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ISL28246FUZ-T7 Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL28246FUZ-T7 Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 16 page 11 FN6321.3 June 23, 2008 Applications Information Introduction The ISL28146 and ISL28246 are single and dual channel rail-to-rail input, output (RRIO) micropower precision operational amplifiers. The parts are designed to operate from single supply (2.4V to 5.0V) or dual supply (±1.2V to ±2.75V). The parts have an input common mode range that extends 0.25V above the positive rail and down to the negative supply rail. The output operation can swing within about 3mV of the supply rails with a 100k Ω load. 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 ISL28146 and ISL28246 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 an undistorted behavior from typically down to the negative rail and up to 0.25V higher than the V+ rail. Rail-to-Rail Output A pair of complementary MOS 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 ISL28146 and ISL28246 with a 100k Ω load will swing to within 3mV of the positive supply rail and within 3mV of the negative supply rail. Results of Over-Driving the Output Caution should be used when over-driving the output for long periods of time. Over-driving the output can occur in two ways: 1. The input voltage times the gain of the amplifier exceeds the supply voltage by a large value. 2. The output current required is higher than the output stage can deliver. These conditions can result in a shift in the Input Offset Voltage (VOS) as much as 1µV/hr. of exposure under these conditions. IN+ and IN- 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 also contain back-to-back diodes across the input terminals (“Pin Descriptions” on page 10 - Circuit 1). For applications where the input differential voltage is expected to exceed 0.5V, an external series resistor must be used to ensure the input currents never exceed 5mA (Figure 36). Enable/Disable Feature The ISL28146 offers 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 10µA at room temperature. The EN pin 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. When not used, the EN pin should either be left floating or connected directly to the -V pin. By disabling the part, multiple ISL28146 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 loading effects of the feedback resistors of the disabled amplifier must be considered when multiple amplifier outputs are connected together. 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. If large signals are required, use series IN+ resistors, or a large value RF, to keep the feed through current low enough to minimize the impact on the active channel. See “Limitations of the Differential Input Protection” on page 11. Limitations of the Differential Input Protection 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 non-inverting 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. Used this way, the IN+ and IN- voltages don’t track, so differentials arise. 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 FIGURE 36. INPUT CURRENT LIMITING - + RIN RL VIN VOUT ISL28146, ISL28246 |
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