Download AN-643 APPLICATION NOTE

AN-643
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
Closed-Loop Control Circuit Implementation of the ADuC832 MicroConverter® IC
and the AD8305 Logarithmic Converter in a Digital Variable Optical Attenuator
By Mark Malaeb
INTRODUCTION
Today’s optical market may not seem very promising; this
could be due mainly to the cyclical nature of the optical
networking infrastructure. However, the major advancements of the recent past in the optical space cannot be
ignored. Such advancements allowed for complete signal processing in the optical domain without the need to
convert to the electrical domain. Also, because of its wide
bandwidth and high density (DWDM) capabilities, optical
fiber continues to be and will remain the medium of choice
for data, voice, and video transport. The need to support
such a medium was the reason behind the birth of countless optical devices. Optical attenuators, optical amplifiers
(EDFA and Raman), laser diode drivers, and photodetectors
are among these devices, just to name a few.
wave-guide, digitally controlled variable optical attenuator (DVOA). Figure 1 shows the physical layout of such a
module, the DVOA by GalayOr, Inc. Figure 2 shows how
the control circuit is implemented in the DVOA module.
This application note focuses on the control circuitry,
using the ADuC832 MicroConverter IC and the AD8305
logarithmic converter, for a MEMS based, silicon, optical
INTEGRATED
COUPLER
INPUT
Figure 1. GalayOr DVOA
INTEGRATED
COUPLER
SILICON VOA 40dB
INTEGRATED
PHOTODIODES
OPTICAL INPUT POWER
MEASUREMENT
LOG AMP
AD8305
OUTPUT
OPTICAL INPUT POWER
MEASUREMENT
LOG AMP
AD8305
MicroConverter
ADuC832
ADC1
VOLTAGE
REF.
AD1584
DAC1
RS-232
INTERFACE
ADM101
I2C/SPI BUS
RS-232 BUS
ADC2
ATTENUATOR
CONTROL VOLTAGE
AD823
36V
VOLTAGE
BOOSTER
CIRCUIT
5V
Figure 2. Overall DVOA Module Block Diagram
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Purchase of licensed I2C components of Analog Devices or one
of its sublicensed Associated Companies conveys a license for
the purchaser under the Philips I2C Patent Rights to use these
components in an I2C system, provided that the system conforms to the I2C Standard Specification as defined by Philips.
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AN-643
SYSTEM FUNCTIONAL DESCRIPTION
This DVOA is designed to attenuate optical signals up to
40 dB. The input and output optical power is measured
using two integrated (1%) optical couplers and their corresponding photodiodes. The output currents from the
photodiodes are fed into the log amps. The log amps
convert the photodiode currents into a voltage proportional to the optical signal power level. These two voltages
are then fed into two different ADC channels, on board
the MicroConverter IC, for processing.
ADuC832 MicroConverter IC
The ADuC832 MicroConverter IC, shown in Figure 3, is
part of an 8052 based ADuC8xx controller family from
Analog Devices. As shown in Figure 3, the MicroConverter
IC integrates an 8-channel multiplexed 12-bit SAR analogto-digital converter and two 12-bit digital-to-analog
converters. Two channels are used to measure the output
voltages from the AD8305 log amps, which correspond
to the optical power levels present at the input and output
of the VOA. One of the two on-board DACs is used to put
out a voltage signal level proportional to the difference
between the DVOA input and output ADC readings.
Once the VOA input and output power levels are known,
an attenuation value is derived from the difference. The
VOA attenuation value is set using an analog control voltage in the range of 5 V to 26 V. The attenuation increases
with increasing voltage levels. This control voltage is
generated from the DAC on board the MicroConverter IC.
The voltage at the DAC output is amplified to the 5 V to
26 V levels using one of Analog Devices’ high voltage op
amps (AD823). A voltage booster, from 5 V to 36 V, is used
to bias the AD823. Also, a high precision voltage reference
(AD1584) is used by the ADC and DAC.
The MicroConverter IC control circuitry consists of an 8052
core with 62 Kbytes of program memory and 4 Kbytes
of data memory. The preset VOA attenuation numbers,
which are used to set the DAC, are stored in data memory.
Two PWM outputs are also available on board in addition
to the standard timers. To add to this high level of integration, the ADuC832 and all the ADuC8xx family include an
on-board power-on reset circuit, a voltage reference, a
temperature sensor, and a phase-locked loop (PLL). The
PLL makes it possible to run the core with a small and
inexpensive industry-standard 32 kHz watch crystal.
In the following paragraphs, a detailed description of the
functions and features of the control loop’s main components is presented.
3
AIN
MUX
13
12-BIT ADC
T/H
14
27
26
25
24
21
20
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
19
18
41
42
DAC
CONTROL
2
DAC0
BUF
11
DAC0
DAC1
BUF
12
DAC1
41
PWM0
42
PWM1
24
T0
25
T1
15
SS 14
MISO 21
MOSI 29
SINGLE-PIN
EMULATOR
EA 43
SYNCHRONOUS
SERIAL INTERFACE
(SPI)
16-BIT
COUNTER
TIMERS
PWM
TIME
INTERVAL
COUNTER
56
T2
1
T2EX
20
INT0
21
INT1
OSC
AND PLL
XTAL2 35
POWER SUPPLY
MONITOR
PSEN 44
ALE 45
TxD 19
50
38
DGND
37
23
51
36
DVDD
22
8
7
AGND
5
6
4
RESET 17
ASYNCHRONOUS
SERIAL PORT
(UART)
POR
256 8
USER RAM
WATCHDOG
TIMER
DOWNLOADER
DEBUGGER
9
AVDD
MCU
PORE
62K 8
PROGRAM
FLASH/EE
BUF
10
8052
SCLOCK 28
(–3mV/C)
2K 8
USER XRAM
XTAL1 34
4K 8
DATA
FLASH/EE
2.5V
BAND GAP
REFERENCE
CREF
ADC
CONTROL
AND
CALIBRATION
1
TEMP
SENSOR
VREF
40
ADuC832
56
RxD 18
ADC0
ADC1
ADC2
ADC3
ADC4
ADC5
ADC6
39
32
HARDWARE
25
CONVST
33
P2.0
P2.1
P2.2
P2.3
P2.4
P2.5
P2.6
P2.7
31
30
16
15
14
3
13
2
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
1
55
56
54
53
52
48
49
P0.0
P0.1
P0.2
P0.3
P0.4
P0.5
P0.6
P0.7
47
46
All these features plus all the standard peripherals that come
with an 8052 based controller, i.e., RS-232 interface and
I2C®/SPI®, are integrated in a 56-lead LFCSP space-saving
package. This will make a perfect fit in the housing of this
47 mm 17 mm 10 mm DVOA module.
Figure 3. ADuC832 MicroConverter Block Diagram
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AN-643
Two such log amps are implemented in the previously
described DVOA. One is used to measure the current
from the input photodiode and the other to measure the
output. (A dual version of the AD8305 will be offered in
the near future.) Through the use of the ADC on board the
MicroConverter IC, these measurements are compared
and processed to provide the corresponding digital value
to the on-board DAC.
AD8305 LOGARITHMIC AMP
The AD8305 logarithmic converter belongs to an Analog
Devices family of converters optimized for converting
optical power, measured as electrical current from a
photodiode, into a voltage level. The AD8305’s operation
is based on a translinear technique to provide a wide
dynamic range of power measurements (over five
decades). The input current, from a photodiode, is applied
to the collector of an NPN transistor, which in turn converts this current into a voltage (VBE). The VBE voltage is
directly proportional to the logarithm of the input current
establishing the basic logarithmic relationship. A second
identical NPN transistor is used to generate a second VBE
to use as a fixed reference for the converter. The difference
of the transistor VBE provides an absolute measurement
of the photodiode current and provides an output logvoltage that is proportional to the dB equivalent of the
incident optical power.
Communication, for calibration purposes, with the DVOA
module is done through the serial communication port
using the ADM101 from ADI. Communication is also done
over the I2C bus, which seems to have become the preferred
choice for optical module designs.
These days, component suppliers must focus on simple
and low cost optical components to be integrated in
existing systems, resulting in short-term revenues. The
Variable Optical Attenuator (VOA) is one of the most basic
but essential optical building blocks in the evolution of
optical networks. It exists in most optical subsystems and
applications, such as optical amplifiers, optical add/drop
modules, equalization modules, and testing elements.
With its various applications and growing use in optical
networking, the VOA component market is showing impressive growth and is expected to reach sales revenues of
$400M to $600M in 2006 (KMI/RHK). With this promising
VOA market forecast, Analog Devices is well positioned
to supply the electronic control circuitry needed to support such a module.
Temperature compensation circuitry is employed to ensure
good log conformity to within 0.1 dB over a wide range
of temperatures. The device is designed to operate with
a single positive supply. However, it can be run with dual
supplies. This feature provides flexibility, especially where
the anode on the photodiode has to be at the ground
level. A functional block diagram of the AD8305 is shown
in Figure 4.
The AD8305 includes an on-board amp to be used as a
buffer or for amplification as required by the specific
application. All these features are packaged in a spacesaving 16-lead 3 mm 3 mm LFCSP package. For more
details on the AD8305, refer to its data sheet.
5V
VPOS
VRDZ
VOUT
VREF
20k⍀
200k⍀
0.5V
80k⍀ 2.5V
COMM
IREF
VBIAS
VBE2
1k⍀
IPD
1k⍀
1nF
Q1
451⍀
VBE1
INPT
6.69k⍀
VSUM
1nF
SCAL
BFIN
14.2k⍀
TEMPERATURE ILOG
COMPENSATION
Q2
1nF
0.2 log10
BIAS
GENERATOR
COMM
0.5V
VNEG
COMM
Figure 4. AD8305 Log Amp Block Diagram
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© 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies.
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