Analog Devices AN643 Application Notes

AN-643

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • Fax: [phone] • 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

2

C system, provided that the system con-

2

C Standard Specifi cation as defi ned by Philips.

2

C Patent Rights to use these

ADuC832

ATTENUATOR

DAC1

CONTROL VOLTAGE

C/SPI BUS

BOOSTER

2

I

OPTICAL INPUT POWER
MEASUREMENT

LOG AMP

AD8305

36V

VOLTAG E

CIRCUIT

AD823

5V

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®

I

/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

V

REF

C

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

BUF

PWM

16-BIT

COUNTER

TIME

OSC

34

XTAL1

P3.7

TIMERS

35

XTAL2

11

12

41

42

24

25

56

20

21

1

DAC0

DAC1

PWM0

PWM1

T0

T1

T2

T2EX

INT0
INT1

2

3

47

46

25

56

1

2

3

13

14

15

10

9

5

6

4

DD

AV

48

49

MUX

TEMP

SENSOR

2.5V

BAND GAP

REFERENCE

7

8

223651

AGND

525354

AIN

DD

DV

T/H

BUF

POR

55

56

12-BIT ADC

(–3mV/C)

233738

DGND

1

131415

4K  8

DATA

FLASH/EE

62K  8
PROGRAM
FLASH/EE

DOWNLOADER

DEBUGGER

ASYNCHRONOUS

SERIAL PORT

(UART)

50

17

RESET

18

RxD

ADC

19

TxD

16

AND

CONTROL

3031323339

ADuC832

CALIBRATION

8052

MCU

PORE

EMULATOR

SINGLE-PIN

45

44

43

EA

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

14

SS

INTERVAL
COUNTER

AND PLL

Figure 3. ADuC832 MicroConverter Block Diagram

–2–

REV. 0

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

BE

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

BE

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

provides an absolute measurement

BE

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

2

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

V

BIAS

200k⍀

1k⍀

1nF

PD

1k⍀

1nF 1nF

5V

VPOS

VRDZ

VREF

IREF

INPTI

VSUM

0.5V

0.5V

20k⍀

80k⍀

COMM

Q2

Q1

VNEG

GENERATOR

2.5V

V

BE2

COMPENSATION

V

BE1

BIAS

TEMPERATURE

Figure 4. AD8305 Log Amp Block Diagram

–3–

14.2k⍀

6.69k⍀

COMM

I

LOG

COMM

451⍀

VOUT

SCAL
BFIN

0.2 log

I

PD

()

10

1nA

E03644–.1–3/03(0)

© 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies.

–4–