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OPA341UA Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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OPA341UA Datasheet(HTML) 10 Page - Burr-Brown (TI) |
10 / 15 page OPA341, 2341 10 SBOS202A FIGURE 3. Input Current Protection for Voltages Exceeding the Supply Voltage. FIGURE 4. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive. A double-folded cascode adds the signal from the two input pairs and presents a differential signal to the class AB output stage. Normally, input bias current is approximately 600fA, however, input voltages exceeding the power supplies by more than 300mV can cause excessive current to flow in or out of the input pins. Momentary voltages greater than 300mV beyond the power supply can be tolerated if the current on the input pins is limited to 10mA. This is easily accomplished with an input resistor, as shown in Figure 3. Many input signals are inherently current-limited to less than 10mA, therefore, a limiting resistor is not required. RAIL-TO-RAIL OUTPUT A class AB output stage with common-source transistors is used to achieve rail-to-rail output. For light resistive loads (> 50k Ω), the output voltage is typically a few millivolts from the supply rails. With moderate resistive loads (2k Ω to 50k Ω), the output can swing to within a few tens of milli- volts from the supply rails and maintain high open-loop gain. See the typical characteristic “Output Voltage Swing vs Output Current.” CAPACITIVE LOAD AND STABILITY OPA341 series op amps can drive a wide range of capacitive loads. However, all op amps under certain conditions may become unstable. Op amp configurations, gain, and load value are just a few of the factors to consider when determin- ing stability. An op amp in unity-gain configuration is the most susceptible to the effects of capacitive load. The capacitive load reacts with the op amp’s output resistance, along with any additional load resistance, to create a pole in the small-signal response which degrades the phase margin. In unity gain, OPA341 series op amps perform well, with a pure capacitive load up to approximately 1000pF. Increasing gain enhances the amplifier’s ability to drive more capaci- tance. See the typical characteristic “Small-Signal Over- shoot vs Capacitive Load.” One method of improving capacitive load drive in the unity- gain configuration is to insert a 10 Ω to 20Ω resistor in series with the output, as shown in Figure 4. This significantly reduces ringing with large capacitive loads. However, if there is a resistive load in parallel with the capacitive load, RS creates a voltage divider. This introduces a DC error at the output and slightly reduces output swing. This error may be insignificant. For instance, with RL = 10kΩ and RS = 20Ω, there is only about a 0.2% error at the output. DRIVING A/D CONVERTERS OPA341 series op amps are optimized for driving medium speed (up to 100kHz) sampling A/D converters. However, they also offer excellent performance for higher-speed con- verters. The OPA341 series provides an effective means of buffering the A/D converter’s input capacitance and result- ing charge injection while providing signal gain. For appli- cations requiring high accuracy, the OPA340 series is rec- ommended. The OPA341 implements a power-saving shutdown feature particularly useful for low-power sampling applications. Figure 5 shows the OPA341 driving the ADS7816, a 12-bit micro-power sampling converter available in the tiny MSOP-8 package. With the OPA341 in non-inverting configuration, an RC network at the amplifier’s output is used as an anti-aliasing filter. By tying the enable of the OPA341 to the shutdown of the ADS7816, addi- tional power-savings can be used for sampling applications. To effectively drive the ADS7816, timing delay was introduced between the two devices, see Figure 5. Alternative applications may need additional timing adjustments. Figure 6 shows the OPA341 configured as a speech band- pass filter. Figure 7 shows the OPA341 configured as a transimpedance amplifier. OPAx341 10mA max V+ V IN V OUT I OVERLOAD 10 Ω to 20 Ω OPAx341 V+ V ENABLE V IN V OUT R S R L C L |
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