AD8307ANZ Analog Devices Inc, AD8307ANZ Datasheet - Page 10

• Manufacturer Part Number: AD8307ANZ
• Description: IC LOGARITHMIC AMP 8-DIP
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Amplifierr

Specifications of AD8307ANZ

• Applications: Receiver Signal Strength Indication (RSSI)
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• No. Of Amplifiers: 1
• Dynamic Range, Decades: 4.6
• Scale Factor V / Decade: 0.5
• Response Time: 500ns
• Supply Voltage Range: 2.7V To 5.5V
• Supply Current: 8mA
• Input Offset Voltage: 50µV
• Amplifier Type: Logarithmic
• Bandwidth: 900 MHz
• Current, Input Bias: 10 μA
• Current, Supply: 8 mA
• Package Type: PDIP-8
• Resistance, Input: 1.1 Kilohms
• Temperature, Operating, Range: -40 to +85 °C
• Voltage, Input: 7.5 V
• Voltage, Input Offset: 50 mV
• Voltage, Noise: 1.5 nV/sqrt Hz
• Voltage, Supply: 2.7 to 5.5 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

AD8307
just dB. The logarithmic function disappears from the formula
because 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 use of dBV (decibels with respect to 1 V rms) is more precise,
though still incomplete, because waveform is involved as well.
Because most users think about and specify RF signals in terms
of power, more specifically, in dBm re: 50 Ω, this convention is
used in specifying the performance of the AD8307.
PROGRESSIVE COMPRESSION
Most high speed, high dynamic range log amps use a cascade of
nonlinear amplifier cells (see Figure 22) to generate the logarithmic
function from a series of contiguous segments, a type of piecewise
linear technique. This basic topology immediately opens up the
possibility of enormous gain bandwidth products. For example,
the AD8307 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 some 500 MHz, resulting
in the incredible gain bandwidth product (GBW) of 10,000 GHz,
about a million times that of a typical op amp. This very high
GBW is an essential prerequisite for accurate operation under
small signal conditions and at high frequencies. In Equation 2,
however, the incremental gain decreases rapidly as V
The AD8307 continues to exhibit an essentially logarithmic
response down to inputs as small as 50 μV at 500 MHz.
To develop the theory, first consider a scheme slightly different
from that employed in the AD8307, but simpler to explain and
mathematically more straightforward to analyze. This approach
is based on a nonlinear amplifier unit, called an A/1 cell, with
the transfer characteristic shown in Figure 23.
The local small signal gain δV
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
large signal gain has a value of A for inputs in the range −E
V
inputs. In logarithmic amplifiers based on this amplifier function,
both the slope voltage and the intercept voltage must be traceable
to the one reference voltage, E
analysis, the calibration accuracy of the log amp is dependent
solely on this voltage. In practice, it is possible to separate the
basic references used to determine V
the AD8307, V
whereas V
temperature corrected.
IN
V
≤ +E
X
K
, but falls asymptotically toward unity for very large
X
STAGE 1
is derived from the thermal voltage kT/q and is later
A
Figure 22. Cascade of Nonlinear Gain Cells
Y
is traceable to an on-chip band gap reference,
STAGE 2
A
OUT
K
K
. Therefore, in this fundamental
, above which the incremental
/δV
STAGE N–1
Y
IN
A
and V
is A, maintained for all
IN
X
less than –E
and, in the case of
STAGE N
A
IN
increases.
K
V
K
. The
W
Rev. D | Page 10 of 24
≤
Let the input of an N-cell cascade be V
be V
six-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
gain in implementing the logarithmic function has been noted;
however, this parameter is only of incidental interest in the design
of log amps.
From this point forward, rather than considering gain, analyze
the overall nonlinear behavior of the cascade in response to a
simple dc input, corresponding to the V
very small inputs, the output from the first cell is V
The output from the second cell is V
V
V
and because there are N − 1 cells of Gain A ahead of this node,
calculate V
the lin-log transition (labeled 1 in Figure 24). Below this input,
the cascade of gain cells acts as a simple linear amplifier, whereas
for higher values of V
lie on a logarithmic approximation (dotted line).
Continuing this analysis, the next transition occurs when the
input to the N − 1 stage just reaches E
E
easily demonstrated (from the function shown in Figure 23) that
the output of the final stage is (2A − 1)E
Thus, the output has changed by an amount (A − 1)E
change in V
At the next critical point (labeled 3 in Figure 24), the input is
again A times larger and V
is, by another linear increment of (A − 1)E
K
N
N − 1
/A
(4A–3) E
(3A–2) E
(2A–1) E
= A
OUT
A/1
, is exactly equal to the knee voltage E
N − 2
V
AE
OUT
N
. For small signals, the overall gain is simply A
. The output of this stage is then exactly AE
V
K
K
K
K
0
IN
IN
IN
. At a certain value of V
= E
from E
AE
(A–1) E
K
0
K
Figure 23. A/1 Amplifier Function
/A
Figure 24. First Three Transitions
K
N − 1
/A
IN
K
E
, it enters into a series of segments that
N − 1
. This unique situation corresponds to
K
1
/A
N–1
OUT
to E
E
K
has increased to (3A − 2)E
K
E
/A
K
/A
2
SLOPE = A
N − 2
N–2
RATIO
IN
OF A
2
, that is, a ratio change of A.
, the input to the Nth cell,
= A
K
K
SLOPE = 1
IN
, that is, when V
E
(labeled 2 in Figure 24).
IN
K
, and the final output
/A
2
3
of Equation 1. For
N–3
K
K
V
.
. Thus, V
IN
, and so on, up to
E
K
/A
3
2
INPUT
N–4
1
K
= AV
, and it is
OUT
LOG V
K
N
for a
IN
. A
= AE
K
=
IN
IN
, that
.
K