ADL5315ACPZ-R7 datasheet(11/20 Pages) AD

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ADL5315

Rev. 0 | Page 11 of 20

APPLICATIONS

The ADL5315 is primarily designed for wide dynamic range

applications, simplifying power monitoring designs where

access is only permitted to the cathode of a PIN photodiode or

receiver module. Figure 22 shows a typical application where

the ADL5315 is used to provide an accurate bias to a PIN diode

while simultaneously mirroring the diode current to be

measured by a translinear logarithmic amplifier.

In this application, the ADL5315 sets the bias voltage on the

PIN diode. This voltage is delivered at the INPT pin and is

controlled by the voltage at the VSET pin. VSET is driven by

the on-board reference VSREF, which is equal to VPOS − 1 V.

The input current, IINPT, is precisely mirrored at a ratio of 1:1 to

the IOUT pin. This interface is optimized for use with any of

ADI’s translinear logarithmic amplifiers (for example, the

AD8304 or AD8305) to offer a precise, wide dynamic range

measurement of the optical power incident upon the PIN.

If a linear voltage output is preferred at IOUT, a single external

resistor to ground is all that is necessary to perform the

conversion.

AVERAGE POWER MONITORING

In applications where a modulated signal is incident upon the

photodiode, the average power of the signal can be measured.

Figure 21 shows the connections necessary for using the

ADL5315 in such a measurement system.

The value of the capacitor to ground should be selected to

eliminate errors due to modulation of the ADL5315 input

current.

VOLTAGE

REFERENCE

CURRENT

LIMITING

COMM

VSET

INPT

DATA PATH

LINEAR

VOLTAGE

OUTPUT

IPD

CURRENT

MIRROR

1:1

ADL5315

SREF

VPOS

20k

RLIM

CSET

VPOS

IOUT

IPD

TIA

Figure 21. Average Power Monitoring Using the ADL5315

VOLTAGE

REFERENCE

CURRENT

LIMITING

COMM

VSET

INPT

DATA PATH

OPTICAL

POWER

TRANSLINEAR LOG AMP

AD8304, AD8305, ETC.

THIS CONNECTION IS NOT NECESSARY,

BUT REDUCES ERRORS DUE TO LEAKAGE

CURRENTS AT LOW SIGNAL LEVELS.

IPD

CURRENT

MIRROR

1:1

ADL5315

SREF

VPOS

20k

RLIM

RLIM = 48V

ILIM

ILIM = 1mA – 16mA

– 3k

IOUT

VSUM

INPT

IPD

TIA

VSREF = VPOS – 1V

VSET = VINPT

NODE VOLTAGES

Figure 22. Typical Application Using the ADL5315

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