LINEAR TECHNOLOGY LTC1592 Technical data
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Multiple Output Range 16-Bit DAC Design Made Simple
Design Note 337
Derek Redmayne
Introduction
Precision 16-bit analog outputs with softwareconfigurable output ranges are often needed in industrial
process control equipment, analytical and scientific
instruments and automatic test equipment. In the past,
designing a universal output module was a daunting task
and the cost and PCB real estate associated with this
function were problematic, if not prohibitive. Figure 1
shows an example of the circuitry formerly required to
produce a programmable 16-bit DAC with a variety of
REF
0.1%
10ppm/°C
TRACKING
5V
10k
10k
10k
+
U3
10V
–
SHDN
+
U4
5V
–
SHDN
+
U5
2.5V
–
SHDN
5k
U6
U6
U7
U7
U8
U8
U6, U7, U8: DG411
U9: 74HC1G4066
10k 10k
10ppm
TRACKING
5V
output ranges. However, with the new LTC
®
1592 multiple
output range DAC, all of this complexity is unnecessary.
Figure 2 shows the compact simplicity of an implementation based on the new LTC1592. All the standard
industrial ranges (0V to 5V, 0V to 10V, ±5V, ±10V, ±2.5V
and –2.5V to 7.5V) are provided, accurately and under
software control.
,LTC and LT are registered trademarks of Linear Technology Corporation.
SoftSpan is a trademark of Linear Technology Corporation.
U7
U6
+
+
1/2 LT
–
U2
®
U10
–
1469
100pF
U1
16-BIT CMOS DAC
U7
50k
1%
U9
U6
15pF
U8
U8
100k
OUT
–
U2
1/2 LT1469
DN337 F01
+
15V
U6 U7 U8
–15V
05/04/337
Figure 1. How NOT to Build a Universal 16-Bit Analog Output
The Old Way
Figure 1 shows a pre-LTC1592 implementation of a multiple output range DAC. The circuit can be made to work,
but only with costly components and a lot of PCB real
estate. The range switching capability requires the addition of analog switches and precision resistors to the
basic DAC. Some of these analog switches are required to
compensate for the resistance of switches at other points
in the circuit. The circuit as shown, even with its considerable complexity, is a compromise as some of these
analog switches are not paired with counterparts in the
same package. The analog switches are expensive. They
also require PCB real estate, bypassing and decoupling to
compensate for poor PSRR to mitigate digital noise. In
addition, since they are not switching at virtual ground,
the analog switches exhibit on-resistance variation with
voltage which will degrade linearity. Leakage can be an
issue at high temperature.
Precision matched resistor pairs are shown, as they are
available from a number of sources. But unless very
expensive devices are used, they will degrade accuracy.
V
REF
5V
5
+
1/2 LT1469
6
–
150pF
7
C2
The New, Easy Way
In contrast is Figure 2 where the LTC1592 contains all of
the circuitry required to perform these functions—all
under processor control. All the ranges are accurate with
low drift, fast settling and low glitch operation right out of
the box. The LTC1592 incorporates all the switches and
precision resistors. A full implementation takes less than
0.5in × 0.5in including the dual operational amplifier,
bypass and compensation. This analog output subsystem
can be reconfigured in real time and the serial interface
makes optoisolation easy.
Conclusion
Building a precision, multiple output range, softwareconfigurable 16-bit DAC is no longer a complicated,
expensive design effort. Now a clean, simple design yields
smaller size, lower cost and much better accuracy. The
LTC1592 can also be used for embedded or fixed range
applications, where its 4-quadrant operation with serial
interface make it compelling even if range changing may
not be required.
1
2
R
R1
COM
R2
5V
0.1µF
R1
9
V
CC
14
CLR
13
CS/LD
12
SCK
11
SDI
10
SDO
Figure 2. Programmable Output Range 16-Bit SoftSpanTM DAC
Data Sheet Download
http://www.linear.com/go/dnLTC1592
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
LTC1592
16
R2
3
15
R
REF
16-BIT DAC WITH
SPAN ADJUST
OFS
4
R
FB
I
OUT1
I
OUT2
AGND
GND
C1
30pF
5
2
6
3
7
8
DN337 F01
15V
8
–
1/2 LT1469
+
4
–15V
0.1µF
0.1µF
For applications help,
call (408) 432-1900, Ext. 2360
dn337f LT/TP 0504 305K • PRINTED IN THE USA
LINEAR TECHNOLOGY CORPORATION 2004
1
V
OUT
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