LINEAR TECHNOLOGY LTC2704 Technical data

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

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 amplifi ers and
four output amplifi ers—seven packages if dual ampli­fi ers are used. Implementing multiple ranges discretely
is prohibitive. Design Note 337 explains the diffi culty in
implementing multiple ranges, including the cos t 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
dri ven by the LTC2704’s outputs, or to compensate slow
booster stages. The V

pins provide a convenient way

OS

to add an of fset to the output voltage. The gain from the

pin to the output is –0.01, –0.02 or –0.04, depend-

V

OS

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 coeffi cient of the external offset ting 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 amplifi er. 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 t o 1A of ou tpu t c ur re nt. Th is can be us ed t o p ow er te st
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 dr ive 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 amplifi ers.

, 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.

12/07/431

–15V

309k

+5V

V

CC

SEL

ADJ1

LTC2909-5

ADJ2

TMR

10.7k

REF

RST

GND

+5V

REF1

VOSB

C1B

2200pF

1Ω

1µF

RFBB

OUTB

AGNDB

VOSA

C1A

RFBA

OUTA

1nF

10k

2N7002

DAC B

20k

–15V

+

V

1

–

V

1,8,15,22,31,36

–

10k

AGND V

–
+
–

DAC A

+

+

+15V+15V

V

CONTROL

MTD
2955

+

2

DAC C

LT3080

IN

+

+
–

SET

+5V

REF2

VOSC

C1C

RFBC

OUTC

AGNDC

IN4003

OUT

C2

4.7µF

0.01µF

POSITIVE
RANGES
ONLY

30k

–15V

–

VOSD

C1D

30pF

DAC D

RFBD

OUTD

SPI INTERFACE

+5V

LDAC

CS/LD

SDI

SCK

SDO

±5V

REF

LTC2601

GND

+5V

1.5k

ISRC

SENSE

–

GND

+15V

TSD

OUT

+

0.1MF

AGNDD

+
–

REFM2

REFG2

POR

ERROR
FLAGS
INDICATE
CURRENT
LIMIT AND
THERMAL
SHUTDOWN

SRO

RS, 1Ω

10MF

+

+

10µF0.1µF

±500mA

dn431 F01

AGNDA

+
–

REFM1

REFG1

COMMAND

DECODE

LTC2704

+5V +15V

50k

150k

450k

450k

150k

V

CC

V

OUT

50k

–

+

4pF

–15V

SCK LDACCLRCS/LD RFLAG

SPI INTERFACE

450k

4pF

LT1991

450k

SDI

INPUT

SHIFT REGS

SOURCE

CURRENT
LIMIT
CONTROLS
(0V–5V)

SINK

BUFFERS

500mV

5V

DAC

VC

SRC

+IN

–IN

COMMON

READBACK

SHIFT REGS

VC

SNK

LT1970

V

DD

EN

V

CC

+

V

ISNK

SENSE

–

V

V

EE

–15V

Figure 1. LTC2704 Application Examples

Data Sheet Download

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