AD8362
OPERATOR IN CONTROLLER MODES
In order to fully understand this section, it is important to first
read the preceding discussion of measurement modes, because
there are only a few differences in operation and connections.
When used in controller applications, the basic objective is to
use the AD8362 as a level-sensing element in such a way that its
output, here VAPC, moves in a direction that increases the
controlled signal when the input sample is too low, and vice
versa. A general scheme is shown in Figure 60.
Every value of VSET maps uniquely to a specific rms value at its
input. Thus, the major loop shown in Figure 60 forces the
system being controlled to deliver exactly this level (which may
be either in voltage form or as a sample of the power output).
This mode of operation is therefore just an extension of the
measurement mode, having exactly the same scaling (slope and
intercept) at the VSET pin.
When the system in Figure 60 is an RF power amplifier (PA), a
practical consideration immediately comes to our attention.
Frequently, the gain (and thus output power) is arranged to
increase in response to an increasing positive voltage applied to
the gain control pin. However, the AD8362’s output tends
toward higher values as its input crosses over the level
corresponding to the setpoint, which would cause the PA’s
output to increase further. In other words, the feedback polarity
is reversed, forcing the control loop to latch up at one of its
power extremes.
CONTROLLED SYSTEM
(OUTPUT POWER
DECREASES AS V
APC
INCREASES)
SYSTEM
OUTPUT
SYSTEM
INPUT
OUTPUT
INPUT
V
APC
R
= 100Ω
SH
V
S
FOR 50Ω
TERMINATION
AD8362
COMM ACOM 16
3.3Ω
1
2
3
4
An increasing number of modular PAs feature a control polarity
that reduces the power output with increases in control voltage.
These can be controlled directly from the VOUT pin of the
AD8362. Elsewhere, it is necessary to provide the sign inversion
using a low noise buffer. This amplifier may also include
provisions to ensure that the PA is never driven beyond its safe
limits. The complete details of such a control system depends
on many factors, and this example shows only generic aspects of
the design.
NC
CHPF
DECL
INHI
VREF 15
VTGT 14
0.1µF
1nF
1nF
VPOS
13
R
1nF
SH
5
6
7
8
INLO
VOUT 12
VSET 11
1nF
1nF
SETPOINT
VOLTAGE INPUT
0.5V TO 3.5V
DECL
PWDN ACOM 10
COMM CLPF
9
C
LPF
Figure 60. Generalized Control Loop Using the AD8362
USE OF AN INPUT BALUN
Because the AD8362 integrates any input error relative to the
setpoint, and ideally would fully null this error over an
appropriate time interval, it follows that VAPC swings rail-
to-rail over a very narrow range of inputs. In practice, a few
millidecibels of amplitude deviation at the input fully swing the
output.
A balun (balance to unbalance) is used either to transform
differential RF signals to single-ended form or in reverse to
convert single-sided signals to differential form. A typical balun
consists of a short length of transmission line (miniature coaxial
or twisted pair) through which the signal passes without
significant degradation, wound on a core (often a ferrite) to
generate a series mode inductor having a high reactive
impedance, compared to the through-mode impedance of the
transmission line, which is often 50 Ω.
The signal input level at which this occurs (the setpoint) is
determined by the control voltage, VSET. This voltage defines
the narrow range of the ac input over which the AD8362’s
output is most sensitive to the absolute input magnitude. In base
stations, for example, VSET is often delivered by the ramp DAC,
and the setpoint is a rapidly varying sequence of levels during
the ramp-up and ramp-down intervals of each burst as well as
with output power demand variations from one channel to
another.
High frequency common-mode voltages applied to the input of
this line are sustained across this series reactance and do not
appear at the loaded side of the line. On the other hand, the
through-mode bandwidth is very high, and the losses incurred
in a short line of this sort are trivial.
Baluns of slightly more elaborate construction can provide an
impedance transformation (usually designated by their
impedance ratio, for example, 4:1, which becomes a 1:4 ratio
when used in reverse) in order to convert a single-sided signal
to the balanced form, as is desirable in driving the AD8362,
while also presenting a 50 Ω input interface.
Rev. B | Page 27 of 36
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