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Download DN294 - 250ksps, 16-Bit Micropower ADC Offers an Excellent Combination of SIze, Power and Speed
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advertisement 250ksps, 16-Bit Micropower ADC Offers an Excellent Combination of Size, Power and Speed – Design Note 294 Guy Hoover MSOP Package for Portable Applications Available in the MSOP package, the LTC1864 is well suited for portable applications where space is limited. As seen in Figure 1, the MSOP package provides considerable space savings over other serial parts available in 20-lead SW or 28-lead SSOP packages. 20-LEAD SW 28-LEAD SSOP MSOP DN294 F01 Figure 1. This 16-Bit ADC—Available in the Tiny MSOP Package—is an Obvious Space Saver Over Other Serial ADCs Low ADC Supply Current for Battery-Operated Applications With a typical supply current of only 850µA at the maximum sample rate of 250ksps, the LTC1864 already has one of the lowest power consumptions of any 16-bit ADC available. After a conversion, the LTC1864 goes into a low power SLEEP mode (ICC = 1nA typ, ICC = 3µA max) further reducing supply current. The LTC1864 can therefore run at true micropower levels in applications that do not require the maximum LTC1864 sampling rate. At 1ksps, the supply current is typically only 2µA as shown in Figure 2. The part’s low power consumption combined with the ability to go into sleep mode after a 09/02/294 1000 100 SUPPLY CURRENT (µA) Introduction The demand for ever smaller portable devices is increasing. In addition, there is a corresponding demand for longer battery life in these devices that quite often have smaller batteries than their predecessors. Optimized for battery-operated, portable, isolated and remote data acquisition systems, the LTC®1864, a 16-bit, 250ksps micropower ADC is remarkably fast and offers µpower operation and small size. 10 1 0.1 0.01 0.01 0.1 10 100 1 SAMPLING FREQUENCY (kHz) 1000 DN294 F02 Figure 2. Micropower Performance conversion, make the LTC1864 ideal for battery-operated applications. Simple Serial I/O Eases Isolated and Remote Applications The simple 3-wire serial I/O used by the LTC1864 is compatible with industry standard SPI/MICROWIRETM interfaces. The LTC1864 has an internal conversion clock so that the shift clock (SCK) rate does not effect the conversion. This allows the shift clock rate to run from DC to 20MHz without concern over sample-and-hold droop at low clock frequencies or clocking the ADC too fast at high clock frequencies. The data transfer requires only 16 clock cycles as shown in the timing diagram of Figure 3. Running the shift clock (SCK) at the maximum rate of 20MHz, an entire conversion can be transferred in only 800ns. Using only three wires in the serial interface makes the LTC1864 easy to use for isolated or remote applications as shown in Figure 4. Real Convert Start Input Allows More Precise Control The rising edge of CONV immediately causes the LTC1864 sample-and-hold to acquire the input voltage and starts a conversion. This compares favorably to some ADCs that require several shift clocks after the convert signal before , LTC and LT are registered trademarks of Linear Technology Corporation. MICROWIRE is a trademark of National Semiconductor Corporation. www.BDTIC.com/Linear CONV t SMPL SLEEP MODE tCONV 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 SCK B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0* SDO Hi-Z Hi-Z *AFTER COMPLETING THE DATA TRANSFER, IF FURTHER SCK CLOCKS ARE APPLIED WITH CONV LOW, THE ADC WILL OUTPUT ZEROS INDEFINITELY DN294 F03 Figure 3. Data Transfer Only Requires 16 Clock Cycles 5V IN 10k OUT 5V 10k 9V LT1021-5 150Ω POWER DOWN 4N28 10k 1Ω Q2 Q1 GND 5V 10µF VREF VCC 5.1k ANALOG INPUT 0V TO 5V 5.1k LTC1864 150Ω SCK Q3 4N28 SCK IN+ 10k 51k SDO 5V CONV IN – 10k 51k GND 150Ω CONV Q4 4N28 Q1-Q4: 2N3906 Q5: 2N3904 51k 300Ω 4N28 5V 10k Q5 SDO 51k 5.1k DN294 F04 Figure 4. Micropower 500V Optoisolated 16-Bit Data Acquisition System acquiring the input voltage. For fast moving signals, the LTC1864 can more precisely acquire the desired input voltage. True Differential Input Rejects Common Mode Noise The LTC1864 has a true differential input with sampleand-holds on both IN+ and IN– inputs. The LTC1864 samples both the IN+ and IN– inputs simultaneously; so common mode noise on the inputs is rejected. The IN+ range of the LTC1864 is ground to VCC. Most competing parts allow the minus input to go a few hundred millivolts Data Sheet Download http://www.linear.com/go/dnLTC1864 Linear Technology Corporation above ground. The IN– range of the LTC1864 is ground to VCC/2 which makes the LTC1864 a good choice for remote applications where large common mode voltages or common mode noise can be present. Conclusion The LTC1864 16-bit 250ksps micropower ADC is a good choice for battery-operated, portable, isolated and remote ADC applications. The features that make this true include its small size, micropower operation, simple serial I/O, real convert start input and true differential inputs. For literature on our A/D Converters, call 1-800-4-LINEAR. For applications help, call (408) 432-1900, Ext. 2360 dn294f LT/TP 0902 316.5K • PRINTED IN THE USA www.BDTIC.com/Linear 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2002