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EL8178FSZ-T7 Datasheet(PDF) 9 Page - Intersil Corporation |
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EL8178FSZ-T7 Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 12 page 9 FN7504.5 March 29, 2007 non-unity gain op amp configurations, then the loading effects of the disabled amplifiers’ feedback networks must be considered when evaluating the active amplifier’s performance in Mux Amp configurations. Note that feed through from the IN+ to IN- pins occurs on any Mux Amp disabled channel where the input differential voltage exceeds 0.5V (e.g., active channel VOUT = 1V, while disabled channel VIN = GND), so the mux implementation is best suited for small signal applications. In any application where two or more amplifier outputs are muxed, use series IN+ resistors, or large value RFs in each amplifier to keep the feed through current low enough to minimize the impact on the active channel. See “Usage Implications” on page 9 for more details. IN+ and IN- Input Protection In addition to ESD protection diodes to each supply rail, the EL8178 has additional back-to-back protection diodes across the differential input terminals (see “Circuit 1” diagram on page 8). If the magnitude of the differential input voltage exceeds the diode’s VF, then one of these diodes will conduct. For elevated temperatures, the leakage of the protection diodes (Circuit 1 pin description table) increases, resulting in the increase in Ibias as seen in Figures 18 and 19. Usage Implications If the input differential voltage is expected to exceed 0.5V, an external current limiting resistor must be used to ensure the input current never exceeds 5mA. For noninverting unity gain applications the current limiting can be via a series IN+ resistor, or via a feedback resistor of appropriate value. For other gain configurations, the series IN+ resistor is the best choice, unless the feedback (RF) and gain setting (RG) resistors are both sufficiently large to limit the input current to 5mA. Large differential input voltages can arise from several sources: 1) During open loop (comparator) operation. The IN+ and IN- input voltages don’t track. 2) When the amplifier is disabled but an input signal is still present. An RL or RG to GND keeps the IN- at GND, while the varying IN+ signal creates a differential voltage. Mux Amp applications are similar, except that the active channel VOUT determines the voltage on the IN- terminal. 3) When the slew rate of the input pulse is considerably faster than the op amp’s slew rate. If the VOUT can’t keep up with the IN+ signal, a differential voltage results, and visible distortion occurs on the input and output signals. To avoid this issue, keep the input slew rate below 0.2V/ μs, or use appropriate current limiting resistors. Large (>2V) differential input voltages can also cause an increase in disabled ICC. EN I nput Protection The EN input has internal ESD protection diodes to both the positive and negative supply rails, limiting the input voltage range to within one diode beyond the supply rails (see “Circuit 2” diagram on page 8). If the input voltage is expected to exceed VS+ or VS-, then an external series resistor should be added to limit the current to 5mA. Output Current Limiting The EL8178 has no internal current-limiting circuitry. If the output is shorted, it is possible to exceed the “Absolute Maximum Rating” for “operating junction temperature”, potentially resulting in the destruction of the device. Power Dissipation It is possible to exceed the +150°C maximum junction temperature (TJMAX) under certain load and power-supply conditions. It is therefore important to calculate 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 as follows: where PDMAX is calculated using: where: •TMAX = Maximum ambient temperature • θ JA = Thermal resistance of the package •PDMAX = Maximum power dissipation of the amplifier •VS = Supply voltage •IMAX = Maximum supply current of the amplifier •VOUTMAX = Maximum output voltage swing of the application •RL = Load resistance Proper Layout Maximizes Precision To achieve the optimum levels of high input impedance (i.e., low input currents) and low offset voltage, 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 paramount concern, the use of guard rings around the amplifier inputs will further reduce leakage currents. Figure 27 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 T JMAX T MAX θ JAxP DMAX () + = (EQ. 1) PD MAX V S I SMAX V S ( - V OUTMAX ) V OUTMAX R L ---------------------------- × + × = (EQ. 2) EL8178 |
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