AD8307-EB Analog Devices Inc, AD8307-EB Datasheet - Page 11

• Manufacturer Part Number: AD8307-EB
• Manufacturer: Analog Devices Inc

Specifications of AD8307-EB

• Lead Free Status / Rohs Status: Not Compliant

Further analysis shows that right up to the point where the input to
the first cell is above the knee voltage, V
for a ratio change of A in V
fraction of a decade, which is simply log
when A = 5, a transition in the piecewise linear output function
occurs at regular intervals of 0.7 decade (log
divided by 20 dB). This insight allows the user to immediately
write the volts per decade scaling parameter, which is also the
scaling voltage, V
Note that only two design parameters are involved in determining
V
the number of stages, is unimportant in setting the slope of the
overall function. For A = 5 and E
a rather awkward 572.3 mV per decade (28.6 mV/dB). A well
designed log amp has rational scaling parameters.
The intercept voltage can be determined by using two pairs of
transition points on the output function (consider Figure 24).
The result is
For the case under consideration, using N = 6, calculate V
4.28 μV. However, be careful about the interpretation of this
parameter, because it was earlier defined as the input voltage at
which the output passes through zero (see Figure 21). Clearly,
in the absence of noise and offsets, the output of the amplifier
chain shown in Figure 23 can be zero when, and only when,
V
amplifier, which results in a failure to maintain the logarithmic
approximation below the lin-log transition (labeled 1 in Figure 24).
Closer analysis shows that the voltage given by Equation 5
represents the extrapolated, rather than actual, intercept.
DEMODULATING LOG AMPS
Log amps based on a cascade of A/1 cells are useful in baseband
applications because they do not demodulate their input signal.
However, baseband and demodulating log amps alike can be
made using a different type of amplifier stage, called an A/0 cell.
Its function differs from that of the A/1 cell in that the gain
above the knee voltage E
line in Figure 25. This is also known as the limiter function, and
a chain of N such cells are often used to generate hard-limited
output in recovering the signal in FM and PM modes.
Y
IN
, namely, the cell gain A and the knee voltage, E
= 0. This anomaly is due to the finite gain of the cascaded
V
V
Y
X
=
=
Decades
Linear
A
(
N
+
E
/ 1
K
Y
(
Change
A
, when using base 10 logarithms, as
Change
−
1
)
)
K
falls to zero, as shown by the solid
in
IN
in
V
. This can be expressed as a certain
OUT
V
IN
K
= 100 mV, the slope would be
=
(
log
OUT
A
10
−
10
changes by (A − 1)E
(A). For example,
1
(
)
A
E
10
K
)
(A), or 14 dB
K
, while N,
Z
=
Rev. D | Page 11 of 24
K
(4)
(5)
The AD640, AD606, AD608, AD8307, and various other Analog
Devices, Inc., communications products incorporating a logarith-
mic IF amplifier all use this technique. It becomes apparent that
the output of the last stage can no longer provide the logarithmic
output because this remains unchanged for all inputs above the
limiting threshold, which occurs at V
logarithmic output is now generated by summing the outputs of
all the stages. The full analysis for this type of log amp is only
slightly more complicated than that of the previous case. It is
readily shown that, for practical purposes, the intercept voltage,
V
voltage is
Preference for the A/0 style of log amp over one using A/1 cells
stems from several considerations. The first is that an A/0 cell
can be very simple. In the AD8307, it is based on a bipolar
transistor differential pair, having resistive loads, R
emitter current source, I
voltage of E
The large signal transfer function is the hyperbolic tangent
(see the dashed line in Figure 25). This function is very precise,
and the deviation from an ideal A/0 form is not detrimental. In
fact, the rounded shoulders of the tanh function result in a
lower ripple in the logarithmic conformance than that obtained
using an ideal A/0 function.
An amplifier composed of these cells is entirely differential
in structure and can thus be rendered very insensitive to
disturbances on the supply lines and, with careful design, to
temperature variations. The output of each gain cell has an
associated transconductance (g
tial output voltage of the cell to a pair of differential currents,
which are summed simply by connecting the outputs of all the
g
back to a voltage by a transresistance stage to generate the
logarithmic output. This scheme is depicted in single-sided
form in Figure 26.
m
X
, is identical to that given in Equation 5, while the slope
(detector) stages in parallel. The total current is then converted
V
A/0
Y
=
Figure 25. A/0 Amplifier Functions (Ideal and Tanh)
log
K
AE
= 2 kT/q and a small signal gain of A = I
10
( )
K
A
AE
0
K
E
. This exhibits an equivalent knee
E
K
m
) cell that converts the differen-
tanh
SLOPE = A
IN
SLOPE = 0
= E
K
/A
N − 1
. Instead, the
INPUT
L
, and an
AD8307
E
R
L
/E
K
.
(6)