AD5384BBC-5 datasheet(33/36 Pages) AD

33 / 36 page

AD5384

Rev. A | Page 33 of 36

Each of the 40 DAC channels on the AD5384 contains an A and

B data register. Note that the B registers can be loaded only

when toggle mode is enabled. The sequence of events when

configuring the AD5384 for toggle mode is

Enable toggle mode for the required channels via the

control register.

Load data to A registers.

Load data to B registers.

Apply LDAC.

The LDAC is used to switch between the A and B registers in

determining the analog output. The first LDAC configures the

output to reflect the data in the A registers. This mode offers

significant advantages if the user wants to generate a square

wave at the output of all 40 channels, as might be required to

drive a liquid crystal-based variable optical attenuator. In this

case, the user writes to the control register and enables the

toggle function by setting CR6 to CR2 = 1, thus enabling the

five groups of eight for toggle mode operation. The user must

then load data to all 40 A and B registers. Toggling LDAC sets

the output values to reflect the data in the A and B registers.

The frequency of the LDAC determines the frequency of the

square wave output.

Toggle mode is disabled via the control register. The first LDAC

following the disabling of the toggle mode updates the outputs

with the data contained in the A registers.

THERMAL MONITOR FUNCTION

The AD5384 contains a temperature shutdown function to

protect the chip if multiple outputs are shorted. The short-

circuit current of each output amplifier is typically 40 mA.

Operating the AD5384 at 5 V leads to a power dissipation of

200 mW per shorted amplifier. With five channels shorted, this

leads to an extra watt of power dissipation. For the 100-lead

CSPBGA, the θJA is typically 44°C/W.

The thermal monitor is enabled by the user via CR8 in the

control register. The output amplifiers on the AD5384 are

automatically powered down if the die temperature exceeds

approximately 130°C. After a thermal shutdown has occurred,

the user can re-enable the part by executing a soft power-up if

the temperature drops below 130°C, or by turning off the

thermal monitor function via the control register.

AD5384 IN A MEMS-BASED OPTICAL SWITCH

In their feed-forward control paths, MEMS based optical

switches require high resolution DACs that offer high channel

density with 14-bit monotonic behavior. The 40-channel, 14-bit

AD5384 DAC satisfies these requirements. In the circuit in

Figure 38, the 0 V to 5 V outputs of the AD5384 are amplified

to achieve an output range of 0 V to 200 V, which is used to

control actuators that determine the position of MEMS mirrors

in an optical switch. The exact position of each mirror is

measured using sensors. The sensor outputs are multiplexed

into a high resolution ADC in determining the mirror position.

The control loop is closed and driven by an ADSP-21065L, a

32-bit SHARC® DSP with an SPI-compatible SPORT interface.

The ADSP-21065L writes data to the DAC, controls the multi-

plexer, and reads data from the ADC via the serial interface.

ACTUATORS

FOR MEMS

MIRROR

ARRAY

SENSOR

AND

MULTIPLEXER

8-CHANNEL ADC

(AD7856)

SINGLE-CHANNEL

ADC (AD7671)

AD5384

14-BIT DAC

REFOUT REFINA AVDD

VOUT1

VOUT40

G = 50

OUTPUT RANGE

0V–200V

ADSP-21065L

0.01

µF

Figure 38. AD5384 in a MEMS-Based Optical Switch

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