AD8304 데이터 시트보기 (PDF) - Analog Devices
부품명
상세내역
제조사
AD8304 Datasheet PDF : 21 Pages
| |||
AD8304
BASIC CONCEPTS
The AD8304 uses an advanced circuit implementation that
exploits the well known logarithmic relationship between the
base-to-emitter voltage, VBE, and collector current, IC, in a
bipolar transistor, which is the basis of the important class of
translinear circuits*:
VBE = VT log(IC /IS )
(1)
There are two scaling quantities in this fundamental equation, namely
the thermal voltage VT = kT/q and the saturation current IS. These
are of key importance in determining the slope and intercept for this
class of log amp. VT has a process-invariant value of 25.69 mV
at T = 25°C and varies in direct proportion to absolute temperature,
while IS is very much a process- and device-dependent parameter,
and is typically 10–16 A at T = 25°C but exhibits a huge variation
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
IS is readily corrected using a second reference transistor, having
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,
IPD, applied to Pin INPT, and the voltage appearing at the inter-
mediate output Pin VLOG is:
VLOG = VY log10(IPD/IZ )
(2)
VY is called the slope voltage (in the case of base-10 logarithms,
it is also the “volts per decade”). The fixed current IZ is called
the intercept. The scaling is chosen so that VY is trimmed to
200 mV/decade (10 mV/dB). The intercept is positioned at
100 pA; the output voltage VLOG would cross zero when IPD is
of this value. However, when using a single supply the actual
VLOG must always be slightly above ground. On the other hand,
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 IPD.
Thus, for an input current of 25 nA,
VLOG = 0.2 V log10(25 nA/100 pA) = 0.4796 V
(3)
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,
B. Gilbert, Analog Integrated Circuits and Signal Processing, 9, pp. 95–118, 1996.
Optical Measurements
When interpreting the current IPD in terms of optical power inci-
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 IPD applied to a resistive load RL results in a power
proportional to the square of the current (that is, IPD2 RL). The
reason for the difference in scaling for a photodiode interface is
that the current IPD flows in a diode biased to a fixed voltage,
VPDB. In this case, the power dissipated within the detector
diode is simply proportional to the current IPD (that is, IPDVPDB)
and the proportionality of IPD to the optical power, POPT, is
preserved.
IPD = ρPOPT
(4)
Accordingly, a reciprocal correspondence can be stated between the
intercept current, IZ, and an equivalent “intercept power,” PZ, thus:
IZ = ρPZ
(5)
and Equation 2 may then be written as:
VLOG = VY log10(POPT /PZ )
(6)
For the AD8304 operating in its default mode, its IZ of 100 pA
corresponds to a PZ of 110 picowatts, for a diode having a
responsivity of 0.9 A/W. Thus, an optical power of 3 mW would
generate:
VLOG = 0.2V log10(3 mW /110 pW ) = 1.487V
(7)
Note that when using the AD8304 in optical applications, the
interpretation of VLOG is in terms of the equivalent optical
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 VY (“volts per decade”)
must now be converted to its decibel value, VY´ = VY/10, because
there are 10 dB per decade in the context of a power measurement.
Then it can be stated that:
( ) VLOG = 20 DOPT − DZ mV /dB
(8)
where DOPT is the optical power in decibels above a reference level,
and DZ is the equivalent intercept power relative to the same level.
This convention will be used throughout this data sheet.
–8–
REV. A
Share Link: 