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The LTC1446/LTC1446L: World’s First Dual 12-Bit DACs in SO-8
Design Note 131
Hassan Malik and Kevin R. Hoskins
The LTC®1446/LTC1446L are the first dual, single supply,
rail-to-rail voltage output 12-bit DACs. Both parts include
an internal reference and two DACs with rail-to-rail output
buffer amplifiers, packaged into a space-saving 8-pin SO
or PDIP package. The LTC1446’s patented architecture is
inherently monotonic and has excellent 12-bit DNL, guaranteed to be less than 0.5LSB. These parts have an easyto-use SPI compatible interface that allows daisy-chaining.
DAC A and the second is for DAC B. Each 12-bit segment is
loaded MSB first and latched into the shift register on the
rising edge of the clock. When all the data has been shifted
in, it is loaded into the DAC registers when the signal on the
CS/LD pin changes to a logic high. This updates both 12bit DACs and internally disables the CLK signal. The DOUT
pin allows the user to daisy-chain several DACs together.
Power-on reset initializes the outputs to zero scale.
Low Power 5V or 3V Single Supply
The LTC1446 has an output swing of 0V to 4.095V, with
each LSB equal to 1mV. It operates from a single 4.5V to
5.5V supply, drawing 1mA. The LTC1446L has an output
swing of 0V to 2.5V, operates on a single 2.7V to 5.5V
supply and draws 650µA.
Rail-to-Rail Outputs
The on-chip output buffer amplifiers can source or sink
over 5mA with a 5V supply. More over, they have true railto-rail performance. This results in excellent load regulation up to the 4.095V full-scale output with a 4.5V supply.
When sinking current with outputs close to zero scale, the
effective output impedance is about 50Ω. The midscale
glitch on the output is 20nV • s and the digital feedthrough
is a negligible 0.15nV • s.
Complete Stand-Alone Performance
Figure 1 shows a block diagram of the LTC1446/LTC1446L.
The data inputs for both DAC A and DAC B are clocked into
one 24-bit shift register. The first 12-bit segment is for
, LTC and LT are registered trademarks of Linear Technology Corporation.
REFERENCE
LD
DAC B
REGISTER
CLK 1
DIN 2
12-BIT
DAC B
+
LD
DOUT 4
DAC A
REGISTER
12-BIT
DAC A
7
VCC
6
GND
5
VOUTA
+
–
POWER-ON
RESET
DN131 F01
Figure 1. Dual 12-Bit Rail-to-Rail Performance in an SO-8 Package
06/96/131
VOUTB
–
24-BIT
SHIFT
REGISTER
CS/LD 3
8
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A Wide Range of Applications
Some of the typical applications for these parts include
digital calibration, industrial process control, automatic
test equipment, cellular telephones and portable batterypowered applications. Figure 2 shows how easy these
parts are to use.
Figure 3 shows how to use one LTC1446 to make an
autoranging ADC. The microprocessor adjusts the ADC’s
reference span and offset by loading the appropriate digital
code into the LTC1446. VOUTA controls the common pin for
the analog inputs to the LTC1296 and VOUTB controls the
reference span by setting the LTC1296’s REF + pin. The
LTC1296 has a Shutdown pin whose output is a logic low in
shutdown mode. During shutdown, this logic low turns off
the PNP transistor that supplies power to the LTC1446. The
resistors and capacitors lowpass filter the LTC1446 outputs, attenuating noise.
output (VOUTA) is used as the offset voltage. Figure 4 also
shows how the circuit’s output voltage changes as a
function of the input digital code.
Conclusion
The LTC1446/LTC1446L are the world’s only DACs that
offer dual 12-bit stand-alone performance in an 8-pin SO
or PDIP package. Along with their amazing density, these
DACs do not compromise performance, offering excellent
12-bit DNL, rail-to-rail voltage outputs and very low power
dissipation. This allows users to save circuit board space
without sacrificing performance.
5V
22µF
VCC
DOUT
µP
VCC
DIN
LTC1446
LTC1446L
GND
CS/LD
µP
DOUT
COM
SSO
LTC1446: 4.5V TO 5.5V
LTC1446L: 2.7V TO 5.5V
LTC1446: 0V TO 4.095V
LTC1446L: 0V TO 2.5V
VOUTB
REF +
REF –
74HC04
50k
50k
5V
0.1µF
CLK
0.1µF
VOUTB
100Ω
0.1µF
VCC
DIN
LTC1446
GND
CS/LD
100Ω
LTC1446: 0V TO 4.095V
LTC1446L: 0V TO 2.5V
VOUTA
8 ANALOG
INPUT CHANNELS
CLK
LTC1296
DIN
CH7
Figure 4 shows how to use an LTC1446 and an LT®1077 to
make a wide bipolar output swing 12-bit DAC with a
digitally programmable offset. The voltage on DAC A’s
CLK
CH0
CS
DOUT
VOUTA
0.1µF
DN131 F02
Figure 2. Easy Stand-Alone Application
for the LTC1446 or LTC1446L
DN131 F03
Figure 3. An Autoranging 8-Channel ADC with Shutdown
5V
8.190
VOUT
0.1µF
CLK
15V
50k
DIN
µP
VCC
DOUT
–
VOUT
2{ VOUTB – VOUTA }
C
–4.096
–15V
50k
DIN
0
LT1077
100k
GND
VOUTA
B
+
LTC1446
CS/LD
A
4.094
VOUTB
100k
–8.190
A: VOUTA ≅ 0V (ZERO SCALE)
B: VOUTA ≅ 2.048V (MID SCALE)
C: VOUTA ≅ 4.095V (FULL SCALE)
DN131 F04
Figure 4. A Wide-Swing, Bipolar Output DAC with Digitally Controlled Offset
For literature on our Digital-to-Analog Converters,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2525
Linear Technology Corporation
LT/GP 0696 155K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1996
www.BDTIC.com/Linear