DAC0830/DAC0832
DAC0830 Series Application Hints
(Continued)
00560808
FIGURE 5. Accommodating a High Speed System
2.0 ANALOG CONSIDERATIONS
The fundamental purpose of any D to A converter is to
provide an accurate analog output quantity which is repre-
sentative of the applied digital word. In the case of the
DAC0830, the output, I
OUT1
, is a current directly proportional
to the product of the applied reference voltage and the digital
input word. For application versatility, a second output,
I
OUT2
, is provided as a current directly proportional to the
complement of the digital input. Basically:
The digital input code to the DAC simply controls the position
of the SPDT current switches and steers the available ladder
current to either I
OUT1
or I
OUT2
as determined by the logic
input level (“1” or “0”) respectively, as shown in
The
MOS switches operate in the current mode with a small
voltage drop across them and can therefore switch currents
of either polarity. This is the basis for the 4-quadrant multi-
plying feature of this DAC.
2.2 Basic Unipolar Output Voltage
To maintain linearity of output current with changes in the
applied digital code, it is important that the voltages at both
of the current output pins be as near ground potential (0V
DC
)
as possible. With V
REF
=+10V every millivolt appearing at
either I
OUT1
or I
OUT2
will cause a 0.01% linearity error. In
most applications this output current is converted to a volt-
age by using an op amp as shown in
The inverting input of the op amp is a “virtual ground” created
by the feedback from its output through the internal 15 kΩ
resistor, R
fb
. All of the output current (determined by the
digital input and the reference voltage) will flow through R
fb
to the output of the amplifier. Two-quadrant operation can be
obtained by reversing the polarity of V
REF
thus causing I
OUT1
to flow into the DAC and be sourced from the output of the
amplifier. The output voltage, in either case, is always equal
to I
OUT1
xR
fb
and is the opposite polarity of the reference
voltage.
The reference can be either a stable DC voltage source or
an AC signal anywhere in the range from −10V to +10V. The
DAC can be thought of as a digitally controlled attenuator:
the output voltage is always less than or equal to the applied
reference voltage. The V
REF
terminal of the device presents
a nominal impedance of 15 kΩ to ground to external circuitry.
12
where the digital input is the decimal (base 10) equivalent of
the applied 8-bit binary word (0 to 255), V
REF
is the voltage
at pin 8 and 15 kΩ is the nominal value of the internal
resistance, R, of the R-2R ladder network (discussed in
Section 2.1).
Several factors external to the DAC itself must be consid-
ered to maintain analog accuracy and are covered in subse-
quent sections.
2.1 The Current Switching R-2R Ladder
The analog circuitry,
consists of a silicon-chromium
(SiCr or Si-chrome) thin film R-2R ladder which is deposited
on the surface oxide of the monolithic chip. As a result, there
are no parasitic diode problems with the ladder (as there
may be with diffused resistors) so the reference voltage,
V
REF
, can range −10V to +10V even if V
CC
for the device is
5V
DC
.
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