AD8315ARM Analog Devices, AD8315ARM Datasheet - Page 8

• Manufacturer Part Number: AD8315ARM
• Description: Controllers, 50dB GSM PA Controller
• Manufacturer: Analog Devices

AD8315
GENERAL DESCRIPTION AND THEORY
The AD8315 is a wideband logarithmic amplifier (log amp)
similar in design to the AD8313 and AD8314. However, it is
strictly optimized for use in power control applications rather
than as a measurement device. Figure 1 shows the main features
in block schematic form. The output (Pin 7, VAPC) is intended
to be applied directly to the automatic power-control (APC) pin
of a power amplifier module.
Basic Theory
Logarithmic amplifiers provide a type of compression in which a
signal having a large range of amplitudes is converted to one of
smaller range. The use of the logarithmic function uniquely results
in the output representing the decibel value of the input. The
fundamental mathematical form is:
Here V
because when V
and thus the result is zero, and V
which is the amount by which the output changes for a certain
change in the ratio (V
denoted by the function log
and since a decade corresponds to 20 dB, V
“volts/dB.” For the AD8315, a nominal (low frequency) slope
TPC 26. Test Setup for Power-On and -Off Response with
VSET Grounded
52.3
GENERATOR
V
R AND S
220pF
SIGNAL
OUT
SMT03
IN
TPC 25. Power-On and -Off Response with
V
SET
is the input voltage, V
RF OUT
=
GND
GND
V
Grounded
SLP
1
2
3
4
NC = NO CONNECT
10MHz REF
OUTPUT
IN
RFIN
ENBL
VSET
FLTR
V
AND
V
ENBL
V
log
APC
= V
AD8315
S
IN
10
Z
/V
COMM
V
VPOS
VAPC
V
the argument of the logarithm is unity
Z
IN
Z
). When BASE-10 logarithms are used,
NC
10
, V
VERTICAL
DIVISION
8
7
6
5
200mV PER
1V PER
VERTICAL
DIVISION
SLP
Z
AVERAGE = 16 SAMPLES
EXT TRIG
SLP
is called the intercept (voltage)
represents the “volts/decade,”
FET PROBE
FET PROBE
is called the slope (voltage),
TEK P6205
TEK P6205
AD811
732
STANFORD DS345
2 s PER
HORIZONTAL
DIVISION
GENERATOR
SLP
PULSE
/20 represents the
PULSE OUT
49.9
TEK TDS694C
SCOPE
TRIG
TRIG
OUT
(1)
–8–
TPC 28. Test Setup for Power-On and -Off Response with
VSET and ENBL Grounded
of 24 mV/dB was chosen, and the intercept V
equivalent of –70 dBV for a sine wave input (316 mV rms). This
corresponds to a power level of –57 dBm when the net resistive
part of the input impedance of the log amp is 50 W. However,
both the slope and the intercept are dependent on frequency (see
TPC 13 and TPC 16).
Keeping in mind that log amps do not respond to power but
only to voltages and that the calibration of the intercept is
waveform dependent and is only quoted for a sine wave signal,
the equivalent power response can be written as:
where the input power P
both expressed in dBm (thus, the quantity in parentheses is
simply a number of decibels), and V
so many mV/dB. For a log amp having a slope V
and an intercept at –57 dBm, the output voltage for an input
power of –30 dBm is 0.024 [–30 – (–57)] = 0.648 V.
Further details about the structure and function of log amps can
be found in data sheets for other log amps produced by Analog
Devices. Refer to data sheets for the AD640 and AD8307, both
of which include a detailed discussion of the basic principles of
operation and explain why the intercept depends on waveform,
an important consideration when complex modulation is
imposed on an RF carrier.
TPC 27. Power-On and -Off Response with V
ENBL Grounded
52.3
GENERATOR
V
R AND S
220pF
SIGNAL
OUT
SMT03
RF OUT
=
GND
GND
V
DB
1
2
3
4
NC = NO CONNECT
10MHz REF
OUTPUT
RFIN
ENBL
FLTR
VSET
(
V
P
APC
V
AD8315
IN
S
–
COMM
AVERAGE = 16 SAMPLES
VPOS
VAPC
P
IN
Z
NC
500mV PER
and the equivalent intercept P
VERTICAL
)
DIVISION
VERTICAL
DIVISION
8
7
6
5
1V PER
EXT TRIG
DB
FET PROBE
FET PROBE
TEK P6205
TEK P6205
732
AD811
is the slope expressed as
STANFORD DS345
2 s PER
HORIZONTAL
DIVISION
GENERATOR
Z
PULSE
was placed at the
PULSE OUT
DB
49.9
TEK TDS694C
of 24 mV/dB
SET
SCOPE
TRIG
and
TRIG
OUT
REV. B
Z
are
(2)