain is controlled by the three-wire

“SPI” compatible inputs. These inputs are called SDATA

(serial data input port), DATEN (data enable low input port)

and CLK (clock input port). See Pin Function Descriptions

and Functional Block diagram. The AD8321 is programmed by

an 8-bit “attenuator” word. When a standard 8-bit word is

used, the first data bit MSB will be shifted out of the 7-bit shift

register during the eighth rising CLK edge. The lower seven

bits will then be loaded into the AD8321’s digital decode sec-

tion when the DATEN input is taken high.

The gain of the AD8321 is linear in steps of 0.7526 dB. The

gain transfer function starts at –27.43 dB (at decimal code 0)

and increases 0.7526 dB/LSB. The gain increases up to decimal

code 71. At this point the gain is at its maximum level of 26 dB.

If a decimal word between 71 and 127 is entered, the gain is no

longer incremented and stays at 26 dB. Since the MSB of an 8-bit

word is a “don’t care” bit, at decimal code 128, the AD8321’s

gain returns to its minimum value. The gain vs. gain control

relationship repeats itself as shown in Figure 23 for the upper

127 codes.

The gain transfer function is as follows:

AV = 26 dB – ((71 – CODE) × 0.7526 dB) for CODE ≤ 71

AV = 26 dB for 71 ≤ CODE ≤ 127

AV = 26 dB + ((199 – CODE) × 0.7526 dB) for 128 ≤

CODE ≤ 199

AV = 26 dB for 199 ≤ CODE ≤ 255

where CODE is the decimal equivalent of the 8-bit word loaded in

the AD8321’s data latch (see Figure 23).

30

20

10

0

–10

–20

–30

0

32

64

96 128 160 192 224 256

GAIN CODE – Decimal

Figure 23. Linear-In dB Gain vs. Gain Control

The AD8321 is composed of four analog functions in the

power-up or forward mode. The input amplifier (preamp) which

can be used single-endedly or differentially and provides a maxi-

mum of 12 dB of attenuation. If the input is used in the differ-

ential configuration, it is imperative that the input signals are

180 degrees out of phase and of equal amplitudes. This will

ensure the proper gain accuracy and harmonic performance.

The preamp stage drives a vernier stage that provides the fine

tune gain adjustment. The 0.7526 dB step resolution is imple-

mented in this stage. After the vernier stage, a DAC provides the

bulk of the AD8321’s attenuation (six bits or 36 dB). The signals

in the preamp and vernier gain blocks are differential to im-

prove the PSRR and linearity. A single-ended current is fed

from the DAC into the output stage, which amplifies this cur-

rent to the appropriate level necessary to drive a 75 Ω load. The

output stage utilizes negative feedback to implement a

75 Ω output impedance. This eliminates the need for an exter-

nal 75 Ω matching resistor needed in typical video (or video

filter) termination requirements.

REV. 0

–7–

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