AD8321
Varying the Gain and SPI Programming
Between Burst On/Off Transients, Asynchronous Power-
Down and DOCSIS
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.
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
noise power in dBmV for 160 KSYM/SEC is:
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
when RLOAD approaches infinity. The relationship between
RLOAD and gain is depicted in Figure 26 and is described by the
following equation:
Ê
Á
ˆ
˜
Ê
Á
ˆ
Ê
Ë
ˆ2
˜
20log
20nV / Hz ¥160E + 3 + 60 or - 41.5 dBmV
¯
Á
Ë
˜
Á
Ë
˜
¯
¯
Gain (dB) = [20 log ((2 ¥ RLOAD)/(RLOAD +75))]+(26–(0.7526 ¥
(71-Code)))
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.
35
30
25
20
15
10
5
Table I.
Rc (�)
Rd (�)
Re (�)
Attenuation (dB)
1304
654.3
432
8.65
17.42
26.1
1304
654.3
432
–1
–2
–3
–4
0
0
100
200
300
– �
400
500
331.5
35.75
331.5
R
LOAD
Figure 26. Maximum Gain vs. RLOAD
REV. A
–10–
ADI [ ADI ]
