AD8315
THEORY OF OPERATION
when the net resistive part of the input impedance of the log
amp is 50 Ω. However, both the slope and the intercept are
dependent on frequency (see Figure 15 and Figure 18).
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 31 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
VOUT = VDB (PIN − PZ)
where:
(2)
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:
PIN, the input power, and PZ, the equivalent intercept, are both
expressed in dBm (thus, the quantity in parentheses is simply a
number of decibels).
VIN
VZ
VDB is the slope expressed as so many mV/dB.
VOUT =VSLP log10
(1)
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.
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 of 24 mV/dB was chosen, and the intercept VZ
was placed at the equivalent of −70 dBV for a sine wave input
(316 μV rms). This corresponds to a power level of −57 dBm
Further details about the structure and function of log amps can
be found in data sheets for other log amps produced by Analog
Devices, Inc. Refer to the AD640 data sheet and AD8307 data
sheet, 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.
(PRECISE GAIN
CONTROL)
(PRECISE SLOPE
CONTROL)
(ELIMINATES
GLITCH)
VPOS
ENBL
LOW NOISE
BAND GAP
REFERENCE
OUTPUT
ENABLE
DELAY
LOW NOISE
GAIN BIAS
(CURRENT-MODE SIGNAL)
DET
VAPC
×1.35
HI-Z
DET
DET
DET
DET
LOW NOISE (25nV/√Hz)
RAIL-TO-RAIL BUFFER
RFIN
FLTR
VSET
10dB
10dB
10dB
10dB
(CURRENT-
NULLING
MODE)
(CURRENT-MODE
FEEDBACK)
V-I
23mV/dB
250mV TO
1.4V = 50dB
OFFSET
COMP’N
INTERCEPT
POSITIONING
(SMALL INTERNAL
FILTER CAPACITOR
FOR GHz RIPPLE)
COMM
(PADDLE)
(WEAK GM STAGE)
Figure 31. Block Schematic
Rev. C | Page 12 of 24
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