Datasheet

DAC7731
11
SBAS249B
www.ti.com
V
CC
REF
OUT
REF
IN
REFADJ
V
REF
R
OFFSET
AGND
RFB2
RFB1
SJ
V
OUT
V
DD
V
SS
REFEN
RSTSEL
SCLK
CS
SDO
SDI
LDAC
RST
NC
TEST
DGND
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
DAC7731
1µF0.1µF
V
CC
1µF0.1µF
V
DD
Control/Data
Bus
1µF0.1µF
V
SS
(–10V to +10V)
FIGURE 4. Basic Operation: V
OUT
= –10V to +10V.
ANALOG OUTPUTS
The output amplifier can swing to within 1.4V of the supply
rails, specified over the –40°C to +85°C temperature range.
This allows for a ±10V DAC voltage output operation from
±12V supplies with a typical 5% tolerance.
When the DAC7731 is configured for a unipolar, 0V to 10V
output, a negative voltage supply is required. This is due to
internal biasing of the output stage. Please refer to the
Electrical Characteristics table (see page 3) for more infor-
mation.
The minimum and maximum voltage output values are de-
pendent upon the output configuration implemented and
reference voltage applied to the DAC7731. Please note that
V
SS
(the negative power supply) must be in the range of
–4.75V to –15.75V for unipolar operation. The voltage on V
SS
sets several bias points within the converter and is required
in all modes of operation. If V
SS
is not in one of these two
configurations, the bias values may be in error and proper
operation of the device is not ensured.
Supply sequence is important in establishing the correct
startup of the DAC. The following supply sequence must be
followed: V
SS
(device substrate) first, then V
DD
followed by
V
CC
. In addition, each supply must reach the values specified
in the Electrical Characteristics table (see page 3) within
100ms of its ramp start.
REFERENCE INPUTS
The DAC7731 provides a built-in +10V voltage reference and
on-chip buffer to allow external component reference drive. To
use the internal reference, REFEN must be LOW, enabling the
reference circuitry of the DAC7731 (as shown in Table I) and
the REF
OUT
pin must be connected to REF
IN
. This is the input
to the on-chip reference buffer. The buffer output is provided
at the V
REF
pin. In this configuration, V
REF
is used to setup the
REFEN ACTION
1 Internal Reference disabled;
REF
OUT
= High Impedance
0 Internal Reference enabled;
REF
OUT
= +10V
TABLE I. REFEN Action.
The internal reference of the DAC7731 can be disabled when
use of an external reference is desired. When using an
external reference, the reference input, REF
IN
, can be any
voltage between 4.75V (or V
SS
+ 14V, whichever is greater)
and V
CC
– 1.4V.
DIGITAL INTERFACE
Table II shows the input data format for the DAC7731 and
Table III illustrates the basic control logic of the device. The
serial interface consists of a chip select input (CS), serial data
clock input (SCLK), serial data input (SDI), serial data output
(SDO), and load control input (LDAC). An asynchronous reset
input (RST), which is active LOW, is provided to simplify start-
up conditions, periodic resets, or emergency resets to a known
state, depending on the status of the reset select (RSTSEL)
signal. Please refer to the
DAC Reset section for additional
information regarding the reset operation.
CONTROL STATUS COMMAND
CS RST RSTSEL LDAC SCLK ACTION
H H X X X Shift Register is disabled on the serial bus.
Enable SDO pin from High Impedance;
L H X X X enables shift operation and I/O bus
(SCLK, SDI, SDO).
LH X X ↑ Serial Data Shifted into Input Register
↑ H X X L Serial Data Shifted into Input Register
(1)
XH X ↑ X Data in Input Register is Loaded into DAC Register.
X L H X X Resets Input and DAC Registers to mid-scale.
X L L X X Resets Input and DAC Registers to min-scale.
NOTE: (1) In order to avoid unwanted shifting of the input register by an
additional bit, care must be taken that a rising edge on CS only occurs
when SCLK is HIGH.
TABLE III. DAC7731 Logic Truth Table.
ANALOG OUTPUT
TABLE II. DAC7731 Data Format.
DIGITAL INPUT Unipolar Configuration Bipolar Configuration
Unipolar Straight Binary Bipolar Offset Binary
0x0000 Zero (0V)
–Full-Scale (–V
REF
or –V
REF
/2)
0x0001 Zero + 1LSB –Full-Scale + 1LSB
:: :
0x8000 1/2 Full-Scale Bipolar Zero
0x8001 1/2 Full-Scale + 1LSB Bipolar Zero + 1LSB
:: :
0xFFFF Full-Scale (V
REF
– 1LSB) +Full-Scale (+V
REF
– 1LSB
or +V
REF
/2 – 1LSB)
DAC7731 output amplifier into one of three voltage output
modes as discussed earlier. V
REF
can also be used to drive
other system components requiring an external reference.