AD8309ARU Analog Devices Inc, AD8309ARU Datasheet - Page 9

• Manufacturer Part Number: AD8309ARU
• Description: IC LOGARITHM AMP 100DB 16-TSSOP
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Amplifierr

Specifications of AD8309ARU

• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP
• Rohs Status: RoHS non-compliant
• Applications: Receiver Signal Strength Indication (RSSI)
• No. Of Amplifiers: 1
• No. Of Pins: 16
• Peak Reflow Compatible (260 C): No
• Bandwidth: 500MHz
• Leaded Process Compatible: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

labeled
cells is acting as a simple linear amplifier, while for higher values
of V
mic approximation.
Continuing this analysis, we find that the next transition occurs
when the input to the (N–1)th stage just reaches E
when V
AE
Figure 21) that the output of the final stage is (2A–1)E
beled
amount (A–1)E
that is, a ratio change of A.
At the next critical point, labeled , the input is A times larger
and V
increment of (A–1)E
the point where the input to the first cell reaches the knee volt-
age, V
Expressed as a certain fraction of a decade, this is simply log
For example, when A = 5 a transition in the piecewise linear
output function occurs at regular intervals of 0.7 decade (log10(A),
or 14 dB divided by 20 dB). This insight allows us to immedi-
ately state the “Volts per Decade” scaling parameter, which is
also the “Scaling Voltage” V
Note that only two design parameters are involved in determin-
ing V
N, the number of stages, is unimportant in setting the slope of
the overall function. For A = 5 and E
would be a rather awkward 572.3 mV per decade (28.6 mV/dB).
A well designed practical log amp will provide more rational
scaling parameters.
The intercept voltage can be determined by solving Equation
(4) for any two pairs of transition points on the output function
(see Figure 22). The result is:
For the example under consideration, using N = 6, V
to 4.28 V, which thus far in this analysis is still a simple dc
voltage.
REV. B
K
(4A-3) E
(3A-2) E
(2A-1) E
IN
. It is easily demonstrated (from the function shown in
V
V
Y
, it enters into a series of segments which lie on a logarith-
OUT
OUT
Y
X
, namely, the cell gain A and the knee voltage E
AE
V
IN
OUT
on Figure 22). Thus, the output has changed by an
K
K
K
K
0
= E
on Figure 22. Below this input, the cascade of gain
Linear Change inV
Decades Change inV
has increased to (3A–2)E
A
changes by (A–1)E
Figure 22. The First Three Transitions
(
N
K
E
1
/A
K
K
/[ – ])
(A-1) E
A
N–2
for a change in V
1
. The output of this stage is then exactly
K
. Further analysis shows that, right up to
K
E
K
/A
N–1
Y
OUT
K
when using base-10 logarithms:
IN
E
for a ratio change of A in V
K
/A
N–2
IN
( – )
RATIO
OF A
K
log ( )
A
, that is, by another linear
from E
K
E
10
= 100 mV, the slope
1
K
/A
E
A
N–3
K
K
/A
E
K
N–1
/A
N–4
K
, that is,
to E
X
K
LOG V
K
, while
evaluates
K
(la-
10
/A
IN
IN
(A).
N–2
.
(4)
(5)
,
–9–
Care is needed in the interpretation of this parameter. It was
earlier defined as the input voltage at which the output passes
through zero (see Figure 19). Clearly, in the absence of noise
and offsets, the output of the amplifier chain shown in Figure 20
can only be zero when V
gain of the cascaded amplifier, which results in a failure to main-
tain the logarithmic approximation below the “lin-log transition”
(Point
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
(pulse) applications, because they do not demodulate their input
signal. Demodulating (detecting) log-limiting amplifiers such as
the AD8309 use a different type of amplifier stage, which we
will call an A/0 cell. Its function differs from that of the A/1 cell
in that the gain above the knee voltage E
by the solid line in Figure 23. This is also known as the limiter
function, and a chain of N such cells is often used alone to
generate a hard limited output, in recovering the signal in FM
and PM modes.
The AD640, AD606, AD608, AD8307, AD8309, AD8313 and
other Analog Devices communications products incorporating a
logarithmic IF amplifier all use this technique. It will be appar-
ent that the output of the last stage cannot now provide a loga-
rithmic output, since this remains unchanged for all inputs
above the limiting threshold, which occurs at V
Instead, the logarithmic output is 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 can be shown that, for practical purpose, the intercept voltage
V
voltage is:
An A/0 cell can be very simple. In the AD8309 it is based on a
bipolar-transistor differential pair, having resistive loads R
an emitter current source I
an equivalent knee-voltage of E
gain of A = I
hyperbolic tangent (see dotted line in Figure 23). 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 func-
tion beneficially result in a lower ripple in the logarithmic con-
formance than that which would be obtained using an ideal A/0
function. A practical amplifier chain built of these cells is differ-
ential in structure from input to final output, and has a low
X
is identical to that given in Equation (5), while the slope
Figure 23. A/0 Amplifier Functions (Ideal and tanh)
V
Y
in Figure 22). Closer analysis shows that the voltage
log ( )
A/0
A E
E
10
R
K
L
A
/E
K
AE
. The large signal transfer function is the
K
0
IN
= 0. This anomaly is due to the finite
E
. This amplifier limiter cell exhibits
E
K
K
SLOPE = 0
= 2kT/q and a small-signal
SLOPE = A
K
falls to zero, as shown
INPUT
IN
AD8309
= E
K
/A
N–1
L
and
.
(6)