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OPA620SG Datasheet(PDF) 9 Page - Texas Instruments |
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OPA620SG Datasheet(HTML) 9 Page - Texas Instruments |
9 / 16 page 9 ® OPA620 Oscillations at frequencies of 200MHz and above can easily occur if good grounding techniques are not used. A heavy ground plane (2 oz. 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 (1 µF to 10 µF) with very short leads are recommended. A parallel 0.1 µF ceramic should be added at the supply pins. Surface mount bypass capacitors will produce excellent results due to their low lead inductance. Additionally, suppression fil- ters can be used to isolate noisy supply lines. Properly bypassed and modulation-free power supply lines allow full amplifier output and optimum settling time performance. Points to Remember 1) Don’t use point-to-point wiring as the increase in wiring inductance will be detrimental to AC performance. How- ever, if it must be used, very short, direct signal paths are required. The input signal ground return, the load ground return, and the power supply common should all be connected to the same physical point to eliminate ground loops, which can cause unwanted feedback. 2) Good component selection is essential. Capacitors used in critical locations should be a low inductance type with a high quality dielectric material. Likewise, diodes used in critical locations should be Schottky barrier types, such as HP5082- 2835 for fast recovery and minimum charge storage. Ordinary diodes will not be suitable in RF circuits. 3) Whenever possible, solder the OPA620 directly into the PC board without using a socket. Sockets add parasitic capacitance and inductance, which can seriously degrade AC performance or produce oscillations. If sockets must be used, consider using zero-profile solderless sockets such as Augat part number 8134-HC-5P2. Alternately, Teflon® stand- offs located close to the amplifier’s pins can be used to mount feedback components. 4) Resistors used in feedback networks should have values of a few hundred ohms for best performance. Shunt capaci- tance problems limit the acceptable resistance range to about 1k Ω on the high end and to a value that is within the amplifier’s output drive limits on the low end. Metal film and carbon resistors will be satisfactory, but wirewound resistors (even “non-inductive” types) are absolutely unacceptable in high-frequency circuits. 5) Surface-mount components (chip resistors, capacitors, etc) have low lead inductance and are therefore strongly recommended. Circuits using all surface-mount components with the OPA620KU (SO-8 package) will offer the best AC performance. The parasitic package inductance and capaci- tance for the SO-8 is lower than the both the Cerdip and 8-lead Plastic DIP. APPLICATIONS INFORMATION DISCUSSION OF PERFORMANCE The OPA620 provides a level of speed and precision not previously attainable in monolithic form. Unlike current feedback amplifiers, the OPA620’s design uses a “classical” 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 many disadvantages. 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. Bias current cancellation through matching of in- verting 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 reasonable. Such amplifiers are completely inadequate for fast settling 12-bit applications. The OPA620’s “classical” operational amplifier architecture employs true differential and fully symmetrical inputs to eliminate these troublesome problems. All traditional circuit configurations and op amp theory apply to the OPA620. The use of low-drift thin-film resistors allows internal operating currents to be laser-trimmed at wafer-level to optimize AC performance such as bandwidth and settling time, as well as DC parameters such as input offset voltage and drift. The result is a wideband, high-frequency monolithic operational amplifier with a gain-bandwidth product of 200MHz, a 0.01% settling time of 25ns, and an input offset voltage of 200 µV. WIRING PRECAUTIONS Maximizing the OPA620’s capability requires some wiring precautions and high-frequency layout techniques. Oscillation, 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 OPA620, as it is with all high-frequency circuits. Teflon® E. I. Du Pont de Nemours & Co. |
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