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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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ISL28486FAZ Fiches technique(PDF) 11 Page - Intersil Corporation |
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ISL28486FAZ Fiches technique(HTML) 11 Page - Intersil Corporation |
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11 / 14 page  11 FN6312.1 June 28, 2007 Applications Information Introduction The ISL28286 and ISL28486 are dual and quad BiCMOS rail-to-rail input, output (RRIO) micropower precision operational amplifiers. These devices are designed to operate from a single supply (2.4V to 5.0V) or dual supplies (±1.2V to ±2.5V) while drawing only 120 μA (ISL28286) of supply current. This combination of low power and precision performance makes these devices suitable for solar and battery power applications. Rail-to-Rail Input Many rail-to-rail input stages use two differential input pairs, a long-tail PNP (or PFET) and an NPN (or NFET). Severe penalties have to be paid for this circuit topology. As the input signal moves from one supply rail to another, the operational amplifier switches from one input pair to the other causing drastic changes in input offset voltage and an undesired change in magnitude and polarity of input offset current. The ISL28286 and ISL28486 achieve input rail-to-rail without sacrificing important precision specifications and degrading distortion performance. The devices’ input offset voltage exhibits a smooth behavior throughout the entire common- mode input range. The input bias current versus the common-mode voltage range gives us an undistorted behavior from typically down to the negative rail to 10% higher than the V+ rail (0.5V higher than V+ when V+ equals 5V). Input Protection All input terminals have internal ESD protection diodes to positive and negative supply rails, limiting the input voltage to within one diode beyond the supply rails. Both parts have additional back-to-back diodes across the input terminals. If overdriving the inputs is necessary, the external input current must never exceed 5mA. External series resistors may be used as an external protection to limit excessive external voltage and current from damaging the inputs. Input Bias Current Compensation The ISL28286 and ISL28486 contain an input bias cancellation circuit which reduces the bias currents down to a typical of 500pA while maintaining an excellent bandwidth for a micro-power operational amplifier. The input stage transistors are still biased with adequate current for speed but the canceling circuit sinks most of the base current, leaving a small fraction as input bias current. 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 swing to within 4mV of the supply rails. Enable/Disable Feature The ISL28286 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 EN pins also have an internal pull-down. If left open, the EN pins will pull to the negative rail and the op amp will be enabled by default. Using Only One Channel The ISL28286 and ISL28486 are dual and quad channel op amps. If the application only requires one channel when using the ISL28286 or less than 4 channels when using the ISL28486, 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 37). Proper Layout Maximizes Performance To achieve the maximum performance of the high input impedance and low offset voltage of the ISL28286 and ISL28486, 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 38 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. FIGURE 37. PREVENTING OSCILLATIONS IN UNUSED CHANNELS - + 1/2 ISL28286 1/4 ISL28486 ISL28286, ISL28486 |
Numéro de pièce similaire - ISL28486FAZ |
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Description similaire - ISL28486FAZ |
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