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 ●