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OPA365 Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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OPA365 Datasheet(HTML) 10 Page - Burr-Brown (TI) |
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10 / 16 page  OPA365 OPA2365 SBOS365A − JUNE 2006 − REVISED JULY 2006 www.ti.com 10 ACHIEVING AN OUTPUT LEVEL OF ZERO VOLTS (0V) Certain single-supply applications require the op amp output to swing from 0V to a positive full-scale voltage and have high accuracy. An example is an op amp employed to drive a single-supply ADC having an input range from 0V to +5V. Rail-to-rail output amplifiers with very light output loading may achieve an output level within millivolts of 0V (or +VS at the high end), but not 0V. Furthermore, the deviation from 0V only becomes greater as the load current required increases. This in- creased deviation is a result of limitations of the CMOS output stage. When a pull-down resistor is connected from the ampli- fier output to a negative voltage source, the OPA365 can achieve an output level of 0V, and even a few milli- volts below 0V. Below this limit, nonlinearity and limiting conditions become evident. Figure 7 illustrates a circuit using this technique. A pull-down current of approximately 500 µA is required when OPA365 is connected as a unity-gain buffer. A practical termination voltage (VNEG) is −5V, but other convenient negative voltages also may be used. The pull-down resistor RL is calculated from RL = [(VO −VNEG)/(500µA)]. Using a minimum output voltage (VO) of 0V, RL = [0V−(−5V)]/(500µA)] = 10kΩ. Keep in mind that lower termination voltages result in smaller pull-down resistors that load the output during positive output voltage excursions. Note that this technique does not work with all op amps and should only be applied to op amps such as the OPA365 that have been specifically designed to oper- ate in this manner. Also, operating the OPA365 output at 0V changes the output stage operating conditions, resulting in somewhat lower open-loop gain and band- width. Keep these precautions in mind when driving a capacitive load because these conditions can affect cir- cuit transient response and stability. ACTIVE FILTERING The OPA365 is well-suited for active filter applications requiring a wide bandwidth, fast slew rate, low-noise, single-supply operational amplifier. Figure 8 shows a 500kHz, 2nd-order, low-pass filter utilizing the multiple− feedback (MFB) topology. The components have been selected to provide a maximally-flat Butterworth response. Beyond the cutoff frequency, roll-off is −40dB/dec. The Butterworth response is ideal for ap- plications requiring predictable gain characteristics such as the anti-aliasing filter used ahead of an ADC. V OUT RP =10kΩ 500 µA OPA365 V IN V+ = +5V Op Amps Negative Supply Grounded −V= −5V (Additional Negative Supply) Figure 7. Swing-to-Ground C 1 1nF C 2 150pF R 3 549 Ω R 1 549 Ω R 2 1.24k Ω V+ V OUT V IN V − OPA365 Figure 8. Second-Order Butterworth 500kHz Low-Pass Filter |
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