ADL5306 Analog Devices, Inc., ADL5306 Datasheet - Page 11

• Manufacturer Part Number: ADL5306
• Description: 60 Db Range 100 Na To 100 Ua Low Cost Logarithmic Converter
• Manufacturer: Analog Devices, Inc.

APPLICATIONS
The ADL5306 is easy to use in optical supervisory systems and
in similar situations where a wide-ranging current is to be
converted to its logarithmic equivalent (i.e., represented in
decibel terms). Basic connections for measuring a single current
input are shown in Figure 22, which includes various
nonessential components, as will be explained.
The 2 V difference in voltage between VREF and INPT, in
conjunction with the external 200 kΩ resistor R
reference current I
reference raises the voltage at VLOG by 0.8 V, effectively
lowering the intercept current I
it at 1 nA. Any temperature variation in R
account when estimating the stability of the intercept. Also, the
overall noise will increase when using very low values of I
fixed-intercept applications, there is little benefit in using a large
reference current, since this only compresses the low current
end of the dynamic range when operated from a single supply,
shown here as 5 V. The capacitor between VSUM and ground is
recommended to minimize the noise on this node and to help
provide a clean reference current.
Since the basic scaling at VLOG is 0.2 V/dec and a swing of 4 V
at the buffer output would therefore correspond to 20 decades,
it will often be useful to raise the slope to make better use of the
rail-to-rail voltage range. For illustrative purposes, the circuit in
Figure 22 provides an overall slope of 0.5 V/dec (25 mV/dB).
Thus, using I
to 0.8 V at I
2.0 V, corresponding to a dynamic range of 60 dB electrical
(30 dB optical) power.
The optional capacitor from VLOG to ground forms a single-
pole low-pass filter in combination with the 4.55 kΩ resistance
at this pin. For example, using a C
1kΩ
1nF
V
BIAS
200kΩ
R
1nF
1kΩ
REF
1nF
I
PD
VSUM
VREF
IREF
INPT
NC
Figure 22. Basic Connections for Fixed Intercept Use
PD
REF
0.5V
= 100 µA. The buffer output runs from 0.5 V to
20kΩ
0.5V
= 100 µA, V
REF
COMM
of 100 µA into Pin IREF. The internal
VNEG
80kΩ
Q2
Q1
LOG
2.5V
V
V
BE2
BE1
INTC
runs from 0.2 V at I
VPOS
COMPENSATION
TEMPERATURE
FLT
GENERATOR
by a factor of 10
of 10 nF, the –3 dB corner
BIAS
+5V
REF
COMM
COMM
0.5 log
14.2kΩ
6.69kΩ
must be taken into
I
LOG
REF
10
, provides a
( )
451Ω
PD
4
1nA
I
PD
VLOG
to position
= 100 nA
03727-0-022
C
10nF
REF
VOUT
SCAL
BFIN
FLT
8kΩ
12kΩ
. In
Rev. 0 | Page 11 of 16
frequency is 3.2 kHz. Such filtering is useful in minimizing the
output noise, particularly when I
more effective in reducing the total noise. For examples, see the
AD8304 Data Sheet.
The dynamic response of this overall input system is influenced
by the external RC networks connected from the two inputs
(INPT, IREF) to ground. These are required to stabilize the
input systems over the full current range. The bandwidth
changes with the input current due to the widely varying pole
frequency. The RC network adds a zero to the input system to
ensure stability over the full range of input current levels. The
network values shown in Figure 22 will usually suffice, but some
experimentation may be necessary when the photodiode’s
capacitance is high.
Although the two current inputs are similar, some care is
needed to operate the reference input at extremes of current
(<100 nA) and temperature (<0°C). Modifying the RC network
to 4.7 nF and 2 kΩ will allow operation to –40°C at 10 nA. By
inspecting the transient response to perturbations in I
representative current levels, the capacitor value can be adjusted
to provide fast rise and fall times with acceptable settling. To
fine-tune the network zero, the resistor value should be
adjusted.
USING A NEGATIVE SUPPLY
Most applications of the ADL5306 require only a single supply
of 3.0 V to 5.5 V. However, to provide further versatility, dual
supplies may be employed, as illustrated in Figure 23.
1kΩ
1nF
V
BIAS
200kΩ
R
1nF
1kΩ
REF
I
PD
VSUM
VREF
IREF
INPT
NC
R
V
V
S
N
F
≤
I
0.5V
sig
I
q
20kΩ
0.5V
V
+ I
Figure 23. Negative Supply Application
= I
N
I
q
sigmax
– V
+ I
PD
sig
COMM
F
+ I
REF
VNEG
80kΩ
Q2
Q1
V
C
2.5V
V
V
SUM
BE2
BE1
1
VPOS
PD
– V
COMPENSATION
TEMPERATURE
GENERATOR
is small. Multipole filters are
F
≤ –0.5V
BIAS
+5V
COMM
COMM
0.5 log
14.2kΩ
6.69kΩ
I
LOG
10
ADL5306
451Ω
( )
1nA
I
VLOG
PD
REF
at
03727-0-023
10nF
C
VOUT
SCAL
BFIN
FLT
8kΩ
12kΩ