Datasheet

AD5724/AD5734/AD5754
Rev. D | Page 20 of 32
LOAD DAC (LDAC)
After data has been transferred into the input register of the
DACs, there are two ways to update the DAC registers and DAC
outputs. Depending on the status of both
SYNC
and
LDAC
, one
of two update modes is selected: individual DAC updating or
simultaneous updating of all DACs.
SYNC
SCLK
V
OUT
DAC
REGISTER
INTERFACE
LOGIC
OUTPUT
AMPLIFIER
LDAC
SDO
SDIN
REFIN
INPUT
REGISTER
12-/14-/16-BIT
DAC
06468-009
Figure 41. Simplified Diagram of Input Loading Circuitry for One DAC
Individual DAC Updating
In this mode,
LDAC
is held low while data is being clocked into
the input shift register. The addressed DAC output is updated
on the rising edge of
SYNC
.
Simultaneous Updating of All DACs
In this mode,
LDAC
is held high while data is being clocked
into the input shift register. All DAC outputs are asynchronously
updated by taking
LDAC
low after
SYNC
has been taken high.
The update now occurs on the falling edge of
LDAC
.
ASYNCHRONOUS CLEAR (CLR)
CLR
is an active low clear that allows the outputs to be cleared
to either zero-scale code or midscale code. The clear code value is
user-selectable via the CLR select bit of the control register (see
the section). It is necessary to maintain Control Register
CLR
low
for a minimum amount of time to complete the operation (see
). When the Figure 2
CLR
signal is returned high, the output
remains at the cleared value until a new value is programmed.
The outputs cannot be updated with a new value while the
CLR
pin is low. A clear operation can also be performed via the clear
command in the control register.
CONFIGURING THE AD5724/AD5734/AD5754
When the power supplies are applied to the AD5724/AD5734/
AD5754, the power-on reset circuit ensures that all registers
default to 0. This places all channels in power-down mode. The
DV
CC
should be brought high before any of the interface lines
are powered. If this is not done, the first write to the device may
be ignored. The first communication to the AD5724/AD5734/
AD5754 should be to set the required output range on all
channels (the default range is the 5 V unipolar range) by writing
to the output range select register. The user should then write to
the power control register to power on the required channels. To
program an output value on a channel, that channel must first
be powered up; any writes to a channel while it is in power-down
mode are ignored. The AD5724/ AD5734/AD5754 operate with a
wide power supply range. It is important that the power supply
applied to the parts provides adequate headroom to support the
chosen output ranges.
TRANSFER FUNCTION
Table 7 to Tabl e 15 show the relationships of the ideal input code
to output voltage for the AD5754, AD5734, and AD5724, respec-
tively, for all output voltage ranges. For unipolar output ranges,
the data coding is straight binary. For bipolar output ranges, the
data coding is user-selectable via the BIN/
2sCOMP
pin and can
be either offset binary or twos complement.
For a unipolar output range, the output voltage expression is
given by
⎥
⎦
⎤
⎢
⎣
⎡
×=
N
REFIN
OUT
D
GainVV
2
For a bipolar output range, the output voltage expression is given by
22
REFIN
N
REFIN
OUT
VGain
D
GainVV
×
−
⎥
⎦
⎤
⎢
⎣
⎡
×=
where:
D is the decimal equivalent of the code loaded to the DAC.
N is the bit resolution of the DAC.
V
REFIN
is the reference voltage applied at the REFIN pin.
Gain is an internal gain whose value depends on the output
range selected by the user, as shown in Table 6.
Table 6. Internal Gain Values
Output Range (V) Gain Value
+5 2
+10 4
+10.8 4.32
±5 4
±10 8
±10.8 8.64