DN431 - Highly Integrated Quad 16-Bit, SoftSpan, Voltage Output DAC for Industrial and Control Applications

Highly Integrated Quad 16-Bit, SoftSpan , Voltage Output DAC
for Industrial and Control Applications
TM
Design Note 431
Mark Thoren
Introduction
Digital-to-analog converters (DACs) are prevalent in
industrial control and automated test applications.
General-purpose automated test equipment often requires many channels of precisely controlled voltages
that span several voltage ranges. The LTC2704 is a
highly integrated 16-bit, 4-channel DAC for high-end
applications. It has a wide range of features designed to
increase performance and simplify design.
Unprecedented Integration
The LTC2704 provides true 16-bit performance over six
software selectable ranges: 0V to 5V, 0V to 10V, –2.5V
to 2.5V, –5V to 5V, –10V to 10V and –2.5V to 7.5V. Four
single-range voltage outputs would normally require
four current-output DACs, two reference amplifiers and
four output amplifiers—seven packages if dual amplifiers are used. Implementing multiple ranges discretely
is prohibitive. Design Note 337 explains the difficulty in
implementing multiple ranges, including the cost of precision-matched resistors and the performance limitations
of analog switches. Control is also complicated, requiring
extra digital lines for each DAC and for range control. The
LTC2704 integrates all of these functions into a single
package with no compromises, and all functions are
controlled via an easy-to-use 4-wire SPI bus.
Ease of Use
The LTC2704 provides many features to aid system design. The voltage output and feedback are separated,
allowing external current booster stages to be added
with no loss in accuracy. The C1A, C1B, C1C and C1D
pins allow external frequency compensation capacitors
to be used, either to allow capacitive loads to be directly
driven by the LTC2704’s outputs, or to compensate slow
booster stages. The VOS pins provide a convenient way
to add an offset to the output voltage. The gain from the
VOS pin to the output is –0.01, –0.02 or –0.04, depend-
12/07/431
ing on the selected range. While this seems like a simple
function to perform externally, implementing it inside
the LTC2704 eliminates concerns about matching the
temperature coefficient of the external offsetting resistor
to the internal resistors.
Example Circuits
Figure 1 shows several ways to use the LTC2704’s features. The offset pin of DAC A is driven by an LTC2601
DAC through an LTC1991 amplifier. This provides ±50mV
of “system offset” adjustment in the ±2.5V and 0V to 5V
ranges, ±100mV of adjustment in the –2.5V to 7.5V, ±5V,
and 0V to 10V ranges and ±200mV of adjustment in the
±10V range. The C1 pin is left open for fast settling. An
LTC2604 quad DAC can be used to drive all four offset
pins, and can share the same SPI bus as the LTC2704.
DAC B drives a 1µF capacitor through a 1Ω resistor, with
2200pF of additional compensation. This is useful for applications where the load has high frequency transients,
such as driving the reference pin of an ADC.
DAC C drives an LT3080 low dropout regulator, providing
up to 1A of output current. This can be used to power test
circuitry directly. Global feedback removes the offset of
the regulator, maintaining accuracy at the output.
DAC D is boosted by an LT1970 power op amp, providing
500mA of drive current, either sourcing or sinking. Once
again, global feedback preserves DC accuracy.
Conclusion
The LTC2704 provides a highly integrated solution for
generating multiple precision voltages. It saves design
time, board space and cost compared to imple-mentations using separate DACs and amplifiers.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
SoftSpan is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
+5V
+15V
+15V
LT3080
IN
VCONTROL
VCC
10k
SEL
ADJ1
10k
20k
ADJ2
309k
+
–
MTD
2955
LTC2909-5
2N7002
TMR
OUT
1nF
10.7k
C2
4.7µF
SET
–15V
RST
REF
POSITIVE
RANGES
ONLY
GND
–15V
IN4003
+5V
V +2
AGND
+5V
REF1
REF2
VOSB
VOSC
C1B
C1C
RFBB
RFBC
2200pF
0.01µF
OUTB
DAC B
AGNDB
OUTC
DAC C
+
–
–
+
1µF
–
1Ω
V–
1,8,15,22,31,36
V +1
+
30k
–15V
AGNDC
+
–
VOSD
VOSA
C1A
C1D
RFBA
RFBD
30pF
OUTA
AGNDA
OUTD
DAC D
DAC A
+
–
+
–
AGNDD
REFM1
REFM2
REFG1
REFG2
COMMAND
DECODE
LTC2704
CS/LD
INPUT
SHIFT REGS
DAC
BUFFERS
CLR
SCK SDI
SRO
READBACK
SHIFT REGS
LDAC
RFLAG
VDD
POR
GND
+5V
+5V
SINK
SOURCE
+15V
50k
450k
150k
450k
4pF
–
450k
+5V
+
SPI INTERFACE
REF
VCC
SDI
LTC2601
SCK
SDO
VOUT
VCSRC
VCSNK
EN
+IN
VCC
LT1970
–IN
4pF
ERROR
FLAGS
INDICATE
CURRENT
LIMIT AND
THERMAL
SHUTDOWN
1.5k
450k
50k
CS/LD
5V
CURRENT
LIMIT
CONTROLS
(0V–5V)
LT1991
150k
LDAC
+15V
500mV
VEE
V+
ISNK
ISRC
TSD
OUT
SENSE+
SENSE–
V–
+
0.1µF
10µF
RS , 1Ω
±500mA
COMMON
–15V
0.1MF
GND
+
SPI INTERFACE
±5V
10MF
–15V
dn431 F01
Figure 1. LTC2704 Application Examples
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2602
www.linear.com
Linear Technology Corporation
431f LT/TP 1207 • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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