ADL5309ACBZ-R7 datasheet(14/25 Pages) AD

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Data Sheet

ADL5309

THEORY OF OPERATION

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Rev. A | 14 of 25

The ADL5309 performance and feature set is optimized for high

dynamic range and high accuracy optical power measurements.

The temperature compensated and factory trimmed logarithmic

transimpedance amplifiers enable accurate measurements over 9

decades of input current, the equivalent to 9 decades of optical

power. The amplifier output voltage can be measured with a

more relaxed dynamic range, without loss of accuracy, due to the

inherent dynamic range compression of the logarithmic transfer.

Therefore, the 14-bit built-in ADC has more than sufficient dynamic

range and resolution to provide an accurate digitized result. The

built-in adaptive photodiode bias function minimizes the impact of

nonidealities such as dark current and diode series resistance on

the measurement accuracy. The built-in I2C interface can be used

to control various internal analog functions and to read out the

ADC. A total of three possible choices for the I2C device address

allow up to three devices to communicate independently over a

single I2C bus.

LOGARITHMIC TRANSFER

The logarithmic transimpedance amplifiers (TIAs) produce an out-

put voltage that is (approximately) linearly related to the logarithm

of the input current (IPD) as follows:

VOUT=SLOPE×log10 IPDIZ

(1)

where:

SLOPE is the logarithmic slope that represents the amount by

which the output voltage (VOUT) changes for each factor of 10

(decade) change in IPD.

IZ is the (extrapolated) IPD for which the VOUT is zero.

The actual device never reaches zero but saturates to the starting

voltage of 17 mV for input currents below 10 pA. Both SLOPE and

IZ can be obtained by linear regression of the measured amplifier

output voltage vs. a range of input current levels. The ADL5309

logarithmic slope and intercept of the VOUT − 1.0 V curve are

accurately factory trimmed to 200 mV/dec and 1 μA, respectively.

The reason 1.0 V is subtracted from the VOUT curve (the ideal

value of VOUT at 1 μA) is to place the x-intercept in the geometric

middle of the specified input current range. As a result, the residual

slope differences have a minimum impact on the x-intercept, and

the equation can be written as follows:

VOUT−1.0=SLOPE×log10 IPDIZ1

(2)

Expressed in dB of input current, Equation 3 can be written as

follows:

VOUT−1.0=SLOPE× IPD, dB−IZ1, dB (3)

where:

IPD, dB is the input current in dBA.

IZ1,dB is the intercept current in dBA (−120 dBA in this case).

The measurement accuracy obtained with a logarithmic amplifier is

determined by the following two factors:

►

The logarithmic conformance error

►

The temperature drift error

The logarithmic conformance error describes the deviation of the

actual TIA transfer from the ideal log-linear relationship (see Equa-

tion 3) and is expressed in dB of input current by the following

equation:

ELC =20×VOUTT

SLOPE

+IZ1, dB−IPD, dB

(4)

where: ELC is the measurement error.

Thus, ELC shows the resulting measurement error when VOUT of a

logarithmic TIA is measured, and Equation 3 is used to determine

the input current that the device is sensing. Since SLOPE and

IZ are usually determined at room temperature only, ELC typically

also contains a contribution due to drift of the TIA transfer over

temperature.

The temperature drift error (EDRIFT) describes the measurement

error introduced solely due to the temperature drift of the TIA

transfer, excluding discrepancies of the actual TIA transfer to the

ideal log-linear relationship (logarithmic conformance).

EDRIFTT = 20SLOPE× VOUTT −VOUTTO (5)

where:

T is the operating temperature.

TO is the reference temperature.

The error, the difference between the VOUT measured at the operat-

ing temperature and the actual VOUT measured at the reference

temperature, usually 25°C, is input referred and expressed in dB (of

IPD) using SLOPE. This is accurate as long as the error is relatively

small and the TIA transfer is approximately logarithmic (linear in

dB).

OPTICAL MEASUREMENTS

A high dynamic range optical power monitor can be constructed

by connecting the anode of a reverse biased photodiode to the

input of the logarithmic TIA, such that the TIA senses the photon-

generated diode current. Therefore, it is important to understand

the transducer aspects of a photodiode to interpret the photodiode

current relative to the incident optical power. In purely electrical

circuits, the power dissipated in a resistive load is proportional to

the square of the current, or, vice versa, the current through the

load is proportional to the square root of the dissipated power:

IR= PDISS/R

(6)

where:

IR is the adaptive photodiode current.

PDISS is the dissipated power from the photodiode.

R is the resistive load from the photodiode.

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