AD8311 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
AD8311 Datasheet PDF : 24 Pages
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THEORY OF OPERATION
The AD8311 is a wideband logarithmic amplifier (log amp)
similar in design to the AD8313, AD8314, and AD8315. Like
the AD8315, it is strictly optimized for use in power control
applications rather than as a measurement device. Figure 25
shows the main features in block schematic form. The output
(Pin 2, 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 a 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
VOUT
= VSLP
log 10
VIN
VZ
(1)
where:
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.
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
AD8311
corresponds to 20 dB, VSLP/20 represents the volts/dB. For the
AD8311, a nominal (low frequency) slope of 24 mV/dB was
chosen, and the intercept VZ was placed at −59 dBm for a sine
wave input (251 μV rms). However, both the slope and the
intercept are dependent on frequency.
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
( ) VOUT = VDB PIN − PZ
(2)
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 in
mV/dB. For a log amp having a slope VDB of 24 mV/dB and an
intercept at −59 dBm, the output voltage for an input power of –
30 dBm is 0.024 [−30 − (−57)] = 0.696 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 the 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.
(PRECISE GAIN (PRECISE SLOPE
CONTROL)
CONTROL)
VPOS
LOW NOISE
GAIN BIAS
LOW NOISE
BAND GAP
REFERENCE
RFIN
COM
(CURRENT-MODE SIGNAL)
⋅ 1.35
VAPC
DET
DET
DET
DET
DET
HI-Z
LOW NOISE
RAIL-TO-RAIL BUFFER
10dB
10dB
OFFSET
COMPENSATION
(WEAK GM STAGE)
10dB
10dB
(CURRENT-
NULLING
MODE)
INTERCEPT
POSITIONING
(CURRENT-MODE
FEEDBACK)
VI
(SMALL INTERNAL
FILTER CAPACITOR
FOR GHz RIPPLE)
FLTR
VSET
23mV/dB
250mV TO
1.4V = 50dB
Figure 25. Block Schematic
Rev. A | Page 11 of 24
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