Circuit Note
CN-0056
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
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ADL5315
Precision Wide-Range High-Side Current
Mirror
AD8304/
AD8305
Logarithmic Amplifier
ADL5306/
ADL5310
Single/Dual Logarithmic Amplifier
Interfacing the ADL5315 High Side Current Mirror to a Translinear Logarithmic
Amplifier in a Photodiode Power Detector Application
CIRCUIT FUNCTION AND BENEFITS
The measured rms noise voltage at the output of the AD8305 vs.
the input current is shown in Figure 2, both for the AD8305 by
itself and in cascade with the ADL5315. The relatively low noise
produced by the ADL5315, 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.
The ADL5315 addresses the need for precision high-side
monitoring of PIN photodiode current in fiber optic systems
and is useful in many nonoptical applications as well. It is
optimized for use with the Analog Devices, Inc., family of
translinear logarithmic amplifiers, which take advantage of the
wide input current range of the ADL5315. The circuit presented
in this document and shown in Figure 1 provides high accuracy
power monitoring using the ADL5315 precision wide-range,
high-side current mirror and the AD8305 translinear logarithmic
amplifier (log amp). This configuration is ideal for high
accuracy power monitoring because the full current mirror
range (3 nA to 3 mA) of the ADL5315 can be used.
ADL5315
VOLTAGE
REFERENCE
VPOS
RLIM = 48V
– 3kΩ
ILIM
CURRENT
LIMITING
RLIM
4
5
200kΩ
4.7nF 2kΩ
IOUT
INPT
8
1
IPD
IPD
3
4
VREF
AD8305
IREF
BFIN
INPT
VLOG
1kΩ
1nF
PIN
TIA
DATA PATH
SCAL
5
6
7
8
12
11
OUTPUT
VOUT = 0.2 × LOG10 (IPDM /1nA)
10
9
0.1µF
AD8305 INPUT
COMPENSATION
NETWORK
3V TO 12V
08621-001
NC
7
VOUT
VPOS
2
VSET
2
VRDZ
13
VNEG
1
14
COMM
VPOS 6
SREF
CSET
15
COMM
16
COMM
3
ILIM = 1mA – 16mA
VNEG
CURRENT
MIRROR
1:1
COMM
20kΩ
VSUM
RLIM
COMM
Figure 1. Connecting the ADL5315 to the AD8305 (Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. A
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CN-0056
Circuit Note
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.
5.5m
5.0m
4.5m
AD8305 AND
ADL5315
3.5m
Figure 3 shows a more generic 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.
3.0m
2.5m
2.0m
1.5m
1.0m
If a linear voltage output is preferred at IOUT, a single external
resistor to ground is all that is necessary to perform the
conversion.
AD8305 ONLY
0.5m
10n
100n
1µ
10µ
1m
100µ
IINPT (A)
Careful consideration should be given to the layout of the circuit
board in this configuration. Leakage current paths in the board
itself could lead to measurement errors at the output of the
translinear log amp, particularly when measuring the low end
of the ADL5315’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 VSUM
pin of the translinear log amp to the NC pin of the ADL5315,
with the VSUM guard trace running on both sides of the IOUT
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 ADL5315 can be used in a similar fashion to
guard the INPT trace.
08621-002
0
1n
Figure 2. RMS Noise of the AD8305 vs. the AD8305 Cascaded with
the ADL5315
CIRCUIT DESCRIPTION
In this application, the ADL5315 sets the bias voltage on the
input PIN photodiode. This voltage is delivered at the INPT pin
of the AD8305 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, INPT, is precisely mirrored at a ratio of 1:1
to the IOUT pin. This interface is optimized for use with any of
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 PIN diode.
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, ADL5315
evaluation board layout, and AD8305 evaluation board layout).
VPOS
RLIM = 48V
– 3kΩ
ILIM
ADL5315
VOLTAGE
REFERENCE
CURRENT
LIMITING
RLIM
4
5
COMM
RLIM
20kΩ
CURRENT
MIRROR
1:1
3
NODE VOLTAGES
VSREF = VPOS – 1V
VSET = VINPT
ILIM = 1mA – 16mA
6
SREF
VPOS
VSET
NC
2
THIS CONNECTION IS NOT NECESSARY,
BUT REDUCES ERRORS DUE TO LEAKAGE
CURRENTS AT LOW SIGNAL LEVELS.
7
VSUM
IOUT
INPT
1
OPTICAL
POWER
8
IPD
IPD
INPT
PIN
TIA
TRANSLINEAR LOG AMP
AD8304, AD8305, ETC.
DATA PATH
Figure 3. Typical Connection of the ADL5315 to the Translinear Logarithmic Amplifier
(Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. A | Page 2 of 3
08621-003
NOISE (V rms)
4.0m
Circuit Note
CN-0056
COMMON VARIATIONS
ADL5306 Data Sheet
The AD8304, ADL5306 and ADL5310 amplifiers can be
connected to the ADL5315 in a similar fashion.
ADL5310 Data Sheet
LEARN MORE
AD8305 Evaluation Board
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
REVISION HISTORY
MT-077 Tutorial, Log Amp Basics, Analog Devices.
MT-078 Tutorial, High Frequency Log Amps, Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Data Sheets
ADL5315 Data Sheet
AD8304 Data Sheet
ADL5315 Evaluation Board
4/10—Rev. 0 to Rev. A
Changes to Circuit Note Title and to Circuit Function and
Benefits Section ................................................................................. 1
Changes to Circuit Description Section......................................... 2
Changes to Learn More Section ...................................................... 3
1/09—Revision 0: Initial Version
AD8305 Data Sheet
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CN08621-0-4/10(A)
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