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| 型号： | ADN2872ACPZ-R7 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 10/20页 |
| 文件大小： | 0K |
| 描述： | IC LASR DVR 3.3GBPS 3.3V 24LFCSP |
| 标准包装： | 1 |
| 类型： | 激光二极管驱动器 |
| 数据速率： | 3.3Gbps |
| 通道数： | 1 |
| 电源电压： | 3 V ~ 3.6 V |
| 电流 - 电源： | 30mA |
| 电流 - 调制： | 90mA |
| 电流 - 偏置： | 100mA |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 24-VFQFN 裸露焊盘，CSP |
| 供应商设备封装： | 24-LFCSP-VQ（4x4） |
| 包装： | 标准包装 |
| 安装类型： | 表面贴装 |
| 产品目录页面： | 788 (CN2011-ZH PDF) |
| 其它名称： | ADN2872ACPZ-R7DKR |
ADN2872
THEORY OF OPERATION
Laser diodes have a current-in to light-out transfer function, as
shown in Figure 23. Two key characteristics of this transfer
MPD
INPUT
OPTICAL COUPLING
BIAS
function are the threshold current, I TH , and slope in the linear
region beyond the threshold current, referred to as slope
efficiency, LI.
P1
ER =
P0
Φ 2
I EX
Gm
1.2V
V BGAP
I PA
Φ 1
SHA
BIAS
CURRENT
HIGH
SPEED
V CC
P1
P AV =
P1 + P0
2
ERSET
PAVSET
MOD
SWITCH
Φ 2
SHA
P AV
Δ P
Φ 2
MOD
P0
Δ I
LI =
Δ P
Δ I
CURRENT 100 2
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
A dual loop is made up of an average power control loop
I TH
CURRENT
(APCL) and the extinction ratio control loop (ERCL), which are
Figure 23. Laser Transfer Function
DUAL-LOOP CONTROL
Typically, laser threshold current and slope efficiency are both
functions of temperature. For FP and DFB type lasers, the
threshold current increases and the slope efficiency decreases
with increasing temperature. In addition, these parameters vary
as the laser ages. To maintain a constant optical average power
and a constant optical extinction ratio over temperature and
laser lifetime, it is necessary to vary the applied electrical bias
current and modulation current to compensate for the laser
changing LI characteristics.
Single-loop compensation schemes use the average monitor
photodiode (MPD) current to measure and maintain the
average optical output power over temperature and laser aging.
The ADN2872 is a dual-loop device, implementing both this
primary average power control loop and a secondary control
loop, which maintains a constant optical extinction ratio. The
dual-loop control of the average power and extinction ratio
implemented in the ADN2872 can be used successfully with
both lasers that maintain good linearity of LI transfer characteris-
tics over temperature, and with those that exhibit increasing
nonlinearity of the LI characteristics over temperature.
Dual Loop
The ADN2872 uses a proprietary patented method to control both
average power and extinction ratio. The ADN2872 is constantly
sending a test signal on the modulation current signal and
reading the resulting change in the MPD current as a means of
detecting the slope of the laser in real time. This information is
used in a servo to control the ER of the laser, which is done in a
time-multiplexed manner at a low frequency, typically 80 Hz.
ADN2872.
separated into two time states. During Time Φ1, the APC loop
is operating, and during Time Φ2, the ER loop is operating.
Average Power Control Loop
The APCL compensates for changes in the laser diode (LD), I TH
and LI, by varying I BIAS . APC control is performed by measuring
the MPD current, I MPD . This current is bandwidth limited by the
MPD. This is not a problem because the APCL must be low
frequency and the APCL must respond to the average current
from the MPD. The APCL compares I MPD × R PAVSET to the BGAP
voltage, V BGAP . If I MPD falls, the bias current is increased until
I MPD × R PAVSET equals V BGAP . Conversely, if the I MPD increases, I BIAS
is decreased.
Modulation Control Loop
The ERCL measures the slope efficiency, LI, of the laser diode
by monitoring the I MPD changes. During the ERCL, I MPD is
temporarily increased by ΔI MOD . The ratio between I MPD and
ΔI MOD is a fixed ratio of 50:1, but during startup, this ratio is
increased to decrease settling time.
During ERCL, switching in ΔI MOD causes a temporary increase
in average optical power, ΔP AV . However, the APC loop is disabled
during ERCL, and the increase is kept small enough so as not to
disturb the optical eye. When ΔI MOD is switched into the laser
circuit, an equal current, I EX , is switched into the PAVSET resis-
tor. The user sets the value of I EX ; this is the ERSET setpoint. If
ΔI MPD is too small, the control loop knows that LI has decreased,
and increases I MPD and, therefore, ΔI MOD accordingly until ΔI MPD
is equal to I EX . The previous control cycle status of the I BIAS and I MOD
settings are stored on the hold capacitors, PAVCAP and ERCAP.
The ERCL is constantly measuring the actual LI curve; it compen-
sates for the effects of temperature and for changes in the LI
curve due to laser aging. Therefore, the laser can be calibrated
once at 25°C so that it can then automatically control the laser
over temperature. This eliminates the expensive and time
consuming temperature calibration of a laser.
Rev. 0 | Page 10 of 20
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