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OPA4348AIDR Datasheet(PDF) 8 Page - Texas Instruments |
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OPA4348AIDR Datasheet(HTML) 8 Page - Texas Instruments |
8 / 17 page OPA348, 2348, 4348 8 SBOS213C www.ti.com In unity-gain inverter configuration, phase margin can be reduced by the reaction between the capacitance at the op amp input, and the gain setting resistors, thus degrading capacitive load drive. Best performance is achieved by using small valued resistors. For example, when driving a 500pF load, reducing the resistor values from 100k Ω to 5kΩ de- creases overshoot from 55% to 13% (see the typical charac- teristic “Small-Signal Overshoot vs. Load Capacitance”). However, when large valued resistors cannot be avoided, a small (4pF to 6pF) capacitor, CFB, can be inserted in the feedback, as shown in Figure 6. This significantly reduces overshoot by compensating the effect of capacitance, CIN, which includes the amplifier's input capacitance and PC board parasitic capacitance. FIGURE 6. Improving Capacitive Load Drive. Normally, input currents are 0.5pA. However, large inputs (greater than 500mV beyond the supply rails) can cause excessive current to flow in or out of the input pins. There- fore, as well as keeping the input voltage below the maxi- mum rating, it is also important to limit the input current to less than 10mA. This is easily accomplished with an input voltage resistor, as shown in Figure 4. R I OPA348 V IN V OUT R F C FB C IN C L FIGURE 4. Input Current Protection for Voltages Exceeding the Supply Voltage. 5k Ω OPA348 10mA max +5V V IN V OUT I OVERLOAD RAIL-TO-RAIL OUTPUT A class AB output stage with common-source transistors is used to achieve rail-to-rail output. This output stage is ca- pable of driving 5k Ω loads connected to any potential be- tween V+ and ground. For light resistive loads (> 100k Ω), the output voltage can typically swing to within 18mV from supply rail. With moderate resistive loads (10k Ω to 50kΩ), the output voltage can typically swing to within 100mV of the supply rails while maintaining high open-loop gain (see the typical characteristic “Output Voltage Swing vs Output Current”). CAPACITIVE LOAD AND STABILITY The OPA348 in a unity-gain configuration can directly drive up to 250pF pure capacitive load. Increasing the gain en- hances the amplifier’s ability to drive greater capacitive loads (see the typical characteristic “Small-Signal Overshoot vs Capacitive Load”). In unity-gain configurations, capacitive load drive can be improved by inserting a small (10 Ω to 20Ω) resistor, RS, in series with the output, as shown in Figure 5. This significantly reduces ringing while maintaining DC per- formance for purely capacitive loads. However, if there is a resistive load in parallel with the capacitive load, a voltage divider is created, introducing a Direct Current (DC) error at the output and slightly reducing the output swing. The error introduced is proportional to the ratio RS/RL, and is generally negligible. FIGURE 5. Series Resistor in Unity-Gain Buffer Configura- tion Improves Capacitive Load Drive. 10 Ω to 20 Ω OPA348 V+ V IN V OUT R S R L C L DRIVING A/D CONVERTERS The OPA348 series op amps are optimized for driving medium-speed sampling Analog-to-Digital Converters (ADCs). The OPA348 op amps buffer the ADCs input capacitance and resulting charge injection while providing signal gain. The OPA348 in a basic noninverting configuration driving the ADS7822, see Figure 7. The ADS7822 is a 12-bit, microPOWER sampling converter in the MSOP-8 package. When used with the low-power, miniature packages of the OPA348, the combination is ideal for space-limited, low- power applications. In this configuration, an RC network at the ADC’s input can be used to provide for anti-aliasing filter and charge injection current. The OPA348 in noninverting configuration driving ADS7822 limited, low-power applications. In this configuration, an RC network at the ADC’s input can be used to provide for anti- aliasing filter and charge injection current. See Figure 8 for the OPA2348 driving an ADS7822 in a speech bandpass filtered data acquisition system. This small, low-cost solution provides the necessary amplification and signal conditioning to interface directly with an electret microphone. This circuit will operate with VS = 2.7V to 5V with less than 250µA typical quiescent current. |
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