ADL5315ACPZ-R7 datasheet(13/20 Pages) AD

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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

100p

10m

IINPT (A)

1.5

1.0

0.5

–0.5

–1.0

–1.5

10m

100

100n

10n

100p

10n

100n

100

–40

°C

+25

°C

+70

°C

+85

°C

+25

°C, +70°C, +85°C,

°C, –40°C

Figure 25. Extended Operating Range of 100 pA to 10 mA for Multiple

Temperatures, Normalized to 25°C and IINPT = 3 μA

2.0

–2.0

10m

IINPT (A)

1.5

1.0

0.5

–0.5

–1.0

–1.5

10m

100

100n

100p

10n

100n

100p

100

IINPT VS. IOUT, ALL

VOLTAGE CONDITIONS

VPOS = 2.7V, VSET = VSREF

VPOS = 5V, VSET = 2V

VPOS = 5V, VSET = VSREF

VPOS = 8V, VSET = 2V

VPOS = 8V, VSET = VSREF

Figure 26. Extended Operating Range of 100 pA to 10 mA for Multiple Supply

Conditions, Normalized to VPOS = 5 V, VSET = VSREF and IINPT = 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 IINPT/40. The voltage generated by this current through

the series combination of an internal 3 kΩ resistor and the

external RLIM is compared to a 1.2 V threshold and fed back to

the mirror bias to limit IINPT.

Figure 27 shows the equivalent circuit and one method for

using RLIM to form a VSET bias proportional to IINPT, 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 >> RLIM minimizes impact on ILIM and allows

the resistor ratio, R2/R1, to be calculated based on maximum

photodiode ESR using the following simplified equation.

LIM

PDmax

where RPDmax is the maximum ESR of the photodiode.

For zero bias at zero input current, the sum of RLIM and R3 must

equal R1. For positive bias at zero input current, the sum of RLIM

and R3 should be greater than R1. The ratio of VPOS to VSET

varies directly.

For example, choosing RLIM = 1.82 kΩ (10 mA ILIM),

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

1.2V

RLIM

VSET

IINPT/40

VPOS

Figure 27. Providing Automatic Photodiode Voltage Biasing Using RLIM Pin

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