adl5304 Analog Devices, Inc., adl5304 Datasheet - Page 16

• Manufacturer Part Number: adl5304
• Description: High Speed, 200 Db Range, Logarithmic Converter
• Manufacturer: Analog Devices, Inc.

ADL5304
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 (P
where typical values for
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, P
PD delivers 1 mA of current, I
When the photodiode current input to the
by the responsivity, the log slope directly represents the change
in input optical power, P
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, I
minimum signal that can be reliably detected. For high speed
InGaAs PIN photodiodes, the I
a photodiode with a responsivity of 1 A/W, a 5 nA I
minimum measureable optical power to −53 dBm.
Logarithmic Function
The logarithmic function is
Logarithmic Slope
Logarithmic slope is the change in output voltage (V
given change in input current usually shown as a semi-log
graph where one input current (I
scale, and the output voltage (V
The other input current is fixed. Typically, slope is denoted as
V
the default configuration, V
Logarithmic Intercept
Logarithmic intercept, I
the input current where V
(the VSMx pins = DCBI = INPS = 1P5V), V
positive, and when the
configuration of I
occurs at I
Y
with units of mV/decade or mV/dB. For the ADL5304, in
I
V
PD
LOG
= ρ × P
=
NUM
V
Y
= 3.162 fA.
OPT
×
OPT
DEN
log
) with PD current (I
= I
10
⎛
⎜
⎜
⎝
REF
ADL5304
I
Z
ρ
OPT
NUM
, is an extrapolated value representing
I
= 100 nA, the logarithmic intercept
Z
of InGaAs p-intrinsic-n (PIN) photo-
LOG
.
Y
⎞
⎟
⎟
⎠
= 200 mV/decade (10 mV/dB).
= 0 V. In single-supply operation
PD
DK
LOG
.
NUM
is operating in the default
is typically around 5 nA. For
) is plotted on a linear scale.
or I
DK
PD
. Dark current limits the
DEN
).
ADL5304
) is plotted on a log
LOG
is always
DK
LOG
is divided
OPT
limits
) for a
, the
Rev. 0 | Page 16 of 32
(1)
(2)
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 I
I
interpret the function log(x).
The most practical way to define intercept is to simply always use
log
that a V
buffer gain and offset is set up.
For the default single-supply setup, as shown in Figure 1, V
0.2 V/decade and V
and I
The relationship between I
Because I
1.500 V, I
calculated value determined by the I
its distribution is the combination of the three parameters and
wider than the original parameters.
The ideal single- and dual-supply
shown in Figure 44.
DEN
10
= I
(I
V
DEN
I
NUM
–0.2
–0.4
–0.6
–0.8
–1.0
Z
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
LOG
NUM
OFS
0
=
1p
is supplied by the on-chip trimmed I
Z
/I
REF
should also be a stable quantity; however, because it is a
needs to be introduced depending on how the output
=
10
DEN
for which log(I
and V
V
I
10p 100p
⎛
⎜
⎜
⎝
) = 0 as the reference point. The only consequence is
DEN
Y
V
V
OFS
×
Y
log
Y
⎞
⎟
⎟
⎠
Figure 44. Ideal Transfer Function
are trimmed for the default setup with V
OFS
=
10
I
I
NUM
NUM
10
⎛
⎜
⎜
⎝
SINGLE SUPPLY V
1n
= 1.5 V (derived from the 1P5V pin),
I
I
DUAL SUPPLY V
I
NUM
I
⎛
⎜
⎜
⎝
= I
= I
NUM
DEN
V
REF
NUM
1
DEN
DEN
V
Z
P
10n 100n
Y
INPUT CURRENT (A)
5
and V
V
⎞
⎟
⎟
⎠
200mV/DEC
200mV/DEC
/I
⎞
⎟
⎟
⎠
+
DEN
=
V
OFS
10
ADL5304
) = 0. This is how most people
OFS
REF
100
SUM
SUM
⎛
⎜
⎝
, V
1µ
is as follows:
0.2V/dec
1.5V
1P5V
nA
= 1.5V
= 0V
10µ
, and V
⎞
⎟
⎠
responses are
REF
100µ
Data Sheet
= 100 nA.
Y
parameters,
1m
10m
OFS
Y
=
=
Z
(3)
(4)
=