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 easy-to-use SPI compatible interface that
allows daisy-chaining.
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.
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 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 12-bit 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.
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
rail-to-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.
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REFERENCE
LD
DAC B
REGISTER
CLK 1
DIN 2
12-BIT
DAC B
+
LD
DAC A
REGISTER
DOUT 4
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_conv
VOUTB
–
24-BIT
SHIFT
REGISTER
CS/LD 3
8
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
SSO
REF +
REF –
74HC04
50k
50k
5V
0.1μF
VOUTB
CLK
VCC
DIN
LTC1446
LTC1446L
GND
CS/LD
μP
COM
LTC1446: 4.5V TO 5.5V
LTC1446L: 2.7V TO 5.5V
LTC1446: 0V TO 4.095V
LTC1446L: 0V TO 2.5V
VOUTB
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
100Ω
0.1μF
VCC
DIN
LTC1446
GND
CS/LD
0.1μF
100Ω
DOUT
DOUT
LTC1446: 0V TO 4.095V
LTC1446L: 0V TO 2.5V
VOUTA
VOUTA
0.1μF
DN131 F03
DN131 F02
Figure 2. Easy Stand-Alone Application
for the LTC1446 or LTC1446L
Figure 3. An Autoranging 8-Channel ADC with Shutdown
5V
8.190
VOUT
0.1μF
15V
50k
VCC
DIN
μP
DOUT
–
VOUT
2{ VOUTB – VOUTA }
C
–4.096
–15V
50k
DIN
0
LT1077
100k
GND
VOUTA
B
+
LTC1446
CS/LD
A
4.094
VOUTB
CLK
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
Data Sheet Download
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