DAC0830LCM National Semiconductor, DAC0830LCM Datasheet - Page 11

• Manufacturer Part Number: DAC0830LCM
• Description: 8-Bit P Compatible/ Double-Buffered D to A Converters
• Manufacturer: National Semiconductor

DAC0830 Series Application Hints
2.0 ANALOG CONSIDERATIONS
The fundamental purpose of any D to A converter is to pro-
vide an accurate analog output quantity which is representa-
tive of the applied digital word. In the case of the DAC0830,
the output, I
product of the applied reference voltage and the digital input
word. For application versatility, a second output, I
provided as a current directly proportional to the complement
of the digital input. Basically:
where the digital input is the decimal (base 10) equivalent of
the applied 8-bit binary word (0 to 255), V
at pin 8 and 15 k
tance, 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, Figure 6 , 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
5V
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
REF
DC
, can range −10V to +10V even if V
.
OUT1
OUT1
, is a current directly proportional to the
is the nominal value of the internal resis-
or I
OUT2
as determined by the logic in-
FIGURE 5. Accommodating a High Speed System
CC
REF
for the device is
is the voltage
OUT2
, is
(Continued)
11
put level (“1” or “0”) respectively, as shown in Figure 6 . The
MOS switches operate in the current mode with a small volt-
age drop across them and can therefore switch currents of
either polarity. This is the basis for the 4-quadrant multiplying
feature of this DAC.
2.2 Basic Unipolar Output Voltage
To maintain linearity of output current with changes in the ap-
plied digital code, it is important that the voltages at both of
the current output pins be as near ground potential (0V
as possible. With V
ther I
applications this output current is converted to a voltage by
using an op amp as shown in Figure 7 .
The inverting input of the op amp is a “virtual ground” created
by the feedback from its output through the internal 15 k re-
sistor, R
input and the reference voltage) will flow through R
output of the amplifier. Two-quadrant operation can be ob-
tained by reversing the polarity of V
flow into the DAC and be sourced from the output of the am-
plifier. The output voltage, in either case, is always equal to
I
age.
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
a nominal impedance of 15 k to ground to external circuitry.
Always use the internal R
age since this resistor matches (and tracks with tempera-
ture) the value of the resistors used to generate the output
current (I
OUT1
OUT1
xR
fb
fb
OUT1
. All of the output current (determined by the digital
or I
and is the opposite polarity of the reference volt-
OUT2
).
REF
will cause a 0.01% linearity error. In most
= +10V every millivolt appearing at ei-
REF
fb
resistor to create an output volt-
terminal of the device presents
REF
DS005608-8
thus causing I
www.national.com
fb
OUT1
to the
DC
to
)