AD5360/AD5361
Rev. A | Page 16 of 28
A/B REGISTERS GAIN/OFFSET ADJUSTMENT
Each DAC channel has seven data registers. The actual DAC
data word can be written to either the X1A or X1B input
register, depending on the setting of the
A
/B bit in the control
register. If the
A
/B bit is 0, data is written to the X1A register. If
the
A
/B bit is 1, data is written to the X1B register. Note that
this single bit is a global control and affects every DAC channel
in the device. It is not possible to set up the device on a per-
channel basis so that some writes are to the X1A register and
some writes are to the X1B register.
MUX
DAC
DAC
REGISTER
MUX
X1A
REGISTER
X1B
REGISTER
M
REGISTER
C
REGISTER
X2A
REGISTER
X2B
REGISTER
05761-023
Figure 21. Data Registers Associated with Each DAC Channel
Each DAC channel also has a gain register (M) and an offset (C)
register, which allow trimming out of the gain and offset errors
of the entire signal chain. Data from the X1A register is oper-
ated on by a digital multiplier and adder by the contents of the
M and C registers. The calibrated DAC data is then stored in the
X2A register. Similarly, data from the X1B register is operated
on by the multiplier and adder and stored in the X2B register.
Although a multiplier and adder symbol are shown for each
channel, there is only one multiplier and one adder in the
device, which are shared among all channels. This has
implications for the update speed when several channels are
updated at once, as described in the Register Update Rates
section.
Each time data is written to the X1A register, or to the M or
C register with the
A
/B control bit set to 0, the X2A data is
recalculated and the X2A register is automatically updated.
Similarly, X2B is updated each time data is written to X1B, or
to M or C with
A
/B set to 1. The X2A and X2B registers are
not readable or directly writable by the user.
Data output from the X2A and X2B registers is routed to the
final DAC register by a multiplexer. An 8-bit
A
/B select register
associated with each group of eight DACs controls whether
each individual DAC takes its data from the X2A or X2B
register. If a bit in this register is 0, the DAC takes its data
from the X2A register; if 1, the DAC takes its data from the
X2B register (Bit 0 through Bit 7 control DAC 0 through
DAC 7, respectively).
Note that because there are 16 bits in two registers, it is possible
to set up, on a per-channel basis, whether each DAC takes its
data from the X2A register or X2B register. A global command
is also provided that sets all bits in the
A
/B select registers to 0
or to 1.
All DACs in the AD5360/AD5361 can be updated simultane-
ously by taking
LDAC
low, when each DAC register is updated
from either its X2A or X2B register, depending on the setting of
the
A
/B select registers. The DAC register is not readable or
directly writable by the user.
OFFSET DACs
In addition to the gain and offset trim for each DAC, there are
two 14-bit offset DACs, one for Group 0, and one for Group 1.
These allow the output range of all DACs connected to them to
be offset within a defined range. Thus, subject to the limitations
of headroom, it is possible to set the output range of Group 0
and/or Group 1 to be unipolar positive, unipolar negative, or
bipolar (either symmetrical or asymmetrical) about 0 V. The
DACs in the AD5360/AD5361 are factory trimmed with the
offset DACs set at their default values. This gives the best offset
and gain performance for the default output range and span.
When the output range is adjusted by changing the value of
the offset DAC, an extra offset is introduced due to the gain
error of the offset DAC. The amount of offset is dependent on
the magnitude of the reference and how much the offset DAC
moves from its default value. This offset is shown in Table 1. The
worst-case offset occurs when the offset DAC is at positive full
scale or negative full scale. This value can be added to the offset
present in the main DAC of a channel to give an indication of
the overall offset for that channel. In most cases, the offset can be
removed by programming the C register of the channel with an
appropriate value. The extra offset caused by the offset DACs
needs to be taken into account only when the offset DAC is
changed from its default value. Figure 22 shows the allowable
code range that can be loaded to the offset DAC, and this is
dependent on the reference value used. Thus, for a 5 V
reference, the offset DAC should not be programmed with
a value greater than 8192 (0x2000).
0 4096 8192 12288 16383
OFFSET DAC CODE
0
1
2
3
4
V
R
E
F
(
V
)
5
RESERVED
5761-005
Figure 22. Offset DAC Code Range
