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TLV240X Datasheet(PDF) 7 Page - Burr-Brown (TI) |
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TLV240X Datasheet(HTML) 7 Page - Burr-Brown (TI) |
7 / 12 page OPA379 OPA2379 OPA4379 SBOS347A − NOVEMBER 2005 − REVISED DECEMBER 2005 www.ti.com 7 APPLICATION INFORMATION The OPA379 family of operational amplifiers minimizes power consumption without compromising bandwidth or noise. Power-supply rejection ratio (PSRR), common-mode rejection ratio (CMRR), and open-loop gain (AOL) typical values are 100dB or better. When designing for ultra-low power, choose system components carefully. To minimize current consumption, select large-value resistors. Any resistors will react with stray capacitance in the circuit and the input capacitance of the operational amplifier. These parasitic RC combinations can affect the stability of the overall system. A feedback capacitor may be required to assure stability and limit overshoot or gain peaking. Good layout practice mandates the use of a 0.1 µF bypass capacitor placed closely across the supply pins. OPERATING VOLTAGE OPA379 series op amps are fully specified and tested from +1.8V to +5.5V. Parameters that vary significantly with supply voltage are shown in the Typical Characteristics curves. INPUT COMMON-MODE VOLTAGE RANGE The input common-mode voltage range of the OPA379 family typically extends 100mV beyond each supply rail. This rail-to-rail input is achieved using a complementary input stage. CMRR is specified from the negative rail to 1V below the positive rail. Between (V+) − 1V and (V+) + 0.1V, the amplifier operates with higher offset voltage because of the transition region of the input stage. See the typical characteristic, Offset Voltage vs Common-Mode Voltage. PROTECTING INPUTS FROM OVER-VOLTAGE Normally, input currents are 5pA. However, large inputs (greater than 500mV beyond the supply rails) can cause excessive current to flow in or out of the input pins. Therefore, as well as keeping the input voltage below the maximum rating, it is also important to limit the input current to less than 10mA. This limiting is easily accomplished with an input voltage resistor, as shown in Figure 1. 5k Ω OPA379 10mA max +5V V IN V OUT I OVERLOAD Figure 1. Input Current Protection for Voltages Exceeding the Supply Voltage NOISE Although micropower amplifiers frequently have high wideband noise, the OPA379 series offer excellent noise performance. Resistors should be chosen carefully because the OPA379 has only 2.8 µVPP of 0.1Hz to 10Hz noise, and 80nV/ √Hz of wideband noise; otherwise, they can become the dominant source of noise. CAPACITIVE LOAD AND STABILITY Follower configurations with load capacitance in excess of 30pF can produce extra overshoot (see typical characteristic, Small-Signal Overshoot vs Capacitive Load) and ringing in the output signal. Increasing the gain enhances the ability of the amplifier to drive greater capacitive loads. In unity-gain configurations, capacitive load drive can be improved by inserting a small (10 Ω to 20 Ω) resistor, RS, in series with the output, as shown in Figure 2. This resistor significantly reduces ringing while maintaining DC performance 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. 10 Ω to 20 Ω OPA379 V+ V IN V OUT R S R L C L Figure 2. Series Resistor in Unity-Gain Buffer Configuration Improves Capacitive Load Drive |
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