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ISL28488FAZ 데이터시트(PDF) 11 Page - Intersil Corporation |
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ISL28488FAZ 데이터시트(HTML) 11 Page - Intersil Corporation |
11 / 13 page 11 FN6339.1 June 28, 2007 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 34). Proper Layout Maximizes Performance To achieve the maximum performance of the high input impedance and low offset voltage of the ISL28288, 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 35 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 ISL28288 (Figure 36) is used to convert the differential thermocouple voltage into single-ended signal with 10X gain. The ISL28288'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 ISL28288 has 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: where: •PDMAXTOTAL is the sum of the maximum power dissipation of each amplifier in the package (PDMAX) •PDMAX for each amplifier is calculated in Equation 2: where: •TMAX = Maximum ambient temperature • θJA = Thermal resistance of the package •PDMAX = Maximum power dissipation of 1 amplifier •VS = Supply voltage (Magnitude of V+ and V-) •IMAX = Maximum supply current of 1 amplifier •VOUTMAX = Maximum output voltage swing of the application •RL = Load resistance FIGURE 34. PREVENTING OSCILLATIONS IN UNUSED CHANNELS - + ISL28288 IN V+ FIGURE 35. GUARD RING EXAMPLE FOR UNITY GAIN AMPLIFIER HIGH IMPEDANCE INPUT - + 5V + V+ V- ISL28X88 K TYPE THERMOCOUPLE 10k Ω R3 10k Ω R2 R4 100k Ω R1 100k Ω 410µV/°C FIGURE 36. THERMOCOUPLE AMPLIFIER T JMAX T MAX θ JAxPDMAXTOTAL () + = (EQ. 1) PD MAX 2*V S I SMAX V S ( - V OUTMAX ) V OUTMAX R L ---------------------------- × + × = (EQ. 2) ISL28288, ISL28488 |
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