AD8315
AVERAGE = 16 SAMPLES
500mV PER
VERTICAL
DIVISION
V
APC
GND
200mV PER
V
APC
VERTICAL
DIVISION
GND
2 s PER
HORIZONTAL
DIVISION
2ꢁs PER
HORIZONTAL
DIVISION
1V PER
VERTICAL
DIVISION
V
AND
1V PER
VERTICAL
DIVISION
S
V
S
V
ENBL
GND
GND
AVERAGE = 16 SAMPLES
TPC 25. Power-On and -Off Response with
TPC 27. Power-On and -Off Response with VSET and
ENBL Grounded
V
SET Grounded
10MHz REF
OUTPUT
10MHz REF
OUTPUT
TRIG
OUT
TRIG
OUT
R AND S
SMT03
R AND S
SMT03
STANFORD DS345
PULSE
GENERATOR
STANFORD DS345
PULSE
GENERATOR
EXTTRIG
EXTTRIG
SIGNAL
SIGNAL
GENERATOR
GENERATOR
PULSE OUT
PULSE OUT
RF OUT
RF OUT
AD811
AD811
49.9
49.9
732
732
AD8315
AD8315
TEK P6205
TEK P6205
1
2
3
4
1
2
3
4
8
7
6
5
8
7
6
5
VPOS
VAPC
NC
VPOS
VAPC
NC
RFIN
ENBL
VSET
FLTR
RFIN
ENBL
VSET
FLTR
FET PROBE
FET PROBE
TRIG
TRIG
52.3
52.3ꢀ
TEK P6205
FET PROBE
TEKTDS694C
SCOPE
TEK P6205
FET PROBE
TEKTDS694C
SCOPE
COMM
COMM
220pF
220pF
NC = NO CONNECT
NC = NO CONNECT
TPC 26. Test Setup for Power-On and -Off Response with
VSET Grounded
TPC 28. Test Setup for Power-On and -Off Response with
VSET and ENBL Grounded
GENERAL DESCRIPTION AND THEORY
of 24 mV/dB was chosen, and the intercept VZ was placed at the
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).
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.
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:
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:
(2)
VOUT = VDB (P – PZ )
IN
where the input power PIN and the equivalent intercept PZ are
both expressed in dBm (thus, the quantity in parentheses is
simply a number of decibels), and VDB is the slope expressed as
so many mV/dB. For a log amp having a slope VDB of 24 mV/dB
and an intercept at –57 dBm, the output voltage for an input
power of –30 dBm is 0.024 [–30 – (–57)] = 0.648 V.
VIN
VZ
VOUT = VSLP log10
(1)
Here VIN is the input voltage, VZ is called the intercept (voltage)
because when VIN = VZ the argument of the logarithm is unity
and thus the result is zero, and VSLP is called the slope (voltage),
which is the amount by which the output changes for a certain
change in the ratio (VIN/VZ). When BASE-10 logarithms are used,
denoted by the function log10, VSLP represents the “volts/decade,”
and since a decade corresponds to 20 dB, VSLP/20 represents the
“volts/dB.” For the AD8315, a nominal (low frequency) slope
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.
–8–
REV. B
ADI [ ADI ]
