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OPA640U Datasheet(PDF) 7 Page - Texas Instruments |
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OPA640U Datasheet(HTML) 7 Page - Texas Instruments |
7 / 14 page ® OPA640 7 TYPICAL PERFORMANCE CURVES (CONT) T A = +25°C, VS = ±5V, RL = 100Ω, CL = 2pF, RFB = 402Ω and all four power supply pins are used, unless otherwise noted. RFB = 25Ω for a gain of +1. APPLICATIONS INFORMATION DISCUSSION OF PERFORMANCE The OPA640 provides a level of speed and precision not previously attainable in monolithic form. Unlike current feedback amplifiers, the OPA640’s design uses a “Classi- cal” operational amplifier architecture and can therefore be used in all traditional operational amplifier applications. While it is true that current feedback amplifiers can provide wider bandwidth at higher gains, they offer some disadvan- tages. The asymmetrical input characteristics of current feedback amplifiers (i.e. one input is a low impedance) prevents them from being used in a variety of applications. In addition, unbalanced inputs make input bias current errors difficult to correct. Cancelling offset errors (due to input bias currents) through matching of inverting and non-inverting input resistors is impossible because the input bias currents are uncorrelated. Current noise is also asymmetrical and is usually significantly higher on the inverting input. Perhaps most important, settling time to 0.01% is often extremely poor due to internal design tradeoffs. Many current feedback designs exhibit settling times to 0.01% in excess of 10 microseconds even though 0.1% settling times are reason- able. Such amplifiers are completely inadequate for fast settling 12-bit applications. The OPA640’s “Classical” operational amplifier architec- ture employs true differential and fully symmetrical inputs to eliminate these troublesome problems. All traditional circuit configurations and op amp theory apply to the OPA640. WIRING PRECAUTIONS Maximizing the OPA640’s capability requires some wiring precautions and high-frequency layout techniques. Oscilla- tion, ringing, poor bandwidth and settling, gain peaking, and instability are typical problems plaguing all high-speed amplifiers when they are improperly used. In general, all printed circuit board conductors should be wide to provide low resistance, low impedance signal paths. They should also be as short as possible. The entire physical circuit should be as small as practical. Stray capacitances should be minimized, especially at high impedance nodes, such as the amplifier’s input terminals. Stray signal coupling from the output or power supplies to the inputs should be minimized. All circuit element leads should be no longer than 1/4 inch (6mm) to minimize lead inductance, and low values of resistance should be used. This will minimize time constants formed with the circuit capacitances and will eliminate stray, parasitic circuits. Grounding is the most important application consideration for the OPA640, as it is with all high-frequency circuits. Oscillations at high frequencies can easily occur if good grounding techniques are not used. A heavy ground plane (2oz copper recommended) should connect all unused areas on the component side. Good ground planes can reduce stray signal pickup, provide a low resistance, low inductance common return path for signal and power, and can conduct heat from active circuit package pins into ambient air by convection. Supply bypassing is extremely critical and must always be used, especially when driving high current loads. Both power supply leads should be bypassed to ground as close as possible to the amplifier pins. Tantalum capacitors (2.2 µF) with very short leads are recommended. A parallel 0.01 µF ceramic must also be added. Surface mount bypass capaci- tors will produce excellent results due to their low lead inductance. Additionally, suppression filters can be used to isolate noisy supply lines. Properly bypassed and modula- tion-free power supply lines allow full amplifier output and optimum settling time performance. –65 –75 –85 –95 0 Output Swing (Vp-p) 10MHz HARMONIC DISTORTION vs OUTPUT SWING (G = +1, R L = 100Ω) 1.0 2.0 3.0 4.0 2f O 3f O |
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