adl5304 Analog Devices, Inc., adl5304 Datasheet - Page 18

• Manufacturer Part Number: adl5304
• Description: High Speed, 200 Db Range, Logarithmic Converter
• Manufacturer: Analog Devices, Inc.

ADL5304
Decibel Scaling
When signal power is expressed in decibels above a reference level
(for example, dBm, when the reference is 1 mW), logarithmic
conversion has already been implicitly performed. Therefore, the
log ratio in the previous expressions becomes a simple difference.
Be careful in assigning variable names, because P is often used to
denote actual power as well as this same power expressed in
decibels. These are very different quantities. Misunderstandings
can be avoided by using D to denote decibel powers.
When V
of a power measurement), it can be written
where:
D
level.
D
Using the previous example and assuming a reference power
of 1 mW, a P
10 log
power of 3.95 fW corresponds to a D
the same result calculated with Equation 12.
CIRCUIT DESCRIPTION
The
to meet the needs of fiber optic supervisory systems, as well as
many nonoptical applications. This section explains the general
structure of this log amp. The
faster than any previous log amp that Analog Devices, Inc., has
made, through careful FET amp design; the key limitation in
the speed at low currents.
Figure 45 is a simplified schematic of the front-end section of
the ADL5304. The numerator current, I
INUM pin. The voltage at this node is equal to that on the two
adjacent guard pins, VSM2 and IDEN, differing only by the offset
voltage of the JFET op amp that supports the operation of the
Translinear Device Q1 that converts the I
logarithmic voltage. VSM2 is needed to provide the collector-
emitter bias for Q1, and it is preset to 1.5 V via the external
connection to Pin 1P5V.
V
Z
Y
OPT
′
denotes the equivalent intercept power relative to the same level.
= V
ADL5304
V
V
is the optical power expressed in decibels above a reference
10
LOG
LOG
(12.5 μW/1 mW) = −19.03 dBm; the equivalent intercept
Y
Y
/10 (because there are 10 dB per decade in the context
= 20 mV(D
= 20 mV{−19.03 − (−114.03)} = 1.900 V
(the volts/decade ) is converted to its decibel value,
OPT
addresses a wide variety of interfacing conditions
of 12.5 μW corresponds to a D
OPT
− D
Z
ADL5304
)
Z
of −114.03 dBm. Therefore,
is an order of magnitude
NUM
NUM
, is received at the
current to a
OPT
of
(13)
(14)
Rev. 0 | Page 18 of 32
In conventional translinear log amps, the collector and base of
Q1 are both held at ground potential, this is not possible in a
single-supply part.
A second transistor, Q2, operates at a collector current of I
In most applications, this is the reference of I
supplied internally and laser trimmed.
The difference between the two V
photodiode current of I
By adding an accurate PTAT voltage of magnitude,
resulting in
The temperature variation of kT/q is then eliminated by an
analog divider that essentially puts a variable proportional to
temperature underneath the T in Equation 17 and raising the
magnitude of kT/q to a stable value of 0.2 V. Therefore, for
photodiode applications,
When the VSM1to VSM4, DCBI, and INPS pins are tied to
ground and V
the more general form.
V
V
V
V
V
BE1
OFS
BE1
LOG
LOG
PD
R
SHIELD
SHIELD
MNTR
− V
− V
= V
= 0.2 V log
= 0.2 V log
BE2
T
BE2
NEG
Figure 45. Simplified Front-End Schematic
log
VSM1
VSM2
VSM3
VSM4
INUM
= V
+ V
IDEN
IREF
< −2 V, the offset (V
10
(I
T
OFS
REF
10
10
2
3
4
5
6
7
8
log
(I
(I
= (kT/q) log
100nA
= V
/I
NUM
PD
NUM
10
Z
1.5V
(I
/I
) = 1.500 V
MONITOR AND
T
/I
PD
Z
= I
30
(1.1× I
[log
)
PD BIAS
DEN
/I
1.5V
IMON
PD
REF
)
10
NUM
is
)
(I
BE
Q1
Q2
10
PD
s with Q1 accepting a
)
VNUM
(I
VDEN
/I
Q3
OFS
PD
REF
/I
) is removed, leaving
) + log
Z
32
9
)
REF
31
10
Data Sheet
V
V
10
INNM
INDN
NUM
= 100 nA,
DEN
(I
REF
COMP
TEMP
VREF
1.5V
26
/I
1P5V
Z
)]
I
DEN
LOG
(15)
(16)
(17)
(18)
(19)
.