AD8321

Varying the Gain and SPI Programming

The gain of the AD8321 can be varied over a range of 53 dB from

approximately –27 dB to +26 dB, in increments of approximately

0.7526 dB per LSB. Programming the gain of the AD8321 is

accomplished using conventional Serial Peripheral Interface or

SPI protocol. Three digital lines,

DATEN, CLK and SDATA,

are used to stream eight bits of data into the serial shift register

of the AD8321. Changing the state of the

DATEN port from

Logic 1-to-0 starts the load sequence by activating the CLK

line. No changes in output signal are realized during this transi­

tion. Subsequently, any data applied to SDATA is clocked into

the serial shift register Most Significant Bit (MSB) first and on

the rising edge of each CLK pulse. The AD8321 may be pro­

grammed to deliver maximum gain (+26 dB) at decimal code

71. As a result, only the last seven bits of a typical 8-bit SPI

word effect the gain resulting in the gain response depicted in

Figure 22. Since the SPI codes from 0 through 71 appear digi­

tally identical to codes 128 through 199 for all bits except the

MSB, the AD8321 repeats the gain vs. decimal code response

twice in the 256 available codes (see Operational Description for

gain equations and Figure 23 for Gain Response). The MSB of

a typical SPI word (i.e., the first data bit presented to the SDATA

line after the

DATEN transition from Logic 1 to 0 and prior to

the rising edge of the first clock pulse) is disregarded or ignored.

Data enters the serial shift register through the SDATA port

on the rising edge of the next seven CLK pulses. Returning the

DATEN line to Logic 1 latches the content of the shift register

into the attenuator core resulting in a well controlled change in

output signal level. The timing diagram for AD8321’s serial

interface is shown in Figure 24.

Gain Dependence on Load Impedance

The AD8321 has a dynamic output impedance of 75

W. This

dynamic output impedance is trimmed to provide a maximum

gain of +26 dB when loaded with 75

W. Operating the AD8321

at load impedances other than 75

W will only change the gain of

the AD8321 while the specified gain range of 53 dB is unchanged.

Varying the load impedance will result in 6 dB of additional gain

following equation:

Gain (dB) = [20 log ((2

(71-Code)))

35

30

25

20

15

10

5

0

0

100

200

300

400

500

Between Burst On/Off Transients, Asynchronous Power-

Down and DOCSIS

A 42% reduction in consumed power may be achieved asynchro­

nously by applying Logic 0 to

PD Pin 6 activating the on-chip

“reverse amplifier.” The supply current is then reduced to

approximately 52 mA and the modem can no longer transmit in

the upstream direction. The on-chip reverse amplifier is designed

to reduce “between burst noise” and maintain a 75

W source

impedance to the low pass port of the modem’s diplexer while

minimizing power consumption. Changing the logic level applied

to the

PD pin will result in a Burst On/Off Transient at the

output of the AD8321. The transient results from switching

between the forward transmit amplifier and the powered down

(reverse) amplifier. Although the resulting transient meets the

DOCSIS transient amplitude requirements at maximum gain, it

is the lower gain range (i.e., 8 dBmV to 31 dBmV) where the

AD8321 may exceed the 7 mV maximum. The diplexer may

further reduce the glitch amplitude. An external RF switch, such

as Alpha Industries AS128-73 GaAs 2 Watt High Linearity

SPDT RF switch, may be used to further reduce the spurious

emissions, improve the isolation between the cable plant and the

upstream line driver and switch in a 75

W back termination

required to maintain proper line termination to the LP port of

the diplexer (see Figure 28).

Noise and DOCSIS

One of the most difficult issues facing designers of DOCSIS

compliant modems is maintaining a quiet output from the PA

during times when no information is being transmitted upstream.

In addition, maintaining proper signal-to-noise ratios serves to

ensure the quality of transmitted data. This is extremely critical

when the output signal of the modem is set to the minimum

DOCSIS specified output level or 8 dBmV. The AD8321 output

noise spectral density at minimum gain (or 8 dBmV) is 20 nV/

÷Hz

measured at 10 MHz. Considering the “Spurious Emissions

in 5 MHz to 42 MHz” of Table 4–8 in DOCSIS, the calculated

Ê

Ê

2

ˆˆ

Á

20 log

Á Ë

20 nV / Hz ¯ ¥160E + 3˜˜ +60 or - 41 5

Á

Á Ê

ˆ

˜˜

. dBmV

Ë

Ë

¯¯

Comparing the computed noise power to the signal at 8 dBmV

yields –49.5 dBc or 3.5 dB higher than the required –53 dBc in

DOCSIS Table 4–8. An attenuator designed to match the

AD8321 75

W source to the 75 W load may be required. Refer­

ring to the schematic of Figure 28 and the evaluation board

silkscreen of Figure 31, the matching attenuator is comprised of

the three resistors referred to as Rc, Rd and Re. Select the at­

tenuation level from Table I such that noise floor is reduced to

levels specified in DOCSIS.

Table I.

Rc (

�)

Rd (

�)

Re (

�)

Attenuation (dB)

1304

8.65

1304

–1

654.3

17.42

654.3

–2

432

26.1

432

–3

331.5

35.75

331.5

–4

–10–

REV. A