DN448 - 12-bit DAC in TSOT-23 Includes Bidirectional REF Pin for Connection to Op Amp or External High Precision Reference

12-bit DAC in TSOT-23 Includes Bidirectional REF Pin for
Connection to Op Amp or External High Precision Reference
Design Note 448
Kevin Wrenner, Troy Seman and Mark Thoren
Introduction
The LTC®2630’s combination of a 12-bit DAC and low-drift
integrated reference in a tiny SC-70 package has proven
popular for a wide variety of applications. Two new DACs,
the LTC2631 and LTC2640, take this winning formula and
further expand its reach by adding a bidirectional REF pin
and an optional I2C interface in a tiny TSOT-23.
The LT1991 precision op amp is a superb choice for
amplifying or attenuating the DAC output to achieve a
desired output range because it requires no precision
external resistors. Its integrated, precision resistors are
matched to 0.04%, allowing gain to be set by simple pin
strapping (see the data sheet for a large variety of gain
options). Figure 1 shows the configuration for a difference
gain of 4, resulting in a ±5V output with 12-bit programmability under I2C control. Integral nonlinearity, seen in
Figure 2, is better than 1LSB.
Like their predecessor, these parts feature 1-bit INL and
DNL, offer excellent load regulation driving up to 10mA
loads, and can operate rail-to-rail. See Table 1 for a list
of options.
Figure 3 shows a negative output system using a similar
setup, this time with the LT1991 configured as an inverting
amplifier with a gain of –0.25. The 0.1μF capacitor at REF
reduces the already low DAC noise by up to 20%.
Applications Using REF Pin
The bidirectional REF pin can be used as an output, where
the accurate 10ppm/°C reference is available to the rest
of the application circuit, or it can be used as an input
for an external reference.
For applications requiring more accuracy at full scale, the
LTC2631 and LTC2640 can be referenced to an external
source. Figure 4 shows how, using an LT1790 low-dropout
reference that’s accurate to 0.05%. Tying REF_SEL low
configures the REF pin as a reference input. If reset-to-zero
is needed, an LTC2640-LZ12 can be substituted. (For that
option, pin 8 is rededicated as a CLR pin, and, upon powering
up, External Reference mode must be selected by software
command before the code is changed from zero.)
To configure REF as an output, simply tie the REF_SEL
pin high. As an output, the REF pin simplifies pairing the
DAC with an op amp. For instance, to achieve an output
range centered at 0V, drive the plus input of the op amp,
with REF connected to the minus input. Avoid loading the
REF pin with DC current; instead, buffer its 500Ω output
with an LTC2054 or similar precision op amp.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
1.0
ZERO SCALE = –5V
FULL SCALE = 5V
LSB = 2.44mV
5V
4
5
–
0.1μF
LTC2054
3
1.7k
1.7k
I2C BUS
CA0
5 6
VCC REF
8
REF_SEL
3
SDA
LTC2631A VOUT 7
2
SCL -LM12
1
CA0
GND
4
+
2
0.1μF
0.5
10V
1
8
M9
9 M3
10
M1
1
P1
2
P3
3
P9
0.1μF
7
08/08/448a
0
VCC
LT1991
OUT
REF
VEE
4
6
VOUT = ±5V
–0.5
5
–1.0
0.1μF
–10V
Figure 1. Programmable ±5V Output
INL (LSB)
5V
DN448 F01
0
1024
2048
CODE
3072
4095
DN F02
Figure 2. Integral Nonlinearity
of Programmable ±5V Output
The REF pin enables the LTC2631 and LTC2640 to share
their full-scale range with another device, as shown in
Figure 5. A 16-bit LTC2453 ADC and LTC2631 DAC are
referenced to the same 5V full scale. This circuit allows
a variety of possible transfer functions to be applied to
an input under computer control. It is easy to implement
functions such as squaring and square root, or time-dependent functions such as integration or proportionalintegral-derivative (PID) control in this manner, resulting
in a circuit that is much simpler and more stable than a
purely analog circuit.
Conclusion
The LTC2631 and LTC2640 add I2C capability and a
bidirectional REF pin to LTC’s family of 12-, 10-, and 8bit DACs with an integrated reference. For applications
requiring a modified output range, the LT1991 op amp
with internal precision resistors is an ideal counterpart.
5V
0.1μF
5 6
VCC REF
8
SERIAL
BUS
CLR
3 SDI
LTC2640
2
SCK -HZ12A
1
CS/LD GND
0.1μF
VOUT 7
8
M9
9 M3
10
M1
1
P1
2
P3
3
P9
4
2.7V TO 5.5V
5V
LT1790A-2.048
0.1μF
VCC
LT1991
REF
VEE
4
0V
6
OUT
SERIAL
BUS
–1.0235V
5
0.1μF
4
–5V
DN448 F03
1μF
1, 2
0.1μF
7
6
5
6
3 SDI VCC
REF
2
SCK LTC2640
VOUT 7
1
-LM12A
CS/LD
0V
8
REF_SEL
GND
4
2.0475V
DN448 F04
Figure 3. Negative Output, 0V to –1.024V
Figure 4. 0V to 2.048V Output Derived from
External Reference
5V
0.1μF
0.1μF
1k
6
VIN = 0V TO 5V
IN+
0.01μF
3
4
REF+
VCC
LTC2453
1k
5
IN–
0.01μF
SDA
SCL
10k
10k
8
7
REF– GND GND
2
1
9
5
6
VCC REF
REF_SEL
3
SDA
LTC2631 VOUT 7
2
SCL -HM12A
1
CA0 GND
4
8
VOUT = 0V TO 5V
I2C BUS
μP
DN448 F05
Figure 5. Electronic Transfer Function Generator
Table 1. Family Characteristics. Each part has a bidirectional REF pin and is available in 12-, 10-, and 8-bit accuracy.
PART NUMBER
TYPE
FULL- SCALE
POWER-ON RESET CODE
PIN 8 FUNCTION
LTC2631-LM
I2C
2.5V
Midscale
Select default REF
LTC2631-LZ
I2C
2.5V
Zero
6 add’l addresses
LTC2631-HM
I2C
4.096V
Midscale
Select default REF
LTC2631-HZ
I2C
4.096V
Zero
6 add’l addresses
LTC2640-LM
SPI
2.5V
Midscale
Select default REF
LTC2640-LZ
SPI
2.5V
Zero
DAC Clear
LTC2640-HM
SPI
4.096V
Midscale
Select default REF
LTC2640-HZ
SPI
4.096V
Zero
DAC Clear
Data Sheet Download
For applications help,
call (408) 432-1900
www.linear.com
Linear Technology Corporation
dn448 LT/TP 0808 246K REV A • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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