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ADL5315
EXTENDED OPERATING RANGE
The ADL5315 is specified over an input current range of 3 nA
to 3 mA, but the device remains fully functional over the full
eight decade range specified for ADI’s flagship translinear
logarithmic amplifier, the AD8304 (100 pA to 10 mA). Figure
25 and Figure 26 show the performance of the ADL5315 for this
extended operating range vs. various temperature and supply
conditions.
This extended dynamic range capability allows the ADL5315 to
be used in optical power measurement systems, precision test
equipment, or any other system that requires accurate, high
dynamic range current monitoring.
USING RLIM AS A SECONDARY MONITOR
The RLIM pin can be used as a secondary linear output for
monitoring input currents near the upper end of the ADL5315
current range. The RLIM pin sinks a current approximately
equal to I INPT /40. The voltage generated by this current through
the series combination of an internal 3 kΩ resistor and the
external R LIM is compared to a 1.2 V threshold and fed back to
the mirror bias to limit I INPT .
using RLIM to form a V SET bias proportional to I INPT , also
referred to as automatic photodiode biasing. This configuration
is useful in PIN photodiode systems to compensate for photo-
=
, R2 >> R LIM , R1 = R3
2.0
1.5
1.0
0.5
0
–0.5
–40 ° C
0 ° C
+25 ° C
+70 ° C
+85 ° C
+25 ° C, +70 ° C, +85 ° C,
0 ° C, –40 ° C
10m
1m
100 μ
10 μ
1 μ
100n
diode equivalent series resistance (ESR) while maintaining low
reverse bias at low signal levels to minimize dark current.
Choosing R2 >> R LIM minimizes impact on I LIM and allows
the resistor ratio, R2/R1, to be calculated based on maximum
photodiode ESR using the following simplified equation.
R2 40 R PDmax
R1 R LIM
where R PDmax is the maximum ESR of the photodiode.
–1.0
10n
–1.5
1n
For zero bias at zero input current, the sum of R LIM and R3 must
equal R1. For positive bias at zero input current, the sum of R LIM
–2.0
100p
1n
10n
100n
1 μ
I INPT (A)
10 μ
100 μ
1m
100p
10m
and R3 should be greater than R1. The ratio of V POS to V SET
varies directly.
Figure 25. Extended Operating Range of 100 pA to 10 mA for Multiple
Temperatures, Normalized to 25°C and I INPT = 3 μA
For example, choosing R LIM = 1.82 kΩ (10 mA I LIM ),
R2 = 100 kΩ, and R1 = 18.2 kΩ compensates for photodiode
2.0
1.5
1.0
I INPT VS. I OUT , ALL
VOLTAGE CONDITIONS
10m
1m
100 μ
ESR up to 250 Ω.
A simple low voltage drop current mirror with a load resistor
although the resulting input current limit is less accurate and
0.5
0
–0.5
10 μ
1 μ
100n
will vary with temperature.
V POS
–1.0
–1.5
V POS = 2.7V, V SET = V SREF
V POS = 5V, V SET = 2V
V POS = 5V, V SET = V SREF
V POS = 8V, V SET = 2V
V POS = 8V, V SET = V SREF
10n
1n
R1
R3
RLIM
VPOS
RLIM
3k Ω
1.2V
–2.0
100p
1n
10n
100n
1 μ
10 μ
100 μ
1m
100p
10m
I INPT (A)
Figure 26. Extended Operating Range of 100 pA to 10 mA for Multiple Supply
Conditions, Normalized to V POS = 5 V, V SET = V SREF and I INPT = 3 μA
R2
R2
VSET
I INPT /40
MIRROR
BIAS
Figure 27. Providing Automatic Photodiode Voltage Biasing Using RLIM Pin
Rev. 0 | Page 13 of 20
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