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OPA4350UA-2K5 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA4350UA-2K5 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 29 page  OPA350 OPA2350 OPA4350 SBOS099C − SEPTEMBER 2000 − REVISED JANUARY 2005 www.ti.com 11 FEEDBACK CAPACITOR IMPROVES RESPONSE For optimum settling time and stability with high-impedance feedback networks, it may be necessary to add a feedback capacitor across the feedback resistor, RF, as shown in Figure 4. This capacitor compensates for the zero created by the feedback network impedance and the OPA350’s input capacitance (and any parasitic layout capacitance). The effect becomes more significant with higher impedance networks. O PA350 V+ V OUT V IN R IN R IN • CIN =RF • CF R F C L C IN C IN C F Where C IN is equal to the OPA350’s input capacitance (approximately 9pF) plus any parasitic layout capacitance. Figure 4. Feedback Capacitor Improves Dynamic Performance It is suggested that a variable capacitor be used for the feedback capacitor since input capacitance may vary between op amps and layout capacitance is difficult to determine. For the circuit shown in Figure 4, the value of the variable feedback capacitor should be chosen so that the input resistance times the input capacitance of the OPA350 (typically 9pF) plus the estimated parasitic layout capacitance equals the feedback capacitor times the feedback resistor: R IN @ C IN + R F @ C F where CIN is equal to the OPA350’s input capacitance (sum of differential and common-mode) plus the layout capacitance. The capacitor can be varied until optimum performance is obtained. DRIVING A/D CONVERTERS OPA350 series op amps are optimized for driving medium speed (up to 500kHz) sampling A/D converters. However, they also offer excellent performance for higher speed converters. The OPA350 series provides an effective means of buffering the A/D’s input capacitance and resulting charge injection while providing signal gain. Figure 5 shows the OPA350 driving an ADS7861. The ADS7861 is a dual, 500kHz, 12-bit sampling converter in the tiny SSOP-24 package. When used with the miniature package options of the OPA350 series, the combination is ideal for space-limited applications. For further information, consult the ADS7861 data sheet (SBAS110A). OUTPUT IMPEDANCE The low frequency open-loop output impedance of the OPA350’s common-source output stage is approximately 1k Ω. When the op amp is connected with feedback, this value is reduced significantly by the loop gain of the op amp. For example, with 122dB of open-loop gain, the output impedance is reduced in unity-gain to less than 0.001 Ω. For each decade rise in the closed-loop gain, the loop gain is reduced by the same amount which results in a ten-fold increase in effective output impedance (see the typical characteristic, Output Impedance vs Frequency). At higher frequencies, the output impedance will rise as the open-loop gain of the op amp drops. However, at these frequencies the output also becomes capacitive due to parasitic capacitance. This prevents the output impedance from becoming too high, which can cause stability problems when driving capacitive loads. As mentioned previously, the OPA350 has excellent capacitive load drive capability for an op amp with its bandwidth. VIDEO LINE DRIVER Figure 6 shows a circuit for a single supply, G = 2 composite video line driver. The synchronized outputs of a composite video line driver extend below ground. As shown, the input to the op amp should be ac-coupled and shifted positively to provide adequate signal swing to account for these negative signals in a single-supply configuration. The input is terminated with a 75 Ω resistor and ac-coupled with a 47 µF capacitor to a voltage divider that provides the dc bias point to the input. In Figure 6, this point is approximately (V−) + 1.7V. Setting the optimal bias point requires some understanding of the nature of composite video signals. For best performance, one should be careful to avoid the distortion caused by the transition region of the OPA350’s complementary input stage. Refer to the discussion of rail-to-rail input. |
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