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OPA658P Datasheet(PDF) 9 Page - Texas Instruments |
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OPA658P Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 18 page  OPA658 9 SBOS045A www.ti.com The feedback resistor value acts as the frequency response compensation element for a current-feedback type amplifier. The 402 Ω used in setting the specification achieves a nomi- nal maximally-flat butterworth response while assuming a 2pF output pin parasitic. Increasing the feedback resistor will overcompensate the amplifier, rolling off the frequency re- sponse, while decreasing it will decrease phase margin, peaking up the frequency response. Note that a noninverting, unity-gain buffer application still requires a feedback resistor for stability (560 Ω for SO-8, 402Ω for DIP, and 324Ω for SOT23). d) Connections to other wideband devices on the board may be made with short direct traces or through onboard transmission lines. For short connections, consider the trace and the input to the next device as a lumped capacitive load. Relatively wide traces (50 mils to 100 mils) should be used, preferably with ground and power planes opened up around them. Estimate the total capacitive load and set RISO from the plot of recommended RISO vs capacitive load. Low parasitic loads may not need an RISO since the OPA658 is nominally compensated to operate with a 2pF parasitic load. If a long trace is required and the 6dB signal loss intrinsic to doubly-terminated transmission lines is acceptable, imple- ment a matched impedance transmission line using microstrip or stripline techniques (consult an ECL design handbook for microstrip and stripline layout techniques). A 50 Ω environ- ment is not necessary onboard, and in fact a higher imped- ance environment will improve distortion as shown in the distortion vs load plot. With a characteristic impedance de- fined based on board material and desired trace dimensions, a matching series resistor into the trace from the output of the amplifier is used as well as a terminating shunt resistor at the input of the destination device. Remember also that the terminating impedance will be the parallel combination of the shunt resistor and the input impedance of the destination device; the total effective impedance should match the trace impedance. Multiple destination devices are best handled as separate transmission lines, each with their own series and shunt terminations. If the 6dB attenuation loss of a doubly-terminated line is unacceptable, a long trace can be series-terminated at the source end only. This will help isolate the line capacitance from the op amp output, but will not preserve signal integrity as well as a doubly-terminated line. If the shunt impedance at the destination end is finite, there will be some signal attenuation due to the voltage divider formed by the series and shunt impedances. e) Socketing a high-speed part like the OPA658 is not recommended. The additional lead length and pin-to-pin capacitance introduced by the socket creates an extremely troublesome parasitic network which can make it almost impossible to achieve a smooth, stable response. Best re- sults are obtained by soldering the part onto the board. If socketing for the DIP package is desired, high-frequency, flush-mount pins (for instance, McKenzie Technology #710C) can give good results. If all terms are divided by the gain (1 + RFB/RFF) it can be observed that input referred offsets improve as gain in- creases. The effective noise at the output can be determined by taking the root sum of the squares of Equation 4 and applying the spectral noise values found in the Typical Characteristics section. This applies to noise from the op amp only. Note that both the noise figure (NF) and the equivalent input offset voltages improve as the closed-loop gain increases (by keeping RFB fixed and reducing RFF with RN = 0Ω). INCREASING BANDWIDTH AT HIGH GAINS The closed-loop bandwidth can be extended at high gains by reducing the value of the feedback resistor RFB. This band- width reduction is caused by the feedback current being split between RS and RFF (refer to Figure 1). As the gain increases (for a fixed RFB), more feedback current is shunted through RFF, which reduces closed-loop bandwidth. CIRCUIT LAYOUT AND BASIC OPERATION Achieving optimum performance with a high-frequency am- plifier such as the OPA658 requires careful attention to layout parasitics and selection of external components. Rec- ommendations for PC board layout and component selection include: a) Minimize parasitic capacitance to any ac ground for all of the signal I/O pins. Parasitic capacitance on the output and inverting input pins can cause instability; on the noninverting input it can react with the source impedance to cause unintentional bandlimiting. To reduce unwanted capacitance, a window around the signal I/O pins should be opened in all of the ground and power planes. Otherwise, ground and power planes should be unbroken elsewhere on the board. b) Minimize the distance (< 0.25") from the two power pins to high-frequency 0.1 µF decoupling capacitors. At the pins, the ground and power-plane layout should not be in close proximity to the signal I/O pins. Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling capacitors. Larger (2.2 µF to 6.8µF) decou- pling capacitors, effective at lower frequencies, should also be used. These may be placed somewhat farther from the device and may be shared among several devices in the same area of the PC board. c) Careful selection and placement of external compo- nents will preserve the high-frequency performance of the OPA658. Resistors should be a very low reactance type. Surface-mount resistors work best and allow a tighter overall layout. Metal film or carbon composition axially-leaded resis- tors can also provide good high-frequency performance. Again, keep their leads as short as possible. Never use wire- wound type resistors in a high-frequency application. Since the output pin and the inverting input pin are most sensitive to parasitic capacitance, always position the feed- back and series output resistor, if any, as close as possible to the package pins. Other network components, such as noninverting input termination resistors, should also be placed close to the package. |
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