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LMC7221AIMX Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMC7221AIMX Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 13 page Application Information (Continued) Direct Sensor Interfacing The wide input voltage range and high impedance of the LMC7221 may make it possible to directly interface to a sensor without the use of amplifiers or bias circuits. In cir- cuits with sensors which can produce outputs in the tens to hundreds of millivolts, the LMC7221 can compare the sensor signal with an appropriately small reference voltage. This may be done close to ground or the positive supply rail. Direct sensor interfacing may eliminate the need for an amplifier for the sensor signal. Eliminating the amplifier can save cost, space, and design time. 2.0 LOW VOLTAGE OPERATION Comparators are the common devices by which analog sig- nals interface with digital circuits. The LMC7221 has been designed to operate at supply voltages of 2.7V without sac- rificing performance to meet the demands of 3V digital sys- tems. At supply voltages of 2.7V, the common-mode voltage range extends 200 mV (guaranteed) below the negative supply. This feature, in addition to the comparator being able to sense signals near the positive rail, is extremely useful in low voltage applications. At V + = 2.7V propagation delays are t PLH =4µsand tPHL = 4 µs with overdrives of 100 mV. Please refer to the performance curves for more extensive characterization. 3.0 OPEN DRAIN OUTPUT Figure 2 shows the difference between push-pull output and open drain output. Push pull outputs will have a conventional high or low digital output, the same as a logic gate. Low will be the negative supply rail (usually ground) and high will be the positive supply rail. This is useful if the chips you are interfacing to run on the same supply voltage as the comparator. An example would be an all +5V system. Open drain outputs will only pull low — for the high output they depend on an external pull-up resistor. This can pull up to a voltage higher or lower than the comparator supply voltage. This voltage can be as high as 15V. This makes the open drain parts useful in mixed voltage systems. An ex- ample would be where the comparator runs at 5V and the logic circuits are at 3.3V. The pull-up resistor would go to the 3.3V supply. Open drain outputs are the CMOS equivalent of open col- lector outputs. 01234616 FIGURE 1. Even at Low-Supply Voltage of 2.7V, an Input Signal which Exceeds the Supply Voltages Produces No Phase Inversion at the Output Output Stage 01234617 01234621 FIGURE 2. www.national.com 8 |
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