Download CIRCUIT FUNCTION AND BENEFITS

Measured rms noise voltage at the output of the AD8305 vs.
input current is shown in Figure 2 for the AD8305 by itself and
in cascade with the ADL5317. The relatively low noise produced
by the ADL5317, combined with the additional noise filtering
inherent in the frequency response characteristics of the
AD8305, results in minimal degradation to the noise
performance of the AD8305.
CIRCUIT FUNCTION AND BENEFITS
This circuit uses the monitor current output, IPDM, of the
ADL5317 to interface directly to an Analog Devices, Inc., translinear logarithmic amplifier such as the AD8304, AD8305,
ADL5306, or ADL5310. Figure 1 shows the basic connections
necessary for interfacing the ADL5317 to the AD8305. In this
configuration, the designer can use the full current mirror
range of the ADL5317 for high accuracy power monitoring of
an avalanche photodiode (APD).
AD8305 INPUT
COMPENSATION
NETWORK
COMM
COMM
COMM
IPDM
ADL5317
3
12
4.7nF 2kΩ
11
NC
VPHV
GARD
6
7
8
4
COMM
COMM
12
11
AD8305
BFIN
INPT
VLOG
9
0.01µF
COMM
SCAL
IREF
IPDM
10nA TO
1mA
10
VPLV
5
VREF
200kΩ
NC
VOUT
0.1µF
VPOS
4
0.01µF
1kΩ
VRDZ
13
VNEG
5
6
7
8
OUTPUT
VOUT = 0.2 ×
LOG10 (IPDM /1nA)
10
9
3V TO 12V
0.01µF
0Ω
IAPD
1kΩ
APD 1nF
0.1µF
VP_HIGH
TIA
DATA
PATH
Figure 1. Typical Connection of the ADL5317 to the AD8305 Translinear Logarithmic Amplifier
(Simplified Schematic: Decoupling and All Connections Not Shown)
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08622-001
0.1µF
2
14
VNEG
0Ω
1
2.5V
15
VSUM
3
VP_LOW
VSET
VAPD
VSET
1nF
13
GARD
2
14
VCLH
10kΩ
FALT
15
VPHV
1
COMM
16
COMM
16
CIRCUIT DESCRIPTION
The ADL5317 is primarily designed for wide dynamic range
applications that simplify APD bias circuit architecture.
Accurate control of the bias voltage across the APD becomes
critical to maintain the proper avalanche multiplication factor
when the temperature and input power vary. Figure 3 shows
how to use the ADL5317 with an external temperature sensor to
monitor the ambient temperature of the APD. Using a lookup
table and DAC to drive the VSET voltage, it is possible to apply
the correct VAPD for the conditions. Note that all connections to
the ADL5317 are not shown for clarity.
In this application, the ADL5317 operates in linear mode.
The bias voltage to the APD, delivered at the VAPD pin, is
controlled by the voltage (VSET) at the VSET pin. The bias
voltage at VAPD is equal to 30 × VSET.
The range of voltages available at VAPD for a given high voltage
supply is limited to approximately 33 V (or less, for VAPD < 41 V).
This is because the GARD and VAPD pins are clamped to
within ~40 V below VPHV, preventing internal device
breakdowns.
The input current, IAPD, is divided by a factor of 5 and
precisely mirrored to the IPDM pin. This interface is optimized
for use with any of the Analog Devices translinear logarithmic
amplifiers (for example, the AD8304 or AD8305) to offer a
precise, wide dynamic range measurement of the optical power
incident upon the APD.
If a voltage output is preferred at IPDM, a single external
resistor to ground is all that is necessary to perform the
conversion. Voltage compliance at IPDM is limited to VPLV
or VAPD/3, whichever is lower.
COMMON VARIATIONS
Careful attention should be paid to the layout of the circuit board
in this configuration. Leakage current paths in the board itself
can lead to measurement errors at the output of the translinear
log amp, particularly when measuring the low end of the
ADL5317’s dynamic range. It is recommended that, when
designing such an interface, a guard potential be used to minimize this leakage. This can be done by connecting the translinear
log amp’s VSUM pin to the NC pin of the ADL5317, with the
VSUM guard trace running on both sides of the IPDM trace, as
shown in Figure 1. Additional details on using VSUM can be
found in the AD8304 and AD8305 data sheets. The VSET pin of
the ADL5317 can be used in a similar fashion to guard the
VAPD trace.
The circuit must be constructed on a multilayer PC board with
a large area ground plane. Proper layout, grounding, and
decoupling techniques must be used to achieve optimum
performance (see MT-031 Tutorial, MT-101 Tutorial, the
ADL5317 evaluation board layout, and the AD8305 evaluation
board layout).
LOGIC
SUPPLY
5.5m
5.0m
FALT
ADL5317
COMM
4.0m
OVERCURRENT
PROTECTION
3.5m
THERMAL
PROTECTION
AD8305 AND
ADL5317
LOOKUP
TABLE
AND DAC
2.5m
VSET
CURRENT
MIRROR
5:1
30 × VSET
TRANSLINEAR
LOG AMP
IPDM
TEMPERATURE
SENSOR
AD8305 ONLY
1.5m
IAPD
5
R
5V
1.0m
0.5m
0
10n
OPTICAL
POWER
29 × R
2.0m
100n
1µ
10µ
100µ
VPLV
IAPD
VPHV
1m
IPDM (A)
Figure 2. RMS Noise of the AD8305 vs. the AD8305 Cascaded with
the ADL5317
VCLH
GARD
VAPD
CGRD
75V
FROM DC–DC
CONVERTER
APD
TIA RECEIVER
DATA
Figure 3. Typical APD Biasing Application Using the ADL5317
(Simplified Schematic: Decoupling and All Connections Not Shown)
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08622-003
3.0m
08622-002
(V rms)
4.5m
LEARN MORE
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND." Analog Devices.
REVISION HISTORY
MT-078 Tutorial, High Frequency Log Amps. Analog Devices.
2/10—Rev. 0 to Rev. A
Updated Format ................................................................. Universal
Changes to Circuit Function and Benefits..................................... 1
Changes to Common Variations ..................................................... 2
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
1/09—Revision 0: Initial Version
MT-077 Tutorial, Log Amp Basics. Analog Devices.
Data Sheets and Evaluation Boards
ADL5317 Data Sheet
ADL5317 Evaluation Board
AD8304 Data Sheet
AD8305 Data Sheet
AD8305 Evaluation Board
ADL5306 Data Sheet
ADL5310 Data Sheet
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