ADL5310-EVAL AD [Analog Devices], ADL5310-EVAL Datasheet - Page 13

• Manufacturer Part Number: ADL5310-EVAL
• Description: 120 dB Range (3 nA to 3 mA) Dual Logarithmic Converter
• Manufacturer: AD [Analog Devices]

APPLICATIONS
The ADL5310 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—that is, represented
in decibel terms. Basic connections for measuring a single
current at each input are shown in Figure 34, which also
includes various nonessential components, as explained next.
The 2 V difference in voltage between the VREF and Input Pins
INP1 and INP2, in conjunction with the external 665 kΩ resis-
tors R
into Pins IRF1 and IRF2. Connecting VRDZ to VREF raises the
voltage at LOG1 and LOG2 by 0.8 V, effectively lowering each
intercept current I
A wide range of other values for I
used. The effect of such changes is shown in Figure 5 and
Figure 8.
Any temperature variation in R
account when estimating the stability of the intercept. Also, the
overall noise increases when using very low values of I
In fixed-intercept applications there is little benefit in using a
large reference current, because doing so only compresses the
low-current-end of the dynamic range when operated from a
single supply. The capacitor between VSUM and ground is
RF1
and R
RF2
, provides 3 µA reference currents I
INTC
by a factor of 10
RF1
V
V
BIAS
BIAS
4.7nF
4.7nF
REF
(R
I
I
2kΩ
PD1
1kΩ
PD2
1kΩ
2kΩ
1nF
1nF
1nF
, from 3 nA to 3 mA, may be
RF2
I
I
RF1
RF2
4
) must be taken into
to position it at 300 pA.
VSUM
IRF1
COMM
VSUM
INP1
IRF2
INP2
20kΩ
665kΩ
665kΩ
Figure 34. Basic Connections for Fixed Intercept Use
0.5V
RF1
VNEG
VNEG
80kΩ
VREF
VREF
RF1
and I
(I
RF2
2.5V
RF2
Rev. A | Page 13 of 20
VRDZ
).
COMPENSATION
COMPENSATION
TEMPERATURE
TEMPERATURE
VNEG
GENERATOR
REFERENCE
VPOS
COMM
COMM
5V
strongly recommended to minimize the noise on this node, to
reduce channel-to-channel crosstalk, and to help provide clean
reference currents.
In addition, each input and reference pin (INP1, INP2, IRF1,
and IRF2) has a compensation network made up of a series
resistor and capacitor. The junction capacitance of the photo-
diode along with the network capacitance of the board artwork
around the input system creates a pole that varies widely with
input current. The RC network stabilizes the system by simul-
taneously reducing this pole frequency and inserting a zero to
compensate an additional pole inherent in the input system. In
general, the 1 nF, 1 kΩ network handles almost any photodiode
interface. In situations where larger active area photodiodes are
used, or when long input traces are used, the capacitor value
may need to be increased to ensure stability. Although the signal
and reference input systems are similar, additional care is
required to ensure stable operation of the reference inputs at
temperature extremes across the full current range of I
It is recommended that filter components of 4.7 nF and 2 kΩ
should be used from Pin IRF1 (IRF2) to ground. Temperature-
stable components should always be used in critical locations
such as the compensation networks; Y5V-type chip capacitors
are to be avoided due to their poor temperature stability.
COMM
6.69kΩ
14.2kΩ
14.2kΩ
6.69kΩ
I
I
LOG
LOG
451Ω
451Ω
LOG2
LOG1
OUT2
SCL2
OUT1
SCL1
BIN2
BIN1
V
V
12kΩ
12kΩ
OUT1
OUT2
C
10 nF
8kΩ
C
10 nF
8kΩ
0.5log
0.5log
FLT1
FLT2
10
10
(
(
I
1nA
I
1nA
PD1
PD2
)
)
ADL5310
RF1
(I
RF2
).