Datasheet

AD5381 Data Sheet
Rev. D | Page 34 of 40
MONITOR FUNCTION
The AD5381 channel monitor function consists of a multiplexer
addressed via the interface, allowing any channel output to be
routed to this pin for monitoring using an external ADC. In
channel monitor mode, VOUT39 becomes the MON_OUT pin,
to which all monitored signals are routed. The channel monitor
function must be enabled in the control register before any
channels are routed to MON_OUT. Table 17 contains the
decoding information required to route any channel to
MON_OUT. Selecting Channel Address 63 three-states
MON_OUT. Figure 41 shows a typical monitoring circuit
implemented using a 12-bit SAR ADC in a 6-lead SOT-23
package. The controller output port selects the channel to be
monitored, and the input port reads the converted data from
the ADC.
AD7476
GND
SDATA
CS
SCLK
VDD
VINVOUT39/MON_OUT
AGND
DIN
SYNC
SCLK
DAC_GND SIGNAL_GND
VOUT0
VOUT38
AVDD
AD5381
OUTPUT PORT
INPUT PORT
CONTROLLER
03732-041
Figure 41. Typical Channel Monitoring Circuit
TOGGLE MODE FUNCTION
The toggle mode function allows an output signal to be gener-
ated using the
LDAC
control signal that switches between two
DAC data registers. This function is configured using the SFR
control register as follows. A write with REG1 = REG0 = 0 and
A5 to A0 = 001100 specifies a control register write. The toggle
mode function is enabled in groups of eight channels using Bit
CR4 to Bit CR0 in the control register. See the AD5381 control
register description. Figure 42 shows a block diagram of toggle
mode implementation. Each of the 40 DAC channels on the
AD5381 contain an A and B data register.
Note that B registers can only be loaded when toggle mode is
enabled. The sequence of events when configuring the AD5381
for toggle mode is
1. Enable toggle mode for the required channels via the
control register.
2. Load data to the A registers.
3. Load data to the B registers.
4. Apply
LDAC
.
LDAC
is used to switch between the A and B registers in
determining the analog output. The first
LDAC
configures the
output to reflect data in the A registers. This mode offers signif-
icant 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 CR4 to CR2 = 0, 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 will update the out-
puts with the data contained in the A registers.
THERMAL MONITOR FUNCTION
The AD5381 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 AD5381 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
LQFP, the θ
JA
is typically 44°C/W.
The thermal monitor is enabled by the user via CR6 in the
control register. The output amplifiers on the AD5381 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 has dropped below 130°C or by turning off the
thermal monitor function via the control register.