12-Bit Rail-to-Rail Micropower DACs in an SO-8 Design Note 96 Hassan Malik and Jim Brubaker The LTC ®1451, LTC1452 and LTC1453 are complete, single supply, rail-to-rail voltage output 12-bit digitalto-analog (DAC) converters. They include an output buffer amplifier and a space saving SPI compatible three-wire serial interface. There is also a data output pin that allows daisy-chaining multiple DACs. These DACs use a proprietary architecture which guarantees a DNL (Differential Nonlinearity) error of less than 0.5LSB. The typical DNL error is about 0.2LSB as shown in Figure 1. There is a built-in power-on reset that resets the output to zero scale. The output amplifier can swing to within 5mV of VCC when unloaded and can source or sink 5mA even at a 4.5V supply. These DACs come in an 8-pin PDIP and SO-8 package. 5V and 3V Operation The LTC1451 has an on-board reference of 2.048V and a nominal output swing of 4.095V. It operates from a single 4.5V to 5.5V supply dissipating 2mW (ICC(TYP) = 400μA). The LTC1452 is a multiplying DAC with no on-board reference and a full-scale output of twice the reference input. It operates from a single supply that can range from 2.7V to 5.5V. It dissipates 1.125mW (ICC(TYP) = 225μA) at a 5V supply and a mere 0.5mW (ICC(TYP) = 160μA) at a 3V supply. The LTC1453 has a 1.22V on-board reference and a convenient full scale of 2.5V. It can operate on a single supply with a wide range of 2.7V to 5.5V as shown in Figure 2. It dissipates 0.75mW (ICC(TYP) = 220μA) at a 3V supply. The digital inputs can swing above VCC for easy interfacing with 5V logic. True Rail-to-Rail Output The output rail-to-rail amplifier can source or sink 5mA over the entire operating temperature range while pulling to within 300mV of the positive supply voltage or ground. The output swings to within a few millivolts of either supply rail when unloaded and has an equivalent output resistance of 50Ω when driving to either rail. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 2.7V TO 5.5V 0.1μF VCC DIN CLK MICROPROCESSOR LTC1453 CS/LD DOUT TO NEXT DAC FOR DAISY-CHAINING VREF VOUT OUTPUT 0V TO 2.5V GND 1.22V DN96 • F02 Figure 2. The 3V LTC1453 is SPI Compatible and Talks to Both 5V and 3V Processors 8 VCC CLK 1 0.5 12-BIT SHIFT REGISTER DAC REGISTER 12-BIT DAC + 7 VOUT – CS/LD 3 POWER-ON RESET DOUT 4 REFERENCE LTC1451: 2.048V LTC1453: 1.22V DNL ERROR (LSB) LD DIN 2 0.0 6 REF –0.5 0 5 GND 512 1024 1536 2048 2560 3072 3584 4095 CODE DN96 • F02B DN96 • F01A Figure 1. Proprietary Architecture Guarantees Excellent DNL 02/95/96_conv The output can drive a capacitive load of up to 1000pF without oscillating. Flexibility, True Rail-to-Rail Performance and Micropower; All In a Tiny SO-8 The LTC1451, LTC1452 and LTC1453 are the most flexible micropower, stand alone DACs that offer true rail-to-rail performance. This flexibility along with the tiny SO-8 package allows these parts to be used in a wide range of applications where size, power, DNL and single supply operation are important. Wide Range of Applications Some of the applications for this family include digital calibration, industrial process control, automatic test equipment, cellular telephones and portable batterypowered applications where low supply current is essential. Figure 3 shows how to use an LTC1453 to make an opto-isolated digitally controlled 4mA to 20mA process controller. The controller circuitry, including the opto-isolator, is powered by the loop voltage that can have a wide range of 3.3V to 30V. The 1.22V reference output of the LTC1453 is used for the 4mA offset current and VOUT is used for the digitally controlled 0mA to 16mA current. RS is a sense resistor and the LT®1077 op amp modulates the transistor Q1 to provide the 4mA to 20mA current through this resistor. The potentiometers allow for offset and full-scale adjustment. The control circuitry consumes well under the 4mA budget at zero scale. Table 1. LTC Serial Voltage Output DACs VCC Range LTC1451 4.5V to 5.5V 2.048V-Internal 4.095V 400μA at 5V LTC1452 2.7V to 5.5V LTC1453 2.7V to 5.5V 1.22V-Internal LTC1257 4.75V to 15.75V Reference Full Scale Part External ICC 2 × REF 225μA at 5V 2.5V 250μA at 3V 2.048V-Internal 2.048V 350μA at 5V (2.5V to (2.5V to 12V-External) 12V) VLOOP 3.3V TO 30V LT1121-3.3 IN 90k 5k 45k 5k OUT 1μF FROM OPTO-ISOLATED INPUTS CLK VCC DIN LTC1453 CS/LD VREF + OUT 1k LT1077 3k – RS 10Ω 3.3V OPTO-ISOLATERS CLK DIN CS/LD 10k 500Ω 4N28 Q1 2N3440 IOUT CLK DIN CS/LD DN96 • F03 Figure 3. 4mA to 20mA Process Controller Has 3.3V Minimum Loop Voltage Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn96f_conv LT/GP 0295 190K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1995