AD8309ARU Analog Devices Inc, AD8309ARU Datasheet - Page 8

• 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

AD8309
As a consequence of this high gain, even very small amounts of
thermal noise at the input of a log amp will cause a finite output
for zero input, resulting in the response line curving away from
the ideal (Figure 19) at small inputs, toward a fixed baseline.
This can either be above or below the intercept, depending on
the design. Note that the value specified for this intercept is
invariably an extrapolated one: the RSSI output voltage will never
attain a value of exactly zero in a single supply implementation.
Voltage (dBV) and Power (dBm) Response
While Equation 1 is fundamentally correct, a simpler formula is
appropriate for specifying the RSSI calibration attributes of a
log amp like the AD8309, which demodulates an RF input. The
usual measure is input power:
V
logarithmic slope, expressed in volts/dB, P
expressed in decibels relative to some reference power level and
P
the same reference level.
The most widely used convention in RF systems is to specify
power in decibels above 1 mW in 50 , written dBm. (However,
that the quantity [P
function disappears from this formula because the conversion
has already been implicitly performed in stating the input in
decibels.
Specification of log amp input level in terms of power is strictly
a concession to popular convention: they do not respond to
power (tacitly “power absorbed at the input”), but to the input
voltage. In this connection, note that the input impedance of the
AD8309 is much higher that 50 , allowing the use of an im-
pedance transformer at the input to raise the sensitivity, by up
to 13 dB.
The use of dBV, defined as decibels with respect to a 1 V rms sine
amplitude, is more precise, although this is still not unambiguous
complete as a general metric, because waveform is also involved
in the response of a log amp, which, for a complex input (such
as a CDMA signal) will not follow the rms value exactly. Since
most users specify RF signals in terms of power—more specifi-
cally, in dBm/50 —we use both dBV and dBm in specifying
the performance of the AD8309, showing equivalent dBm levels
for the special case of a 50
Progressive Compression
High speed, high dynamic range log amps use a cascade of
nonlinear amplifier cells (Figure 20) to generate the logarithmic
function from a series of contiguous segments, a type of piece-
wise-linear technique. This basic topology offers enormous gain-
bandwidth products. For example, the AD8309 employs in its
main signal path six cells each having a small-signal gain of
12.04 dB ( 4) and a –3 dB bandwidth of 850 MHz, followed by
a final limiter stage whose gain is typically 18 dB. The overall
gain is thus 100,000 (100 dB) and the bandwidth to –10 dB
point at the limiter output is 525 MHz. This very high gain-
bandwidth product (52,500 GHz) is an essential prerequisite to
accurate operation under small signal conditions and at high
frequencies: Equation (2) reminds us that the incremental gain
decreases rapidly as V
rithmic response over most of the range from the noise floor of
–91 dBV, or 28 V rms, (or –78 dBm/50 ) to a breakdown-
limited peak input of 4 V (requiring a balanced drive at the
differential inputs INHI and INLO).
0
OUT
is the logarithmic intercept, expressed in decibels relative to
V
is the demodulated and filtered RSSI output, V
OUT
= V
SLOPE
IN
(P
IN
IN
– P
increases. The AD8309 exhibits a loga-
– P
0
] is simply dB). The logarithmic
0
environment.
)
IN
is the input power,
SLOPE
is the
(3)
–8–
Theory of Logarithmic Amplifiers
To develop the theory, we will first consider a somewhat differ-
ent scheme to that employed in the AD8309, but which is sim-
pler to explain, and mathematically more straightforward to
analyze. This approach is based on a nonlinear amplifier unit,
which we may call an A/1 cell, having the transfer characteristic
shown in Figure 21. We here use lowercase variables to define
the local inputs and outputs of these cells, reserving uppercase
for external signals.
The small signal gain V
inputs up to the knee voltage E
gain drops to unity. The function is symmetrical: the same drop
in gain occurs for instantaneous values of V
The large signal gain has a value of A for inputs in the range
–E
large inputs.
In logarithmic amplifiers based on this simple function, both the
slope voltage and the intercept voltage must be traceable to the
one reference voltage, E
sis, the calibration accuracy of the log amp is dependent solely on
this voltage. In practice, it is possible to separate the basic refer-
ences used to determine V
able to an on-chip band-gap reference, while V
the thermal voltage kT/q and later temperature-corrected by a
precise means.
Let the input of an N-cell cascade be V
V
stage system in which A = 5 (14 dB) has an overall gain of
15,625 (84 dB). The importance of a very high small-signal ac
gain in implementing the logarithmic function has already been
noted. However, this is a parameter of only incidental interest in
the design of log amps; greater emphasis needs to be placed on
the nonlinear behavior.
Thus, rather than considering gain, we will analyze the overall
nonlinear behavior of the cascade in response to a simple dc
input, corresponding to the V
inputs, the output from the first cell is V
second, V
value of V
the knee voltage E
cells of gain A ahead of this node, we can calculate that V
E
OUT
K
K
/A
V
< V
X
. For small signals, the overall gain is simply A
N–1
A/1
. This unique point corresponds to the lin-log transition,
Figure 20. Cascade of Nonlinear Gain Cells
IN
STAGE 1
2
IN
A
= A
< +E
Figure 21. The A/1 Amplifier Function
, the input to the Nth cell, V
2
V
K
, but falls asymptotically toward unity for very
IN
K
AE
. Thus, V
, and so on, up to V
0
K
STAGE 2
A
K
OUT
. Therefore, in this fundamental analy-
Y
and V
/ V
OUT
IN
K
E
of Equation (1). For very small
IN
K
, above which the incremental
= AE
X
is A, and is maintained for
STAGE N –1
. In the AD8309, V
SLOPE = A
A
K
SLOPE = 1
IN
N
1
N–1
and since there are N–1
, and the final output
= AV
= A
, is exactly equal to
IN
N
less than –E
STAGE N
X
IN
V
A
is derived from
IN
; from the
. At a certain
INPUT
N
. A six-
Y
REV. B
is trace-
V
IN
K
W
.
=