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ISL28478 Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL28478 Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 13 page 11 FN6145.1 September 28, 2006 Applications Information Introduction The ISL28278 and ISL28478 are Dual and Quad channel CMOS rail-to-rail input, output (RRIO) micropower precision operational amplifier with an enable feature. The parts are designed to operate from single supply (2.4V to 5.0V) or dual supply (±1.2V to ±2.5V) while drawing only 120 μA of supply current. The device has an input common mode range that extends 10% above the positive rail and up to 100mV below the negative supply rail. The output operation can swing within about 4mV of the supply rails with a 100k Ω load (reference Figures 27 through 30). This combination of low power and precision performance makes them suitable for solar and battery power applications. Rail-to-Rail Input The input common-mode voltage range of the ISL28278 and ISL28478 is from the negative supply to 10% greater than the positive supply without introducing additional offset errors or degrading performance associated with a conventional rail-to-rail input operational amplifier. 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 ISL28278 and ISL28487 achieve input rail-to-rail operation 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 100mV below the negative rail and 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 both positive and negative supply rails, limiting the input voltage to within one diode beyond the supply rails. They have additional back-to-back diodes across the input terminals. For applications where the input differential voltage is expected to exceed 0.5V, external series resistors must be used to ensure the input currents never exceed 5mA. 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. The ISL28278 and ISL28478 with a 100k Ω load will swing to within 4mV of the positive supply rail and within 3mV of the negative supply rail. Enable/Disable Feature The ISL28278 has an EN pin that disables the device 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 ISL28278 parts can be connected together as a MUX. In this configuration, the outputs are tied together in parallel and a channel can be selected by the EN pin. 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. The loading effects of the feedback resistors of the disabled amplifier must be considered when multiple amplifier outputs are connected together. Using Only One Channel The ISL28278 and ISL28478 are Dual and Quad channel opamps. 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 37). Proper Layout Maximizes Performance To achieve the maximum performance of the high input impedance 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 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, FIGURE 37. PREVENTING OSCILLATIONS IN UNUSED CHANNELS - + 1/2 ISL28278 1/4 ISL28478 ISL28278, ISL28478 |
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