Datasheet

V
OUT
V
REFIN
/
V
REFOUT
V
REF
EXT
+6V
±5V
-6V
OPA703
DAC7678
10 Fm 0.1 Fm
R
1
10kW
R
2
10kW
Serial Interface
GND
AV
DD
AV
DD
|
REF_PRE REF_POST
6
HYST
REF_NOM
|V V
V = 10 (ppm/°C)
V
æ ö-
´ç ÷
ç ÷
è ø
IN
OUT
n
10 D
V = 5V
2
´
æ ö
-
ç ÷
è ø
IN 1 2
O UT REF REF
n
1
D R + R R2
V = V Gain V
R R1
2
æ ö
æ ö
æ ö æ ö
´ ´ ´ - ´
ç ÷
ç ÷
ç ÷ç ÷
ç ÷
è øè ø
è ø
è ø
DAC7678
www.ti.com SBAS493B –FEBRUARY 2010–REVISED JULY 2012
Long-Term Stability
Long-term stability/aging refers to the change of the
output voltage of a reference over a period of months
(5)
or years. This effect lessens as time progresses (see
Where:
Figure 3, the typical long-term stability curve). The
typical drift value for the internal reference is 100ppm
D
IN
= decimal equivalent of the binary code that
from 0 hours to 2160 hours. This parameter is
is loaded to the DAC register. It can range from 0
characterized by powering-up 19 units and measuring
to 4095 (12 bit)
them at regular intervals for a period of 2160 hours.
n = resolution in bits
Gain = 1 when External Reference is used and 2
Thermal Hysteresis
when internal reference is used.
Thermal hysteresis for a reference is defined as the
change in output voltage after operating the device at
(6)
25°C, cycling the device through the operating
temperature range, and returning to 25°C. Hysteresis
This result has an output voltage range of ±5V with
is expressed by Equation 4:
000h corresponding to a -5V output and FFFh
corresponding to a +5V output for the 12 bit
DAC7678.
(4)
Where:
V
HYST
= thermal hysteresis
V
REF_PRE
= output voltage measured at 25°C pre-
temperature cycling
V
REF_POST
= output voltage measured after the
device cycles through the temperature range of
–40°C to 125°C, and returns to 25°C.
DAC NOISE PERFORMANCE
Typical noise performance for the DAC7678 with the
Figure 96. Bipolar Output Range Using External
internal reference enabled is shown in Figure 47.
Reference at 5V
Output noise spectral density at the V
OUTX
pin versus
frequency is depicted in Figure 47 for full-scale,
midscale, and zero-scale input codes. The typical
MICROPROCESSOR INTERFACING
noise density for midscale code is 290nV/√Hz at
A basic connection diagram to the SCL and SDA pins
1kHz and 117nV/√Hz at 100 kHz when internal
of the DAC7678 is shown in Figure 97. The DAC7678
reference is enabled. The typical noise density
interfaces directly to standard mode, fast mode and
reduces to 104nV/√Hz at 1kHz for mid scale code
high speed mode of 2-Wire compatible serial
with external reference as shown in Figure 48. High-
interfaces. The DAC7678 does not perform clock
frequency noise can be improved by filtering the
stretching (pulling SCL low), as a result it is not
reference noise. Integrated output noise between
necessary to provide for this function unless other
0.1Hz and 10Hz is close to 3µVPP (midscale), as
devices on the same bus require this function. Pull-up
shown in Figure 49.
resistors are required on both the SDA and SCL lines
as the bus-drivers are open-drain. The size of these
BIPOLAR OPERATION USING THE DAC7678
pull-up resistors depends on the operating speed and
The DAC7678 family of products is designed for
capacitance of the bus lines. Higher value resistors
single-supply operation, but a bipolar output range is
consume less power but increase transition time on
also possible using the circuit in Figure 96. Rail-to-rail
the bus limiting the bus speed. Long bus lines have
operation at the amplifier output is achievable using
higher capacitance and require smaller pull-up
an OPA703 as the output amplifier.
resistors to compensate. The resistors should not be
too small; if they are, bus drivers may not be able to
The output voltage for any input code can be
pull the bus lines low.
calculated with Equation 5.
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