AD5337/AD5338/AD5339
DOUBLE-BUFFERED INTERFACE
The AD5337/AD5338/AD5339 DACs all have a double-buffered
interface consisting of two banks of registers—an input register
and a DAC register per channel. The input register is directly
connected to the input shift register, and the digital code is
transferred to the relevant input register upon completion of a
valid write sequence. The DAC register contains the digital code
used by the resistor string.
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Access to the DAC register is controlled by the LDAC bit. When
the LDAC bit is set high, the DAC register is latched and therefore
the input register may change state without affecting the DAC
register. This is useful if the user requires simultaneous updating
of all DAC outputs. The user may write to three of the input
registers individually; by setting the LDAC bit low when writing
to the remaining DAC input register, all outputs will update
simultaneously.
These parts contain an extra feature whereby the DAC register
is only updated if its input register has been updated since the
last time that LDAC was brought low, thereby removing
unnecessary digital crosstalk.
POWER-DOWN MODES
The AD5337/AD5338/AD5339 have very low power
consumption, typically dissipating 0.75 mW with a 3 V supply
and 1.5 mW with a 5 V supply. Power consumption can be
further reduced when the DACs are not in use by putting them
into one of three power-down modes, which are selected by
Bits 15 and 14 (PD1 and PD0) of the data byte. Table 8 shows
how the state of the bits corresponds to the mode of operation
of the DAC.
Table 8. PD1/PD0 Operating Modes
PD1
PD0
Operating Mode
0
0
Normal Operation
0
1
Power-Down (1 k Load to GND)
1
0
Power-Down (100 k Load to GND)
1
1
Power-Down (3-State Output)
When both bits are 0, the DAC works with its normal power
consumption of 300 μA at 5 V. However, for the three power-
down modes, the supply current falls to 200 nA at 5 V (80 nA at
3 V). Not only does the supply current drop, but the output
stage is also internally switched from the output of the amplifier
to a resistor network of known values. This is advantageous in
that the output impedance of the part is known while the part is
in power-down mode, which provides a defined input condition
for whatever is connected to the output of the DAC amplifier.
There are three options. The output may be connected internally
to GND through a 1 k resistor, a 100 k resistor, or may be
left open-circuited (3-state). Resistor tolerance = ±20%. The
output stage is illustrated in Figure 35.
0
AMPLIFIER
POWER-DOWN
CIRCUITRY
RESISTOR
NETWORK
RESISTOR
STRING DAC
V
OUT
Figure 35. Output Stage during Power-Down
The bias generator, the output amplifiers, the resistor string, and
all other associated linear circuitry are shut down when power-
down mode is activated. However, the contents of the DAC
registers remain unchanged when power-down mode is activated.
The time to exit power-down is typically 2.5 μs for V
DD
= 5 V
and 5 μs when V
DD
= 3 V. This is the time from the rising edge
of the eighth SCL pulse to the time when the output voltage
deviates from its power-down voltage. See Figure 23 for a plot.