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OPA622AP Datasheet(PDF) 13 Page - Burr-Brown (TI) |
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OPA622AP Datasheet(HTML) 13 Page - Burr-Brown (TI) |
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13 / 18 page  OPA622 13 ® FIGURE 6. Op Amp Configurations for OPA622. Amplifiers with an external compensation capacitor allow optimal frequency adjustment versus closed-loop gain, but nevertheless do not significantly improve large-signal be- havior. The most effective solution is to make the open-loop gain (GOL) externally adjustable. The widely-used current-feedback op amp type designed with real complementary circuit techniques overcomes the internal compensation capacitor and allows the feedback network to set the open-loop gain. The ratio of the feedback resistors determines the low-frequency closed-loop gain, and the parallel impedance defines the amplifier’s open-loop gain for stable operation and flat frequency response. A nearly constant bandwidth can be achieved over a wide range of closed-loop gains. However, current-feedback op amps suffer from nonidentical inputs and poor input offset and CMRR. The voltage-feedback op amp OPA622 with its complementary topology features two identical high-imped- ance inputs, lower input offset values, and improved CMRR. The ratio of the feedback resistors determines the low- frequency closed-loop gain, and the external resistor ROG sets the open-loop gain to achieve a flat frequency response over a wide range of closed-loop gains. Since ROG can be selected, optimized pulse responses are possible even with larger load capacitances. The OPA622 combines the slew rate enhancements of a complementary amplifier design with the precision of a voltage-feedback system. The hybrid model shown in Figure 9 describes the AC behavior of a noncompensated wide-band differential op amp. The open-loop frequency response, which is illustrated in Figure 10 for various ROG values, is determined by two without the feedback buffer produces the wider bandwidth of the current-feedback concept. The additional signal delay time through the feedback buffer determines the difference in AC performance between voltage and current feedback. The specifications for offset voltage, CMMR, and settling times are the compromise for higher speed. The open-loop gain for the current-feedback amplifier varies directly with the closed-loop gain and can be adjusted by changing the size of R2||R1. For gains of less than 10V/V, the open-loop gain can be adjusted to achieve bandwidth independent of gain, but the effects of this adjustment become limited when second-order effects start to dominate. Figure 6 gives an overview of the OPA622 inverting and non-inverting amplifier configurations and shows the equa- tions for the closed-loop gains. OPTIMAL FREQUENCY RESPONSE ADJUSTMENT Conventional voltage-feedback op amps use a compensation capacitor for stable unity-gain operation. During transitions, the quiescent current charges and discharges this capacitor, and both parameters determine the slew rate according to: SR = = This method is not appropriate for wide-band op amps. The slew rate and thus the large-signal behavior are significantly reduced, and the bandwidth decreases with increasing closed- loop gains according to the gain-bandwidth product. ∆ t C I ∆ V OUT –V IN FB OB R 2 R 1 G CL = 1 + R 1 R 2 R OG +V IN FB OB R 2 R 1 G CL = – R 1 R 2 R OG V OUT Voltage-Feedback OB R 2 R 1 G CL = 1 + R 1 R 2 +V IN OB R 2 R 1 V OUT –V IN Current-Feedback FB G CL = – R 1 R 2 FB Non-inverting Inverting Non-inverting Inverting V OUT V OUT OTA OTA OTA OTA |
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