adl5304 Analog Devices, Inc., adl5304 Datasheet - Page 25

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

Data Sheet
APPLICATIONS INFORMATION
USING THE ADL5304
The basic connections for single-supply operation are shown in
Figure 55. Supply decoupling is not critical and the suggested values
are conservative; however, it is recommended that a ferrite bead
be placed in the supply lines together with a 0.1 μF decoupling
capacitor. Ferrite beads are preferable to resistors because they do
not produce a dc voltage drop that can affect reference levels. In
Figure 55, the slope is 10 mV/dB or 0.2 V/decade, and the intercept
is 3.162 fA. For the full dynamic range of 200 dB (100 dB optical),
V
in Figure 55) with V
the IDEN pin is connected to the IREF pin, I
Figure 55 also shows the setup for the adaptive photodiode bias.
If this is not desired, ground the IMON pin, remove R
provide the desired bias voltage greater than 1.5 V to the cathode of
the PD. As noted in the Photodiode Bias section, the on-chip 2 V
reference can be used for this purpose and provides an exact 0.5 V
reverse bias together with the 1.5 V that is forced by the FET
amp to the anode via the INUM pin.
LOG
varies from 0.5 V to 2.5 V (see small diagram at the output
R
PD
SHIELD
SHIELD
MNTR
LOG
= V
IDEN
IREF
INUM
VSM1
VSM2
VSM3
VSM4
OFS
1.5V
1.5V
1.5V
1.5V
= 1.5 V, when I
2
3
4
5
6
7
8
MONITOR AND
30
(1.1× I
IMON
PD BIAS
NUM
NUM
DEN
Figure 55. Basic Connections for Single-Supply Operation
VDEN
0.1µF
= I
)
= 100 nA.
VNUM INNM
DEN
32
9
0.1µF
. Because
MNTR
31
10
V
V
INDN
NUM
DEN
100nA
1.5V
, and
1µF
26
COMM
1P5V
4.02Ω
Rev. 0 | Page 25 of 32
COMPENSATION
TEMPERATURE
11
ADL5304
28
BIAS AND
NMFS
BSDC
0.1µF
12
V
R
REF
NMFS
VNEG DNFS
2V
17
13
Using the Adaptive Bias
The positive bias on the photodiode cathode must be adequate
to support the peak current, which is limited by its internal series
resistance, R
sentative photodiode (JDSU EPM 605) is shown in Figure 54.
It is desirable to use a small bias at very low levels of
illumination to minimize the error due to current leakage
across the diode terminals. The adaptive bias achieves this
automatically even for larger currents through the addition
of the external resistor, R
uncertainty in R
R
all, the voltage at the IMON pin increases until the current
source saturates and absorbs the excess 10% of current that the
IMON output generates. However, this defeats the purpose of
the adaptive bias; therefore, users must ensure that R
present when using the adaptive bias.
2VLT
S
is recommended. In the limit, when R
I
LOG
14
0.1µF
R
DNFS
PD
29
7.5kΩ
ACOM
VPOS
5kΩ
15
S
. Typical values of R
27
HFCP
FB
DCBI
S
16
5kΩ
, an R
0.1µF
0.55pF
24
23
22
21
20
19
18
R
Figure 54. Photodiode Model
s
INPS
INMS
VLOG
SCL1
SCL2
SCL3
ACOM
V
MNTR
POS
1.5nH
1nH
MNTR
that is slightly greater than 10 times
0.13pF
, that is 10 times R
IMPORTANT: ~1mA
BIAS CURRENT FLOWS OUT
OF DCBI. NEEDS TO BE
CONNECTED TO 1P5V.
2.5V
1.5V
0.5V
S
R
are 5 Ω. A model of a repre-
LOAD
1p 100n 10m
0.5pF
0.5pF
MNTR
is not present at
5nH
5nH
S
. In case of
ADL5304
MNTR
2
CASE
1
is