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MAX378CJE Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX378CJE Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page High-Voltage, Fault-Protected Analog Multiplexers 6 _______________________________________________________________________________________ _______________Typical Applications Figure 6 shows a typical data acquisition system using the MAX378 multiplexer. Since the multiplexer is driving a high-impedance input, its error is a func- tion of its own resistance (RDS(ON)) times the multi- plexer leakage current (IOUT(ON)) and the amplifier bias current (IBIAS): VERR = RDS(ON) x (IOUT(ON) + IBIAS (MAX420)) = 2.0k Ω x (2nA + 30pA) = 18.0µV maximum error In most cases, this error is low enough that preamplifi- cation of input signals is not needed, even with very low-level signals such as 40µV/°C from type J thermo- couples. In systems with fewer than eight inputs, an unused chan- nel can be connected to the system ground reference point for software zero correction. A second channel connected to the system voltage reference allows gain correction of the entire data acquisition system as well. A MAX420 precision op amp is connected as a pro- grammable-gain amplifier, with gains ranging from 1 to 10,000. The guaranteed 5µV unadjusted offset of the MAX420 maintains high signal accuracy, while program- mable gain allows the output signal level to be scaled to the optimum range for the remainder of the data acqui- sition system, normally a Sample/Hold and A/D. Since the gain-changing multiplexer is not connected to the external sensors, it can be either a DG508A multiplexer or the fault-protected MAX358 or MAX378. Truth Table—MAX378 A2 A1 A0 EN ON SWITCH X 0 0 0 0 1 1 1 1 X 0 0 1 1 0 0 1 1 X 0 1 0 1 0 1 0 1 0 1 1 1 1 1 1 1 1 NONE 1 2 3 4 5 6 7 8 Truth Table—MAX379 A1 A0 EN ON SWITCH X 0 0 1 1 X 0 1 0 1 0 1 1 1 1 NONE 1 2 3 4 Note: Logic “0” = VAL ≤ 0.8V, Logic “1” = VAH ≥ 2.4V MAX420 +15V -15V V+ +15V 1M 100k 10k OUT OUT 1k 111 Ω IN1 IN1 IN2 IN3 IN4 IN5 V+ THERMOCOUPLE +15V -15V V- GND DG508A MAX358 OR MAX378 MAX378 IN2 STRAIN GUAGE IN7 +10V GAIN REFERENCE IN8 ZERO REFERENCE IN3 4-20mA LOOP TRANSMITTER IN4 IN5 IN6 -15V V- GND Figure 6. Typical Data Acquisition Front End |
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