ADL5315 Analog Devices, ADL5315 Datasheet - Page 13

• Manufacturer Part Number: ADL5315
• Description: Precision Wide Range (3 nA to 3 mA) High-Side Current Mirror
• Manufacturer: Analog Devices

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
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.
Figure 26. Extended Operating Range of 100 pA to 10 mA for Multiple Supply
–0.5
–1.0
–1.5
–2.0
–0.5
–1.0
–1.5
–2.0
Figure 25. Extended Operating Range of 100 pA to 10 mA for Multiple
2.0
1.5
1.0
0.5
2.0
1.5
1.0
0.5
100p
100p
0
0
Conditions, Normalized to V
Temperatures, Normalized to 25°C and I
1n
1n
10n
–40°C
0°C
+25°C
+70°C
+85°C
10n
100n
VOLTAGE CONDITIONS
100n
+25°C, +70°C, +85°C,
AD8304
I
INPT
I
I
INPT
INPT
1μ
1μ
POS
VS. I
(A)
(A)
= 5 V, V
0°C, –40°C
V
V
V
V
V
OUT
POS
POS
POS
POS
POS
10μ
10μ
(100 pA to 10 mA). Figure
, ALL
= 2.7V, V
= 5V, V
= 5V, V
= 8V, V
= 8V, V
SET
100μ
100μ
= V
SET
SET
SET
SET
SREF
SET
INPT
= 2V
= V
= 2V
= V
1m
and I
1m
= V
SREF
SREF
= 3 μA
SREF
INPT
10m
10m
= 3 μA
10m
1m
100μ
10μ
1μ
100n
10n
1n
100p
10m
1m
100μ
10μ
1μ
100n
10n
1n
100p
Rev. 0 | Page 13 of 20
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
the series combination of an internal 3 kΩ resistor and the
external R
the mirror bias to limit I
Figure 27 shows the equivalent circuit and one method for
using RLIM to form a V
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
the resistor ratio, R2/R1, to be calculated based on maximum
photodiode ESR using the following simplified equation.
where R
For zero bias at zero input current, the sum of R
equal R1. For positive bias at zero input current, the sum of R
and R3 should be greater than R1. The ratio of V
varies directly.
For example, choosing R
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.
Figure 27. Providing Automatic Photodiode Voltage Biasing Using RLIM Pin
R2
R1
PDmax
=
R1
INPT
LIM
40
R2
/40. The voltage generated by this current through
is the maximum ESR of the photodiode.
is compared to a 1.2 V threshold and fed back to
R
R
LIM
PDmax
V
POS
LIM
,
R3
R2
minimizes impact on I
R2
SET
INPT
LIM
RLIM
>>
bias proportional to I
.
= 1.82 kΩ (10 mA I
R
LIM
VPOS
RLIM
VSET
,
R1
=
R3
I
INPT
3kΩ
LIM
/40
MIRROR
BIAS
and allows
LIM
LIM
INPT
POS
),
ADL5315
and R3 must
, also
to V
1.2V
SET
LIM