6-Channel SAR ADCs for Industrial Monitoring and Portable Instruments Design Note 426 Guy Hoover and Steve Logan The 14-bit LTC2351-14 is a 1.5Msps, low power SAR ADC with six simultaneously sampled differential input channels. It operates from a single 3V supply and features six independent sample-and-hold amplifiers and a single ADC. The single ADC with multiple S/HAs enables excellent range match (1mV) between channels and channel-tochannel skew (200ps). The versatile LTC2351-14 is ideally suited for industrial monitoring applications such as 3-phase power line monitoring to ensure line voltage compliance, portable power line instrumentation, power factor correction, motor control, and data acquisition. These applications may be battery powered, and it is here that the LTC2351-14’s low power and small size are desirable. Power consumption is a mere 16.5mW, which extends battery life. The 3-wire serial interface means fewer pins than parallel output devices, allowing the LTC2351-14 to fit in a 32-pin, 5mm × 5mm QFN package. Power Line Monitoring Application Figure 1 shows a typical power line monitoring application. Current is sensed by a CR Magnetics CR8348-2500-N current transformer. An LT1790-1.25 biases the output of the transformer to the middle of the LTC2351-14 input range, giving the inputs maximum swing. A 6:1 transformer and 41:1 attenuator scale the line voltage, and the transformer output is similarly biased. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. LT1790-1.25 IN OUT 3V 0.1MF 3V 1MF GND 10MF 100Ω CH0 49.9Ω 4 TURNS + 47pF 47pF 49.9Ω CR MAGNETICS CR8348-2500-N 0.1MF VCC VDD OVDD LTC2351-14 VREF S&H 100Ω 47pF 2k 6:1 MUX 49.9Ω + 47pF 47pF 49.9Ω 49.9Ω 14-BIT ADC CH1 49.9Ω 120VAC LINE 2k 10MF – S&H TO PROCESSOR – 47pF CHANNELS 2-5 TO MEASURE TWO OTHER VI PAIRS Figure 1. Typical Power Line Monitoring Application 10/07/426 CONV SCK SDO DN426 F01 Figure 2 shows the AC line voltage in Linear Technology’s Mixed Signal lab. The flattened peaks are typical of the voltage in an office building where many of the loads are nonlinear, such as computer power supplies. Figure 3 shows the current through a 50W incandescent bulb. Figure 4 shows the current through a 15W compact fluorescent bulb, and Figure 5 is the current through a 4W LED-based bulb. The 5MHz full linear bandwidth of the LTC2351-14 allows analysis of high frequency components of the line voltage and current, limited in this case by the bandwidth of the sense transformers. Conclusion With PCB real estate getting tighter and designers always searching for lower power ICs, fast data acquisition can be a challenge. The LTC2351-14 and other low power SAR converters make it possible to optimize solution size, power and cost. 200 1.0 150 0.8 0.6 0.4 50 CURRENT (A) VOLTAGE (V) 100 0 –50 0.2 0 –0.2 –100 –0.4 –150 –0.6 –200 –0.8 –1.0 –250 0 10 20 30 40 50 TIME (ms) 60 70 0 80 10 20 30 40 50 TIME (ms) 60 Figure 3. 50W Incandescent Bulb Current 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 CURRENT (A) CURRENT (A) Figure 2. Line Voltage 0.2 0 –0.2 0.2 0 –0.2 –0.4 –0.4 –0.6 –0.6 –0.8 –0.8 –1.0 10 20 30 40 50 TIME (ms) 60 80 dn426 F03 dn426 F02 0 70 70 80 dn426 F04 Figure 4. 15W Compact Fluorescent Bulb Current –1.0 0 10 20 30 40 50 TIME (ms) 60 70 80 dn426 F05 Figure 5. 4W LED Bulb Current Data Sheet Download For applications help, call (408) 432-1900, Ext. 2602 www.linear.com Linear Technology Corporation dn426f LT/TP 1007 305K • 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 ●