Datasheet

V
REF
R
R
R
R
V
REF
2
R
DIVIDER
To Output Amplifier
(2x Gain)
V =
OUT
V´
REFIN
D
IN
4096
V =
OUT
2 V´ ´
REFOUT
D
IN
4096
DAC
Register
REF(+)
Resistor String
REF( )-
V
REFIN
/V
REFOUT
V
OUT
X
178kW
150kW 150kW
DAC7678
www.ti.com SBAS493B –FEBRUARY 2010–REVISED JULY 2012
THEORY OF OPERATION
will be un-shorted if external reference is used. Thus
the overall gain will be one and allows the user to
DIGITAL-TO-ANALOG CONVERTER (DAC)
provide an external reference value of 0 to AVDD. If
The DAC7678 architecture consists of eight string
internal reference is used R
DIVIDER
is shorted and the
DACs each followed by an output buffer amplifier.
overall gain will be two.
The DAC7678 also includes an internal 2.5V
reference with a maximum 25ppm/°C temperature
drift performance, offering a 5V, full-scale output
voltage. Figure 86 shows a principal block diagram of
the DAC architecture.
Figure 86. Device Architecture
For the TSSOP package, the input coding to the
DAC7678 is straight binary. For the QFN package,
the TWOC pin controls the code format.
When using the internal reference, the ideal output
voltage is given by Equation 1:
(1)
When using an external reference, the ideal output
voltage is given by Equation 2:
(2)
Where:
D
IN
= decimal equivalent of the binary code that
Figure 87. Resistor String
is loaded to the DAC register. It can range from 0
to 4095.
V
REFOUT
= internal reference voltage of 2.5V (typ),
OUTPUT AMPLIFIER
supplied at the V
REFIN
/V
REFOUT
pin.
The output buffer amplifier is capable of generating
V
REFIN
= external reference voltage of 0V to 5V
rail-to-rail voltages on its output, giving a maximum
(typ), supplied at the V
REFIN
/V
REFOUT
pin.
output range of 0V to AV
DD
. It is capable of driving a
load of 2kΩ in parallel with 1000pF to GND. The
RESISTOR STRING
source and sink capabilities of the output amplifier
The resistor string circuitry is shown in Figure 87. It is
can be seen in the Typical Characteristics. The
a string of resistors, each of value R. The code
typical slew rate is 0.75V/μs, with a typical full-scale
loaded into the DAC register determines at which
settling time of 7μs with the output unloaded.
node on the string the voltage is tapped off to be fed
into the output amplifier by closing one of the
switches connecting the string to the amplifier. It is
monotonic because it is a string of resistors. R
DIVIDER
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