AD8310ARMZ-REEL71 AD [Analog Devices], AD8310ARMZ-REEL71 Datasheet - Page 9

• Manufacturer Part Number: AD8310ARMZ-REEL71
• Description: Fast, Voltage-Out DC-440 MHz, 95 dB Logarithmic Amplifier
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
Logarithmic amplifiers perform a more complex operation than
classical linear amplifiers, and their circuitry is significantly
different. A good grasp of what log amps do and how they do it
can help users avoid many pitfalls in their applications. For a
complete discussion of the theory, see the AD8307 data sheet.
The essential purpose of a log amp is not to amplify (though
amplification is needed internally), but to compress a signal of
wide dynamic range to its decibel equivalent. It is, therefore, a
measurement device. An even better term might be logarithmic
converter, because the function is to convert a signal from one
domain of representation to another via a precise nonlinear
transformation:
where:
V
V
base ten, in which case V
V
V
Log amps implicitly require two references (here V
that determine the scaling of the circuit. The accuracy of a log
amp cannot be any better than the accuracy of its scaling
reference s . In the AD8310, these are provided by a band gap
reference.
While Equation 1, plotted in Figure 21, is fundamentally correct,
a different formula is appropriate for specifying the calibration
attributes or demodulating log amps like the AD8310, operating
in RF applications with a sine wave input.
V
OUT
OUT
Y
IN
X
is the slope voltage. The logarithm is usually taken to
is the intercept voltage.
V
–2V
is the input voltage.
V
4V
OUT
5V
3V
2V
= 0
V
IN
is the output voltage.
V
–40dBc
Y
Y
Y
Y
Y
Y
OUT
= 10
–2
Figure 21. General Form of the Logarithmic Function
V
=
X
V
Y
LOWER INTERCEPT
log
V
⎛
⎜
⎜
⎝
IN
0dBc
V
V
= V
IN
X
Y
X
is also the volts-per-decade.
⎞
⎟
⎟
⎠
V
IN
+40dBc
V
= 10
SHIFT
2
V
X
V
IN
+80dBc
= 10
X
4
V
and V
X
LOG V
Y
)
IN
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(1)
where:
V
output.
V
(25 mV/dB for the AD8310).
P
reference power level.
P
same reference level.
A widely used referenc
50 Ω, a level of 0 dBm. Note that the quantity ( P – P ) is dB.
The logarithmic function disappears from the formula, becaus
the conversion has already been implicitly performed in stating
the input in decibels. This is strictly a concession to popular
convention. Log amps manifestly do not respond to power
(tacitly, power absorbed at the input), but rather to input
voltage. The input is specified in dBV (decibels with respect t
1 V rms) throughout this data sheet. This is more precise,
although still incomplete, because the signal waveform is also
involved. Many users specify RF signals in terms of power
(usually in dBm/50 Ω) and this convention is used in this data
sheet when specifying the performance of the AD8310.
PROGRESSIVE COMPRESSION
High speed, high dynamic-range log am
nonlinear amplifier cells to generate the logarithmic functio
a series of contiguous segments, a type of piecewise linear
technique. The AD8310 employs six cells in its main signal path,
each having a small-signal gain of 14.3 dB (×5.2) and a −3 dB
bandwidth of about 900 MHz. The overall gain is about 20,000
(86 dB) and the overall bandwidth of the chain is approximatel
500 MHz, resulting in a gain-bandwidth product (GBW) of
10,000 GHz, about a million times that of a typical op amp. This
very high GBW is essential to accurate operation under small-
signal conditions and at high frequencies. The AD8310 exhibits
a logarithmic response down to inputs as small as 40 µV
at 440 MHz.
Progressive co
video response or accept an RF input and demodulate this
signal to develop an output that is essentially the envelope of t
input represented on a logarithmic or decibel scale. The
AD8310 is the latter kind. Demodulation is performed in a total
of nine detector cells. Six are associated with the amplifier
stages, and three are passive detectors that receive a progres-
sively attenuated fraction of the full input. The maximum sig
frequency can be 440 MHz, but, because all the gain stages ar
dc-coupled, operation at very low frequencies is possible.
IN
O
OUT
SLOPE
is the logarithmic intercept, expressed in dB relative t
is the input power, expressed in dB relative to some
V
is th
is the logarithmic slope, now expressed in V/dB
OUT
e demodulated and filtered baseband (video or RSSI)
=
V
mpression log amps either provide a baseband
SLOPE
(
P
e in RF systems is dB above 1 mW in
IN
−
P
O
)
ps use a cascade of
IN
AD8310
O
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