DAC7678_10 Datasheet, PDF (23/52 Pages) Texas Instruments – 12-Bit, Octal-Channel, Ultra-Low Glitch, Voltage Output, Two-Wire Interface Digital-to-Analog Converter with 2.5V Internal Reference

DAC7678
www.ti.com
SBAS493A – FEBRUARY 2010 – REVISED AUGUST 2010
THEORY OF OPERATION
DIGITAL-TO-ANALOG CONVERTER (DAC)
The DAC7678 architecture consists of eight string
DACs each followed by an output buffer amplifier.
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.
will be un-shorted if external reference is used. Thus
the overall gain will be one and allows the user to
provide an external reference value of 0 to AVDD. If
internal reference is used R_DIVIDER is shorted and the
overall gain will be two.
V_REF
R_DIVIDER
DAC
Register
V_REFIN/V_{REFOUT 150kW}
178kW
REF(+)
Resistor String
REF(-)
150kW
V_OUTX
V_REF
2
R
R
To Output Amplifier
(2x Gain)
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:
V_OUT
=
D_IN
4096
´
2´
V_REFOUT
(1)
When using an external reference, the ideal output
voltage is given by Equation 2:
V_OUT
=
D_IN
4096
´
V_REFIN
(2)
Where:
D_IN = decimal equivalent of the binary code that
is loaded to the DAC register. It can range from 0
to 4095.
V_REFOUT = internal reference voltage of 2.5V (typ),
supplied at the V_REFIN/V_REFOUT pin.
V_REFIN = external reference voltage of 0V to 5V
(typ), supplied at the V_REFIN/V_REFOUT pin.
RESISTOR STRING
The resistor string circuitry is shown in Figure 87. It is
a string of resistors, each of value R. The code
loaded into the DAC register determines at which
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
R
R
Figure 87. Resistor String
OUTPUT AMPLIFIER
The output buffer amplifier is capable of generating
rail-to-rail voltages on its output, giving a maximum
output range of 0V to AV_DD. It is capable of driving a
load of 2kΩ in parallel with 1000pF to GND. The
source and sink capabilities of the output amplifier
can be seen in the Typical Characteristics. The
typical slew rate is 0.75V/ms, with a typical full-scale
settling time of 7ms with the output unloaded.
Copyright © 2010, Texas Instruments Incorporated
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