ADL5304ACPZ-R2 datasheet(18/32 Pages) AD

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ADL5304

Data Sheet

Rev. A | Page 18 of 32

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 VY (the volts/decade ) is converted to its decibel value,



= VY/10 (because there are 10 dB per decade in the context

of a power measurement), it can be written

VLOG = 20 mV(DOPT − DZ)

(13)

where:

DOPT is the optical power expressed in decibels above a reference

level.

DZ denotes the equivalent intercept power relative to the same level.

Using the previous example and assuming a reference power

of 1 mW, a POPT of 12.5 μW corresponds to a DOPT of

10 log10(12.5 μW/1 mW) = −19.03 dBm; the equivalent intercept

power of 3.95 fW corresponds to a DZ of −114.03 dBm. Therefore,

VLOG = 20 mV (−19.03 − (−114.03)) = 1.900 V

(14)

the same result calculated with Equation 12.

CIRCUIT DESCRIPTION

The ADL5304 addresses a wide variety of interfacing conditions

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 ADL5304 is an order of magnitude

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, INUM, is received at the

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 INUM current to a

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.

MONITOR AND

PD BIAS

(1.1× INUM)

VSM1

VSM2

RMNTR

VSM4

VSM3

IREF

IDEN

INUM

100nA

1.5V

VDEN

INDN

VNUM

IMON

INNM

1P5V

1.5V

VREF

TEMP

COMP

VNUM

ILOG

VDEN

SHIELD

Figure 45. Simplified Front-End Schematic

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 IDEN.

In most applications, this is the reference of IREF = 100 nA,

supplied internally and laser trimmed.

The difference between the two VBEs with Q1 accepting a

photodiode current of INUM = IPD is

VBE1 − VBE2 = VT log10(IPD/IREF)

(15)

By adding an accurate PTAT voltage of magnitude,

VOFS = VT log10(IREF/IZ) = 1.500 V

(16)

resulting in

VBE1 − VBE2 + VOFS = VT [log10(IPD/IREF) + log10(IREF/IZ)]

= (kT/q) log10(IPD/IZ)

(17)

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,

VLOG = 0.2 V log10(IPD/IZ)

(18)

When the VSM1to VSM4, DCBI, and INPS pins are tied to

ground and VNEG < −2 V, the offset (VOFS) is removed, leaving

the more general form.

VLOG = 0.2 V log10(INUM/IDEN)

(19)

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