Datasheet

21
®
DAC7744
DIGITAL INTERFACE
Table I shows the basic control logic for the DAC7744. Note
that each DAC register is edge triggered and not level
triggered. When the LOADDACS signal is transitioned to
HIGH, the digital word currently in the DAC register is
latched. The first set of registers (the input registers) are
triggered via the A0, A1, R/W, and CS inputs. Only one of
these registers is transparent at any given time.
The double-buffered architecture is designed mainly so that
each DAC input register can be written to at any time and
then all DAC voltages updated simultaneously by the rising
edge of LOADDACS. It also allows a DAC input register to
be written to at any point then the DAC output voltages can
be synchronously changed via a trigger signal connected to
LOADDACS.
DIGITAL TIMING
Figure 11 and Table II provide detailed timing for the digital
interface of the DAC7744.
DIGITAL INPUT CODING
The DAC7744 input data is in Straight Binary format. The
output voltage is given by Equation 1.
where N is the digital input code. This equation does not
include the effects of offset (zero scale) or gain (full scale)
errors.
INPUT DAC
A1 A0 R/W CS RST RSTSEL
LOADDACS REGISTER REGISTER MODE DAC
LLLLXXX Write Hold Write Input A
L H L L X X X Write Hold Write Input B
H L L L X X X Write Hold Write Input C
H H L L X X X Write Hold Write Input D
L L H L X X X Read Hold Read Input A
L H H L X X X Read Hold Read Input B
H L H L X X X Read Hold Read Input C
H H H L X X X Read Hold Read Input D
XXXHXX↑ Hold Write Update All
X X X H X X H Hold Hold Hold All
XXXX↑ L X Reset to Zero Reset to Zero All
XXXX↑ H X Reset to Midscale Reset to Midscale All
TABLE I. DAC7744 Logic Truth Table.
DIGITALLY-PROGRAMMABLE
CURRENT SOURCE
The DAC7744 offers a unique set of features that allows a
wide range of flexibility in designing applications circuits
such as programmable current sources. The DAC7744 offers
both a differential reference input as well as an open-loop
configuration around the output amplifier. The open-loop
configuration around the output amplifier allows transistor
to be placed within the loop to implement a digitally-
programmable, uni-directional current source. The availabil-
ity of a differential reference also allows programmability
for both the full-scale and zero-scale currents. The output
current is calculated as:
Figure 12 shows a DAC7744 in a 4-to-20mA current output
configuration. The output current can be determined by
Equation 3:
At full scale, the output current is 16mA plus the 4mA for
the zero current. At zero scale, the output current is the offset
current of 4mA (1V/250Ω).
(1)
VVL
VHVLN
OUT REF
REF REF
=+
()
–•
,65 536
(3)
I
VV N V
OUT
=
















+




51
250 65 536
1
250
–
•
,ΩΩ
(2)
I
VHVL
R
N
VLR
OUT
REF REF
SENSE
REF SENSE
=


















+
()
–
•
,
/
65 536