AD8304ARU-REEL Analog Devices Inc, AD8304ARU-REEL Datasheet - Page 8

• Manufacturer Part Number: AD8304ARU-REEL
• Description: IC LOGARITHMIC CONV 14-TSSOP T/R
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Converterr

Specifications of AD8304ARU-REEL

• Rohs Status: RoHS non-compliant
• Design Resources: Interfacing ADL5315 to Translinear Logarithmic Amplifier (CN0056) Interfacing ADL5317 High Side Current Mirror to a Translinear Logarithmic Amplifier in an Avalanche Photodiode Power Detector
• Applications: Fiber Optics
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP

AD8304
BASIC CONCEPTS
The AD8304 uses an advanced circuit implementation that
exploits the well known logarithmic relationship between the
base-to-emitter voltage, V
bipolar transistor, which is the basis of the important class of
translinear circuits*:
There are two scaling quantities in this fundamental equation, namely
the thermal voltage V
are of key importance in determining the slope and intercept for this
class of log amp. V
at T = 25°C and varies in direct proportion to absolute temperature,
while I
and is typically 10
over the temperature range, by a factor of about a billion.
While these variations pose challenges to the use of a transistor as
an accurate measurement device, the remarkable matching and
isothermal properties of the components in a monolithic process
can be applied to reduce them to insignificant proportions, as will
be shown. Logarithmic amplifiers based on this unique property
of the bipolar transistor are called translinear log amps to distin-
guish them from other Analog Devices products designed for RF
applications that use quite different principles.
The very strong temperature variation of the saturation current
I
an identical variation, to stabilize the intercept. Similarly, propri-
etary techniques are used to ensure that the logarithmic slope is
temperature-stable. Using these principles in a carefully scaled
design, the now accurate relationship between the input current,
I
mediate output Pin VLOG is:
V
it is also the “volts per decade”). The fixed current I
the intercept. The scaling is chosen so that V
200 mV/decade (10 mV/dB). The intercept is positioned at
100 pA; the output voltage V
of this value. However, when using a single supply the actual
V
by using a negative supply, this voltage can actually cross zero at
the intercept value.
Using Equation 2, one can calculate the output for any value of I
Thus, for an input current of 25 nA,
In practice, both the slope and intercept may be altered, to either
higher or lower values, without any significant loss of calibration
accuracy, by using one or two external resistors, often in conjunc-
tion with the trimmed 2 V voltage reference at Pin VREF.
*For a basic discussion of the topic, see Translinear Circuits: An Historical Overview,
S
PD
B. Gilbert, Analog Integrated Circuits and Signal Processing, 9, pp. 95–118, 1996.
Y
LOG
is readily corrected using a second reference transistor, having
, applied to Pin INPT, and the voltage appearing at the inter-
is called the slope voltage (in the case of base-10 logarithms,
V
V
V
must always be slightly above ground. On the other hand,
BE
LOG
LOG
S
is very much a process- and device-dependent parameter,
=
=
=
V
V
T
0 2
Y
.
log( / )
log (
V
–16
I I
T
10
log (
C
T
has a process-invariant value of 25.69 mV
A at T = 25°C but exhibits a huge variation
I
= kT/q and the saturation current I
10
S
PD
25
/ )
BE
I
Z
, and collector current, I
nA
LOG
/
100
would cross zero when I
pA
)
=
0 4796
.
Y
is trimmed to
V
C
Z
, in a
is called
S
. These
PD
is
PD
(1)
(2)
(3)
.
–8–
Optical Measurements
When interpreting the current I
dent on a photodetector, it is necessary to be very clear about the
transducer properties of a biased photodiode. The units of this
transduction process are expressed as amps per watt. The param-
eter , called the photodiode responsivity, is often used for this
purpose. For a typical InGaAs p-i-n photodiode, the responsivity
is about 0.9 A/W.
It is also important to note that amps and watts are not usually
related in this proportional manner. In purely electrical circuits,
a current I
proportional to the square of the current (that is, I
reason for the difference in scaling for a photodiode interface is
that the current I
V
diode is simply proportional to the current I
and the proportionality of I
preserved.
Accordingly, a reciprocal correspondence can be stated between the
intercept current, I
and Equation 2 may then be written as:
For the AD8304 operating in its default mode, its I
corresponds to a P
responsivity of 0.9 A/W. Thus, an optical power of 3 mW would
generate:
Note that when using the AD8304 in optical applications, the
interpretation of V
power, the logarithmic slope remains 10 mV/dB at this output.
This can be a little confusing since a decibel change on the
optical side has a different meaning than on the electrical side.
In either case, the logarithmic slope can always be expressed in
units of mV per decade to help eliminate any confusion.
Decibel Scaling
In cases where the power levels are already expressed as so many
decibels above a reference level (in dBm, for a reference of 1 mW),
the logarithmic conversion has already been performed, and the
“log ratio” in the above expressions becomes a simple differ-
ence. One needs to be careful in assigning variable names here,
because “P” is often used to denote actual power as well as this
same power expressed in decibels, while clearly these are numeri-
cally different quantities.
Such potential misunderstandings can be avoided by using “D”
to denote decibel powers. The quantity V
must now be converted to its decibel value, V
there are 10 dB per decade in the context of a power measurement.
Then it can be stated that:
where D
and D
This convention will be used throughout this data sheet.
PDB
V
V
V
. In this case, the power dissipated within the detector
I
I
Z
PD
Z
LOG
LOG
LOG
is the equivalent intercept power relative to the same level.
OPT
= ρ
= ρ
=
=
=
PD
P
is the optical power in decibels above a reference level,
V
P
0 2
20
Z
OPT
.
Y
applied to a resistive load R
log (
V
(
D
Z
PD
, and an equivalent “intercept power,” P
log (
OPT
10
Z
LOG
flows in a diode biased to a fixed voltage,
of 110 picowatts, for a diode having a
P
10
OPT
−
is in terms of the equivalent optical
3
D
/
mW
P
Z
Z
)
PD
)
mV dB
PD
/
110
to the optical power, P
/
in terms of optical power inci-
pW
)
=
Y
L
1 487
(“volts per decade”)
PD
.
Y
results in a power
´ = V
(that is, I
V
PD
Y
Z
/10, because
2
of 100 pA
R
Z
OPT
, thus:
L
PD
REV. A
). The
V
, is
PDB
(4)
(5)
(6)
(7)
(8)
)