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ISL28278FAZ Scheda tecnica(PDF) 11 Page - Intersil Corporation |
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ISL28278FAZ Scheda tecnica(HTML) 11 Page - Intersil Corporation |
11 / 13 page 11 FN6145.2 July 11, 2007 Rail-to-Rail Output A pair of complementary MOSFET devices are used to achieve the rail-to-rail output swing. The NMOS sinks current to swing the output in the negative direction. The PMOS sources current to swing the output in the positive direction. Both parts, with a 100k Ω load, will typically swing to within 4mV of the positive supply rail and within 3mV of the negative supply rail. Enable/Disable Feature The ISL28278 offers two EN pins (EN_A and EN_B) which disable the op amp when pulled up to at least 2.0V. In the disabled state (output in a high impedance state), the part consumes typically 4µA. By disabling the part, multiple parts can be connected together as a MUX. The outputs are tied together in parallel and a channel can be selected by the EN pins. The loading effects of the feedback resistors of the disabled amplifier must be considered when multiple amplifier outputs are connected together. The EN pin also has an internal pull-down. If left open, the EN pin will pull to the negative rail and the device will be enabled by default. Using Only One Channel The ISL28278 and ISL28478 are dual and quad channel op amps. If the application only requires one channel when using the ISL28278 or less than 4 channels when using the ISL28478, the user must configure the unused channel(s) to prevent them from oscillating. The unused channel(s) will oscillate if the input and output pins are floating. This will result in higher than expected supply currents and possible noise injection into the channel being used. The proper way to prevent this oscillation is to short the output to the negative input and ground the positive input (as shown in Figure 36). Proper Layout Maximizes Performance To achieve the maximum performance of the high input impedance and low offset voltage of the ISL28278 and ISL28478, care should be taken in the circuit board layout. The PC board surface must remain clean and free of moisture to avoid leakage currents between adjacent traces. Surface coating of the circuit board will reduce surface moisture and provide a humidity barrier, reducing parasitic resistance on the board. When input leakage current is a concern, the use of guard rings around the amplifier inputs will further reduce leakage currents. Figure 37 shows a guard ring example for a unity gain amplifier that uses the low impedance amplifier output at the same voltage as the high impedance input to eliminate surface leakage. The guard ring does not need to be a specific width, but it should form a continuous loop around both inputs. For further reduction of leakage currents, components can be mounted to the PC board using Teflon standoff insulators. Example Application Thermocouples are the most popular temperature-sensing device because of their low cost, interchangeability, and ability to measure a wide range of temperatures. The ISL28X78 (Figure 38) is used to convert the differential thermocouple voltage into single-ended signal with 10X gain. The ISL28X78's rail-to-rail input characteristic allows the thermocouple to be biased at ground and the amplifier to run from a single 5V supply. . Current Limiting The ISL28278 and ISL28478 have no internal current- limiting circuitry. If the output is shorted, it is possible to exceed the Absolute Maximum Rating for output current or power dissipation, potentially resulting in the destruction of the device. Power Dissipation It is possible to exceed the +150°C maximum junction temperatures under certain load and power-supply conditions. It is therefore important to calculate the maximum junction temperature (TJMAX) for all applications to determine if power supply voltages, load conditions, or package type need to be modified to remain in the safe operating area. These parameters are related in Equation 1: FIGURE 36. PREVENTING OSCILLATIONS IN UNUSED CHANNELS - + 1/2 ISL28278 1/4 ISL28478 IN V+ FIGURE 37. GUARD RING EXAMPLE FOR UNITY GAIN AMPLIFIER HIGH IMPEDANCE INPUT - + 5V + V+ V- ISL28X78 K TYPE THERMOCOUPLE 10k Ω R3 10k Ω R2 R4 100k Ω R1 100k Ω 410µV/°C FIGURE 38. THERMOCOUPLE AMPLIFIER T JMAX T MAX θ JAxPDMAXTOTAL () + = (EQ. 1) ISL28278, ISL28478 |
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