Datasheet AN643 Application Notes (Analog Devices)

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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 ﬁ ber 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 ampliﬁ ers (EDFA and Raman), laser diode drivers, and photodetectors are among these devices, just to name a few.

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

INPUT OUTPUT

COUPLER

SILICON VOA 40dB

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.

Figure 1. GalayOr DVOA

INTEGRATED COUPLER

INTEGRATED

OPTICAL INPUT POWER MEASUREMENT

LOG AMP

AD8305

VOLTAG E

REF.

AD1584

RS-232 BUS

RS-232

INTERFACE

ADM101

PHOTODIODES

MicroConverter

ADC1 ADC2

Figure 2. Overall DVOA Module Block Diagram

REV. 0

Purchase of licensed I2C components of Analog Devices or one of its sublicensed Associated Companies conveys a license for the purchaser under the Philips I components in an I forms to the I

C system, provided that the system con-

C Standard Speciﬁ cation as deﬁ ned by Philips.

C Patent Rights to use these

ADuC832

ATTENUATOR

DAC1

CONTROL VOLTAGE

C/SPI BUS

BOOSTER

OPTICAL INPUT POWER MEASUREMENT

LOG AMP

AD8305

36V

VOLTAG E

CIRCUIT

AD823

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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 vol tages are then fed into two different ADC channels, on board the MicroConverter IC, for processing.

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 ampliﬁ ed 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.

In the following paragraphs, a detailed description of the functions and features of the control loop’s main components is presented.

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.

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.

All these features plus all the standard peripherals that come with an 8052 based controller, i.e., RS-232 interface and

2C®

/SPI®, are integrated in a 56-lead LFCSP space-saving package. This will make a perfect ﬁ t in the housing of this 47 mm  17 mm  10 mm DVOA module.

HARDWARE

CONVST

ADC0 ADC1 ADC2 ADC3 ADC4 ADC5 ADC6

REF

P0.0

P0.1

P0.2

P0.3

P0.4

P0.5

P0.6

P0.7

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

P2.0

P2.1

P2.2

P2.3

P2.4

P2.5

P2.6

P2.7

P3.0

P3.1

P3.2

P3.3

P3.4

P3.5

P3.6

BUF

PWM

16-BIT

COUNTER

TIME

OSC

XTAL1

P3.7

TIMERS

XTAL2

DAC0

DAC1

PWM0

PWM1

T2EX

INT0 INT1

MUX

TEMP

SENSOR

2.5V

BAND GAP

REFERENCE

223651

AGND

525354

AIN

T/H

BUF

POR

12-BIT ADC

(–3mV/C)

233738

DGND

131415

4K  8

DATA

FLASH/EE

62K  8 PROGRAM FLASH/EE

DOWNLOADER

DEBUGGER

ASYNCHRONOUS

SERIAL PORT

(UART)

RESET

RxD

ADC

TxD

AND

CONTROL

3031323339

ADuC832

CALIBRATION

8052

MCU

PORE

EMULATOR

SINGLE-PIN

ALE

PSEN

4041421819202124252627

DAC0

DAC

CONTROL

DAC1

2K  8

USER XRAM

256  8

USER RAM

WATCHDOG

TIMER

POWER SUPPLY

MONITOR

SYNCHRONOUS

SERIAL INTERFA CE

(SPI)

282921

MOSI

SCLOCK

MISO

INTERVAL COUNTER

AND PLL

Figure 3. ADuC832 MicroConverter Block Diagram

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AN-643

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 photo diode, 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 (V

). The VBE voltage is

directly propor tional to the logarithm of the input current establishing the basic logarithmic relationship. A second identical NPN transistor is used to generate a second V

to use as a ﬁ xed refer ence for the co nverter. The dif ference of the transistor V

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.

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 ﬂ exibility, 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 ampliﬁ cation as required by the speciﬁ c 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.

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.

Communication, for calibration purposes, with the DVOA module is done through the serial communication port using the ADM101 from ADI. Communication is also done

choice for optical module designs.

over the I

C bus, which seems to have become the preferred

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 Var iab le Op tic al A tt enu ato r (VOA ) is one of the most basic but essential optical building blocks in the evolution of optical net works. It exist s in m o s t o p t i c a l s u b s y s t e m s a n d applications, such as optical ampliﬁ ers, 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 expec ted 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.

REV. 0

BIAS

200k⍀

1k⍀

1nF

1k⍀

1nF 1nF

VPOS

VRDZ

VREF

IREF

INPTI

VSUM

0.5V

20k⍀

80k⍀

COMM

VNEG

GENERATOR

2.5V

BE2

COMPENSATION

BE1

BIAS

TEMPERATURE

Figure 4. AD8305 Log Amp Block Diagram

–3–

14.2k⍀

6.69k⍀

COMM

LOG

COMM

451⍀

VOUT

SCAL BFIN

0.2 log

()

1nA

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E03644–.1–3/03(0)

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