ADL5315
Rev. 0 | Page 13 of 20
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.
2.0
–2.0
1n100p
10m
I
INPT
(A)
LINEARITY (%)
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
10m
1n
I
OUT
(A)
1m
100μ
10μ
1μ
100n
10n
100p
10n 100n 1μ 10μ 100μ 1m
–40°C
+25°C
+70°C
+85°C
0°C
+25°C, +70°C, +85°C,
0°C, –40°C
05694-030
Figure 25. Extended Operating Range of 100 pA to 10 mA for Multiple
Temperatures, Normalized to 25°C and I
INPT
= 3 µA
2.0
–2.0
1n 10m
I
INPT
(A)
LINEARITY (%)
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
10m
1n
I
OUT
(A)
1m
100μ
10μ
1μ
100n
100p
10n
10n 100n100p 1μ 10μ 100μ 1m
I
INPT
VS. I
OUT
, ALL
VOLTAGE CONDITIONS
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
05694-031
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
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
.
Figure 27 shows the equivalent circuit and one method for
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-
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.
R3R1RR2
R
R40
R1
R2
LIM
LI
PDmax
=>>= ,,
where
R
PDmax
is the maximum ESR of the photodiode.
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
and R3 should be greater than R1. The ratio of V
POS
to V
SET
varies directly.
For example, choosing R
LIM
= 1.82 kΩ (10 mA I
LIM
),
R2 = 100 kΩ, and R1 = 18.2 kΩ compensates for photodiode
ESR up to 250 Ω.
A simple low voltage drop current mirror with a load resistor
can replace the differential amplifier shown in
Figure 27,
although the resulting input current limit is less accurate and
will vary with temperature.
VPOS
MIRROR
BIAS
05964-035
1.2V
R2R2
3kΩ
RLIM
RLIM
VSET
R3R1
I
INPT
/40
V
POS
Figure 27. Providing Automatic Photodiode Voltage Biasing Using RLIM Pin
