Datasheet

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 V
SREF
, which is equal to V
POS
− 1 V.
The input current, I
INPT
, 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 NC
INPT
DATA PATH
LINEAR
VOLTAGE
OUTPUT
I
PD
CURRENT
MIRROR
1:1
ADL5315
4
2 7
SREF
3
1
6
VPOS
PIN
20kΩ
5
RLIM
C
SET
V
POS
8
IOUT
I
PD
TIA
05694-010
Figure 21. Average Power Monitoring Using the ADL5315
VOLTAGE
REFERENCE
CURRENT
LIMITING
COMM
VSET NC
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.
I
PD
CURRENT
MIRROR
1:1
ADL5315
4
2 7
SREF
3
1
6
VPOS
V
POS
PIN
20kΩ
5
RLIM
R
LIM
R
LIM
= 48V
I
LIM
I
LIM =
1mA – 16mA
– 3kΩ
8
IOUT
VSUM
INPT
I
PD
TIA
V
SREF
= V
POS
– 1V
V
SET
= V
INPT
NODE VOLTAGES
05694-009
Figure 22. Typical Application Using the ADL5315