adl5317 Analog Devices, Inc., adl5317 Datasheet - Page 9

• Manufacturer Part Number: adl5317
• Description: Avalanche Photodiode Bias Controller And Wide Range 5 Na To 5 Ma Current Monitor
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The ADL5317 is designed to address the need for high voltage
bias control and precision optical power monitoring in optical
systems using avalanche photodiodes. It is optimized for use
with the Analog Devices, Inc. family of translinear logarithmic
amplifiers that take advantage of the wide input current range
of the ADL5317. This arrangement allows the anode of the
photodiode to connect directly to a transimpedance amplifier
for the extraction of the data stream without need for a separate
optical power monitoring tap. Figure 19 shows the basic
connections for the ADL5317.
LOW VOLTAGE
At the heart of the ADL5317 is a precision attenuating current
mirror with a voltage following characteristic that provides
precision biasing at the monitor input. This architecture uses a
JFET-input amplifier to drive the bipolar mirror and maintain
stable V
the VAPD pin. The mirror attenuates the current sourced
through VAPD by a factor of 5 to limit power dissipation under
high voltage operation and delivers the mirrored current to the
IPDM monitor output pin. Proprietary mirroring and cascoding
techniques maintain the linearity vs. the input current and
stability of the mirror ratio over a very wide range of supply and
V
BIAS CONTROL INTERFACE
In the linear operating mode, the voltage at VAPD is referenced
to ground, and follows the simplified equation
GARD is driven to the same potential as VAPD for use in
shielding the highly sensitive VAPD pin from leakage currents.
The GARD and VAPD pins are clamped to within approxi-
mately 40 V below the VPHV supply to prevent internal device
breakdowns, and VAPD is clamped to within a volt of GARD.
APD
SUPPLY
FALT
V
voltages.
APD
0.1μF
APD
10kΩ
= 30 × V
voltage, while offering very low leakage current at
0Ω
V
SET
HIGH VOLTAGE
0.01μF
0.01μF
0.1μF
SET
SUPPLY
Figure 19. Basic Connections
1
2
3
4
0Ω
FALT
VSET
VPLV
VPHV
ADL5317
APD
I
APD
GARD
1kΩ
IPDM
NC
NC
1nF
12
11
10
9
MIRROR CURRENT
0.01μF
OUTPUT
Rev. 0 | Page 9 of 16
The VAPD adjustment range for a given high voltage supply,
VPHV, is limited to approximately 33 V (or less, for V
41 V). For example, VAPD is specified from 40 V to 73.5 V for
a 75 V supply, and 6 V (the minimum allowed) to 28.5 V for a
30 V supply. When VAPD is driven to its lower clamp voltage
via the VSET pin, the mirror can continue to operate, but the
VAPD bias voltage no longer responds to incremental changes
in V
GARD INTERFACE
The GARD pins primarily shield the VAPD trace from leakage
currents and filter noise from the bias control interface. GARD
is driven by the V
resistor forms an RC network with an external capacitor from
GARD to ground that filters the thermal noise of the amplifier’s
feedback network and provides additional power supply
rejection. The series components, R
Figure 20, are necessary to ensure essential high frequency
compensation at the VAPD input pin over the full operating
range of the ADL5317.
The cutoff frequency of the GARD interface for small signals
and noise is defined by
where:
F
on-board 20 kΩ and C
C
A larger value for C
superior noise performance at the lowest input current levels,
but also slows the response time to changes in V
The pull-up of the V
2.5 mA, resulting in a slew limited region for large signals,
followed by an RC decay for the final 700 mV. This decay
corresponds to the above single-pole equation. The pull-down
of the V
approximately 90 kΩ in parallel with 70 μA to ground.
3dB
GRD
is the cutoff frequency of the low-pass filter formed by the
SET
is the filter capacitor installed from GARD to ground.
F
3dB
.
SET
=
Figure 20. Filtering VAPD Using the GARD Interface
amplifier is largely resistive, equivalent to
2π
V
SET
×
20
X30
AMPLIFIER
SET
kΩ
GRD
1
amplifier through a 20 kΩ resistor. This
SET
×
GRD
(up to approximately 0.01 μF) provides
amplifier is limited to approximately
C
.
GRD
20kΩ
ADL5317
COMP
GARD
VAPD
and C
R
C
COMP
C
COMP
GRD
COMP
SET
ADL5317
.
, shown in
PHV
<