ADL5306

GENERAL STRUCTURE

The ADL5306 addresses a wide variety of interfacing conditions

to meet the needs of fiber optic supervisory systems, and is

useful in many nonoptical applications. This section explains

the structure of this unique style of translinear log amp. The

simplified schematic in Figure 21 shows the key elements.

Q2

Q1

451

Ω

14.2k

Ω

80k

Ω

20k

Ω

6.69k

Ω

PHOTODIODE

INPUT

CURRENT

BIAS

GENERATOR

TEMPERATURE

COMPENSATION

(SUBTRACT AND

DIVIDE BY T°K)

2.5V

COMM

COMM

VLOG

VNEG (NORMALLY GROUNDED)

VSUM

INPT

VREF

IREF

44

µA/dec

2.5V

0.5V

0.5V

0.5V

03727-0-021

Figure 21. Simplified Schematic

at this node is essentially equal to the voltage on the two

adjacent guard pins, VSUM and IREF, due to the low offset

corresponding logarithmic voltage, as shown in Equation 1. A

for the usual case of a single-supply voltage. This is internally

set to 0.5 V, one fifth of the 2.5 V reference voltage appearing on

Pin VREF. The resistance at the VSUM pin is nominally 16 kΩ;

this voltage is not intended as a general bias source.

The ADL5306 also supports the use of an optional negative

negative, VSUM may be connected to ground; thus, INPT and

IREF assume this potential. This allows operation as a voltage-

input logarithmic converter by the inclusion of a series resistor

need to be adjusted to maintain the intercept value. It should

also be noted that the collector-emitter voltages of Q1 and Q2

errors at large input currents.

generated externally to a recommended value of 10 µA.

However, other values over a several-decade range can be used

with a slight degradation in law conformance (see Figure 8).

THEORY

The base-emitter voltage of a BJT (bipolar junction transistor)

can be expressed by the following equation, which immediately

shows its basic logarithmic nature:

(1)

where:

kT/q is the thermal voltage, proportional to absolute

temperature (PTAT), and is 25.85 mV at 300 K.

temperature dependence, varying by a factor of roughly a

billion between –35°C and +85°C. Thus, to make use of the BJT

as an accurate logarithmic element, both of these temperature-

dependencies must be eliminated.

The difference between the base-emitter voltages of a matched

(2)

that appears in Equation 1 has therefore been eliminated. To

eliminate the temperature variation of kT/q, this difference

voltage is processed by what is essentially an analog divider.

Effectively, it puts a variable under Equation 2. The output of

this process, which also involves a conversion from voltage

mode to current mode, is an intermediate, temperature-

corrected current:

(3)

determines the slope of the function (change in current per

temperature-independent slope of 44 µA/decade for all values

Rev. 0 | Page 9 of 16