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OPA347UA2K5G4 Datasheet(PDF) 9 Page - Texas Instruments |
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OPA347UA2K5G4 Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 33 page  OPA347, 2347, 4347 9 SBOS167D www.ti.com 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 5mV from supply rail. With moderate resistive loads (10k Ω to 50kΩ), the output can swing to within a few tens of millivolts from the supply rails while maintaining high open-loop gain (see the typical characteristic Output Voltage Swing vs Output Current). CAPACITIVE LOAD AND STABILITY The OPA347 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 characteristic curve 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 6. This significantly reduces ringing while maintaining Direct Current (DC) performance for purely capacitive loads. How- ever, if there is a resistive load in parallel with the capacitive load, a voltage divider is created, introducing a 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. load, reducing the resistor values from 100k Ω to 5kΩ de- creases overshoot from 40% to 8% (see the characteristic curve Small-Signal Overshoot vs Load Capacitance). How- ever, when large-valued resistors can not be avoided, a small (4pF to 6pF) capacitor, CFB, can be inserted in the feedback, as shown in Figure 7. This significantly reduces overshoot by compensating the effect of capacitance, CIN, which includes the amplifier input capacitance and PC board parasitic capacitance. FIGURE 6. Series Resistor in Unity-Gain Buffer Configura- tion Improves Capacitive Load Drive. 10 Ω to 20 Ω OPA347 V+ V IN V OUT R S R L C L FIGURE 7. Adding a Feedback Capacitor In the Unity-Gain Inverter Configuration Improves Capacitative Load. R I OPA347 V IN V OUT R F C FB C IN C L DRIVING ADCs The OPA347 series op amps are optimized for driving medium-speed sampling Analog-to-Digital Converters (ADCs). The OPA347 op amps buffer the ADC’s input capacitance and resulting charge injection while providing signal gain. See Figure 8 for the OPA347 in a basic noninverting configu- ration driving the ADS7822. The ADS7822 is a 12-bit, microPower sampling converter in the MSOP-8 package. When used with the low-power, miniature packages of the OPA347, the combination is ideal for space-limited, low- power applications. In this configuration, an RC network at the ADC input can be used to provide for anti-aliasing filter and charge injection current. See Figure 9 for the OPA2347 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 micro- phone. This circuit will operate with VS = 2.7V to 5V with less than 250 µA typical quiescent current. 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 |
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