Datasheet

Data Sheet AD5381
Rev. D | Page 35 of 40
12-BIT DAC
DAC
REGISTER
INPUT
DATA
INPUT
REGISTER
DATA
REGISTER
B
DATA
REGISTER
A
A/B
VOUT
LDAC
CONTROL INPUT
03732-042
Figure 42. Toggle Mode Function
OPTICAL ATTENUATORS
Based on its high channel count, high resolution, monotonic
behavior, and high level of integration, the AD5381 is ideally
targeted at optical attenuation applications used in dynamic
gain equalizers, variable optical attenuators (VOAs), and optical
add-drop multiplexers (OADMs). In these applications, each
wavelength is individually extracted using an arrayed wave
guide; its power is monitored using a photodiode, transimped-
ance amplifier and ADC in a closed-loop control system. The
AD5381 controls the optical attenuator for each wavelength,
ensuring that the power is equalized in all wavelengths before
being multiplexed onto the fiber. This prevents information loss
and saturation from occurring at amplification stages further
along the fiber.
UTILIZING FIFO
The AD5381 FIFO mode optimizes total system update rates
in applications where a large number of channels need to be
updated. FIFO mode is only available when parallel interface
mode is selected. The FIFO EN pin is used to enable the FIFO.
The status of FIFO EN is sampled during the initialization
sequence. Therefore, the FIFO status can only be changed by
resetting the device.
In a telescope that provides for the cancel-lation of atmospheric
distortion, for example, a large number of channels need to be
updated in a short period of time. In such systems, as many as
400 channels need to be updated within 40 µs. Four-hundred
channels require the use of 10 AD5381s. With FIFO mode
enabled, the data write cycle time is 40 ns; therefore, each group
consisting of 40 channels can be fully loaded in 1.6 µs. In FIFO
mode, a complete group of 40 chan-nels will update in 14.4 µs.
The time taken to update all 400 channels is
14.4 µs + 9 × 1.6 µs = 28.8 µs.
Figure 44 shows the FIFO operation scheme.
ATTENUATOR
ATTENUATOR
ATTENUATOR
ATTENUATOR
AWG
AWG
FIBREFIBRE
DWDM
OUT
OPTICAL
SWITCH
11
12
1n–1
1n
DWDM
IN
AD5381,
40-CHANNEL,
12-BIT DAC
N:1 MULTIPLEXER
16-BIT ADCCONTROLLER
TIA/LOG AMP
(AD8304/AD8305)
ADG731
(40:1 MUX)
AD7671
(0V TO 5V, 1MSPS)
PHOTODIODES
ADD
PORTS
DROP
PORTS
03732-043
Figure 43. OADM Using the AD5381 as Part of an Optical Attenuator