ADL5310
Rev. A | Page 13 of 20
APPLICATIONS
TEMPERATURE
COMPENSATION
REFERENCE
GENERATOR
451
14.2k
80k
665k
665k
20k
1k
1nF
1k
1nF
2k
4.7nF
2k
4.7nF
1nF
6.69k
12k
8k
8k
CFLT2
10 nF
CFLT1
10 nF
COMM
VREF
VNEG
COMM
VREF
VRDZ
VPOS
VNEG
VSUM
INP2
IRF2
2.5V
0.5V
ILOG
IRF2
IRF1
OUT2
SCL2
BIN2
LOG2
04415-
0-
034
VBIAS
TEMPERATURE
COMPENSATION
451
14.2k
6.69k
12k
COMM
5V
VNEG
VSUM
INP1
IRF1
ILOG
OUT1
VOUT1
VOUT2
SCL1
BIN1
LOG1
VBIAS
IPD1
IPD2
0.5log10(
)
IPD2
1nA
0.5log10(
)
IPD1
1nA
Figure 34. Basic Connections for Fixed Intercept Use
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 RRF1 and RRF2, provides 3 A reference currents IRF1 and IRF2
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 IINTC by a factor of 104 to position it at 300 pA.
A wide range of other values for IREF, from 3 nA to 3 mA, may be
used. The effect of such changes is shown in
Figure 5 and
Any temperature variation in RRF1 (RRF2) must be taken into
account when estimating the stability of the intercept. Also, the
overall noise increases when using very low values of IRF1 (IRF2).
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
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 IRF1 (IRF2).
It is recommended that filter components of 4.7 nF and 2 k
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.