dac1054 National Semiconductor Corporation, dac1054 Datasheet - Page 9

• Manufacturer Part Number: dac1054
• Description: Quad 10-bit Voltage-output Serial D/a Converter With Readback
• Manufacturer: National Semiconductor Corporation

Applications Information
FUNCTIONAL DESCRIPTION
The DAC1054 is a monolithic quad 10-bit digital-to-analog
converter that is designed to operate on a single 5V supply
Each of the four units is comprised of an input register a
DAC register a shift register a current output DAC and an
output amplifier In addition the DAC1054 has an onboard
bandgap reference and a logic unit which controls the inter-
nal operation of the DAC1054 and interfaces it to micro-
processors
Each of the four internal 10-bit DACs uses a modified R-2R
ladder to effect the digital-to-analog conversion (Figure 5)
The resistances corresponding to the 2 most significant bits
are segmented to reduce glitch energy and to improve
matching The bottom of the ladder has been modified so
that the voltage across the LSB resistor is much larger than
the input offset voltage of the buffer amplifier The input
digital code determines the state of the switches in the lad-
der network An internal EEPROM which is programmed at
the factory is used to correct for linearity errors in the resis-
tor ladder of each of the four internal DACs The codes
stored in the EEPROM’s memory locations are converted to
a current I
rents I
OUT1
I
OUT1
EEPROM
and I
a
FIGURE 6 Generating a V
OUT2
I
OUT2
with a small trim DAC The sum of cur-
is fixed and is given by
e

FIGURE 5 Equivalent Circuit of R-2R Ladder and Output Amplifier
V
REF
b
R
V
BIAS
J
BIAS
1023
1024
e
1 40V from the Internal Reference Typical Application
9
The current output I
rent I
converted to a voltage The output voltage of each DAC is a
function of V
given by
V
The output voltage range for each DAC is 0 3V – 2 8V This
range can be achieved by using the internal 2 65V reference
and a voltage divider network which provides a V
1 40V (Figure 6) In this case the DAC transfer function is
The output impedance of any external reference that is
used will affect the accuracy of the conversion In order that
this error be less than
external reference must be less than 2X
OUT
EEPROM
e
2 (V
REF
BIAS
V
is applied to the internal output amplifier and
OUT
b
V
V
BIAS
e
OUT2
REF
2 5
)
DATA
1024
and the digital input word and is
LSB the output impedance of the
(DATA)
summed with the correction cur-
1024
TL H 11437 – 17
a
2047
512
a
0 30244
V
BIAS
b
1023
512
TL H 11437 – 16
V
BIAS
REF
of