ADL5304ACPZ-R2 datasheet(16/32 Pages) AD

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ADL5304

Data Sheet

Rev. A | Page 16 of 32

TERMINOLOGY

Optical Power

Optical power is defined as photon energy per unit of time

measured as radiant flux (Φ) or radiant power, which is radiant

energy (Q) per unit time.

Photodiode Responsivity

Photodiode (PD) responsivity, ρ, is a constant that correlates

optical power (POPT) with PD current (IPD).

IPD = ρ × POPT

(1)

where typical values for ρ of InGaAs p-intrinsic-n (PIN) photo-

diodes are in the range of 0.6 A/W to 1 A/W. In the case of 1 A/W,

this means that for 1 mW of incident optical power, POPT, the

PD delivers 1 mA of current, IPD.

When the photodiode current input to the ADL5304 is divided

by the responsivity, the log slope directly represents the change

in input optical power, POPT.

Dark Current

All reverse-biased diodes develop a current due to the random

generation of electrons/holes in the depletion region. In photo-

diodes, this current occurs with no incident light falling on the

diode and is called the dark current, IDK. Dark current limits the

minimum signal that can be reliably detected. For high speed

InGaAs PIN photodiodes, the IDK is typically around 5 nA. For

a photodiode with a responsivity of 1 A/W, a 5 nA IDK limits

minimum measureable optical power to −53 dBm.

Logarithmic Function

The logarithmic function is













NUM

LOG

log

(2)

Logarithmic Slope

Logarithmic slope is the change in output voltage (VLOG) for a

given change in input current usually shown as a semi-log

graph where one input current (INUM or IDEN) is plotted on a log

scale, and the output voltage (VLOG) is plotted on a linear scale.

The other input current is fixed. Typically, slope is denoted as

VY with units of mV/decade or mV/dB. For the ADL5304, in

the default configuration, VY = 200 mV/decade (10 mV/dB).

Logarithmic Intercept

Logarithmic intercept, IZ, is an extrapolated value representing

the input current where VLOG = 0 V. In single-supply operation

(the VSMx pins = DCBI = INPS = 1P5V), VLOG is always

positive, and when the ADL5304 is operating in the default

configuration of IDEN = IREF = 100 nA, the logarithmic intercept

occurs at INUM = 3.162 fA.

In the case of dual-supply operation of the device, the intercept

can be shifted to different values depending on where the VSM1 to

VSM4, DCBI, and INPS pins are biased; the only recommended

values are either 1.5 V via connection to the 1P5V pin, as in the case

of single-supply operation, or ground when a dual supply is used.

For example, if connected to ground, then the intercept is at IZ =

IDEN = INUM for which log(INUM/IDEN) = 0. This is how most

people interpret the function log(x).

The most practical way to define intercept is to simply always use

log10(INUM/IDEN) = 0 as the reference point. The only consequence is

that a VOFS needs to be introduced depending on how the output

buffer gain and offset is set up.

OFS

DEN

NUM

LOG













log

(3)

For the default single-supply setup, as shown in Figure 1, VY =

0.2 V/decade and VOFS = 1.5 V (derived from the 1P5V pin),

and IDEN is supplied by the on-chip trimmed IREF = 100 nA.

The relationship between IZ and VOFS is as follows:





































0.2V/dec

1.5V

100

OFS

REF

DEN

(4)

Because IREF and VY are trimmed for the default setup with VOFS =

1.500 V, IZ should also be a stable quantity; however, because it is a

calculated value determined by the IREF, V1P5V, and VY parameters,

its distribution is the combination of the three parameters and

wider than the original parameters.

The ideal single- and dual-supply ADL5304 responses are

shown in Figure 44.

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

–0.2

–0.4

–0.6

–1.0

–0.8

10p

100p

10n

100n

1µ

10µ

100µ

10m

INUM INPUT CURRENT (A)

INUM = IDEN

SINGLE SUPPLY VSUM = 1.5V

DUAL SUPPLY VSUM = 0V

200mV/DEC

INUM = IDEN

Figure 44. Ideal Transfer Function

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