AD8326ARE-EVAL datasheet(10/24 Pages) AD

номер детали	AD8326ARE-EVAL

подробное описание детали	High Output Power Programmable CATV Line Driver

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REV. 0

AD8326

–10–

APPLICATIONS

General Applications

The AD8326 is primarily intended for use as the upstream

power amplifier (PA), also known as a line driver, in DOCSIS

(Data Over Cable Service Interface Specification) certified

cable modems, cable telephony systems, and CATV set-top

boxes. The upstream signal is either a QPSK or QAM signal

generated by a DSP, a dedicated QPSK/QAM modulator, or a

DAC. In all cases the signal must be low-pass filtered before

being applied to the PA in order to filter out-of-band noise and

higher order harmonics from the amplified signal. Due to the

varying distances between the cable modem and the headend,

the upstream PA must be capable of varying the output power

by applying gain or attenuation. The varying output power of

the AD8326 ensures that the signal from the cable modem will

have the proper level once it arrives at the headend. The upstream

signal path also contains a transformer, a diplexer, and cable split-

ters. The AD8326 has been designed to overcome losses associated

with these passive components in the upstream cable path, particu-

larly in modems that support cable telephony.

AD8326ARP Applications

The AD8326ARP is in a thermally enhanced PSOP2 package,

and designed for single 12 V supply and output power applica-

tions up to +69 dBmV. The AD8326ARP will provide maximum

performance in 12 V systems.

AD8326ARE Applications

The AD8326ARE is in a TSSOP package with an exposed ther-

mal pad. It is designed for dual

±5 V or single 10 V supplies. For

applications requiring up to 65 dBmV of output power, lower

cost, smaller package, and lower power dissipation, the TSSOP

package is most appropriate.

Operational Description

The AD8326 consists of four analog functions in the transmit

enable or forward mode. The input amplifier (preamp) can be

used single-ended or differentially. If the input is used in the

differential configuration, it is imperative that the input signals be

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

ensure proper gain accuracy and harmonic performance. The

preamp stage drives a vernier stage that provides the fine tune

gain adjustment. The approximate step resolution of 0.75 dB is

implemented in this stage and provides a total of approximately

5.25 dB of accumulated attenuation. After the vernier stage, a

DAC provides the bulk of the AD8326’s attenuation (8 bits or

48 dB). The signals in the preamp and vernier gain blocks are

differential to improve the PSRR and linearity. A differential

current is fed from the DAC into the output stage, which

amplifies these currents to the appropriate levels necessary to

drive a 75

Ω load.

The output stage utilizes negative feedback to implement a

differential 75

Ω output impedance, which eliminates the need

for external matching resistors needed in typical video (or

video filter) termination requirements.

SPI Programming

The AD8326 is controlled through a serial peripheral interface

(SPI) of three digital data lines, CLK,

DATEN, and SDATA.

Changing the gain requires 8 bits of data to be streamed into the

SDATA port. The sequence of loading the SDATA register

begins on the falling edge of the

DATEN pin, which activates

the CLK line. With the CLK line activated, data on the SDATA

line is clocked into the serial shift register, Most Significant Bit

(MSB) first, on the rising edge of the CLK pulses. Since a 7-bit

shift register is used in the AD8326, the MSB of the 8-bit word

is a “don’t care” bit and is shifted out of the register on the eighth

clock pulse. The data is latched into the attenuator core on the

rising edge of the

DATEN line. This provides control over the

changes in the output signal level. The serial interface timing for

the AD8326 is shown in Figures 2 and 3. The programmable

gain range of the AD8326 is –25.75 dB to +27.5 dB with steps

of 0.75 dB. This provides a total gain range of 53.25 dB. The

AD8326 was characterized with a TOKO transformer (TOKO

#617DB-A0070), and the stated gain values include the losses

due to the transformer.

For gain codes from 0 through 71 the gain transfer function is:

A

dB

CODE

V

=×

[]

27 5

0 75

71

.

– (.

(

–

)

where AV is the gain in dB and CODE is the decimal equivalent

of the 8-bit word. Gain codes 0 to 71 provide linear changes in

gain. Figure 4 shows the gain characteristics of the AD8326 for

all possible values in an 8-bit word. Note that maximum gain is

achieved at Code 71. From Code 72 through 127 the 5.25 dB

of attenuation from the vernier stage is being applied over every

eight codes, resulting in the saw tooth characteristic at the top

of the gain range. Because the eighth bit is shifted out of the

register, the gain characteristics for Codes 128 through 255 are

identical to Codes 0 through 127, as depicted in Figure 4.

28

21

14

7

0

–7

–14

–21

–28

0

32

64

96

128

160

192

224

256

GAIN CODE – Decimal

Figure 4. Gain Code vs. Gain

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