Feb 1999 The LTC1658 and LTC1655: Smallest Rail-to-Rail 14-Bit and 16-Bit DACs

DESIGN INFORMATION
The LTC1658 and LTC1655: Smallest
Rail-to-Rail 14-Bit and 16-Bit DACs
by Hassan Malik
Expanding the rail-to-rail, voltage
output DAC family, Linear Technology introduces two new voltage output
DACs that break the size/bits barrier. The LTC1658 is a 14-bit
rail-to-rail voltage output DAC in a
tiny MSOP-8 package and the
LTC1655 is a 16-bit voltage output
DAC in an SO-8 package. Both of
these DACs also provide a convenient
upgrade path for users of LTC’s 12bit voltage output DAC family. The
LTC1658 draws only 270µA from a
3V or 5V supply and is 14-bit monotonic over temperature. The LTC1655
draws 600µA from a 5V supply and is
16-bit monotonic over temperature.
2.7V TO 5.5V
8
These DACs have a flexible 3-wire
serial interface that is SPI/QSPI and
MICROWIRE compatible.
Figures 1 demonstrates the ease of
using the LTC1658. The output swings
from 0V to VREF at full-scale. VREF
should be less than or equal to VCC to
prevent the loss of codes and degradation of PSRR near full-scale. The input
serial data is loaded as one 16-bit
word with two dummy bits. The digital
inputs are TTL/CMOS level compatible and the CLK input has an internal
Schmitt trigger for noise immunity.
This allows direct optocoupler interfacing to the part. Figure 2 plots the
part’s 0.25LSB typical DNL.
6
VCC
REF
2 DIN
µP
3 CS/LD
+
16-BIT
SHIFT
REG
AND
DAC
LATCH
1.0
VOUT
14
14-BIT
DAC
RAIL-TO-RAIL
VOLTAGE
OUTPUT
7
–
4 DOUT
POWER-ON
RESET
TO
OTHER
DACS
0.8
0.6
DNL ERROR (LSB)
1 CLK
A typical application for the
LTC1655 is shown in Figure 3. The
LTC1655 has the same interface as
the LTC1658 and is also capable of
being daisy chained. There is an
onboard 2.048V bandgap reference
connected internally to the 16-bit
DAC. The rail-to-rail output nominally swings from 0V to 4.096V, since
there is a gain of two in the output
amplifier. The reference pin can be
overdriven to a value higher than
2.048V if a larger output swing is
desired. Since there is a gain of 2 from
the reference pin to the output at fullscale, the voltage on the REF pin
must always be less than VCC /2.
Figure 4 plots the typical DNL of the
LTC1655.
0.4
0.2
0
– 0.2
– 0.4
– 0.6
GND
1658 TA01
– 0.8
5
– 1.0
0
4096
Figure 1. LTC1658 block diagram
8192
CODE
12288
16383
1658 TA02
4.5V TO 5.5V
8
1 CLK
µP
3 CS/LD
2.048V
6
REF
1.0
REF
+
16-BIT
SHIFT
REG
AND
DAC
LATCH
16
16-BIT
DAC
–
0.8
VOUT
7
RAIL-TO-RAIL
VOLTAGE
OUTPUT
4 DOUT
TO
OTHER
DACS
POWER-ON
RESET
0.6
DNL ERROR (LSB)
VCC
2 DIN
Figure 2. The LTC1658 14-bit rail-to-rail
DAC in MSOP has 0.25LSB typical DNL.
0.4
0.2
0
– 0.2
– 0.4
– 0.6
– 0.8
GND
5
1658 TA01
– 1.0
0
16384
32768
CODE
49152
65535
1658 TA02
Figure 3. LTC1655 block diagram
36
Figure 4. LTC1655 typical DNL plot
Linear Technology Magazine • February 1999