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LM7171 Datasheet(PDF) 13 Page - National Semiconductor (TI) |
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LM7171 Datasheet(HTML) 13 Page - National Semiconductor (TI) |
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13 / 20 page  LM7171 Circuit Operation (Continued) ers the inverting input. The triple-buffered output stage iso- lates the gain stage from the load to provide low output im- pedance. LM7171 Slew Rate Characteristic The slew rate of LM7171 is determined by the current avail- able to charge and discharge an internal high impedance node capacitor. This current is the differential input voltage divided by the total degeneration resistor R E. Therefore, the slew rate is proportional to the input voltage level, and the higher slew rates are achievable in the lower gain configura- tions. A curve of slew rate versus input voltage level is pro- vided in the “Typical Performance Characteristics”. When a very fast large signal pulse is applied to the input of an amplifier, some overshoot or undershoot occurs. By plac- ing an external resistor such as 1 k Ω in series with the input of LM7171, the bandwidth is reduced to help lower the over- shoot. Slew Rate Limitation If the amplifier’s input signal has too large of an amplitude at too high of a frequency, the amplifier is said to be slew rate limited; this can cause ringing in time domain and peaking in frequency domain at the output of the amplifier. In the “Typical Performance Characteristics” section, there are several curves of A V = +2 and AV = +4 versus input sig- nal levels. For the A V = +4 curves, no peaking is present and the LM7171 responds identically to the different input signal levels of 30 mV, 100 mV and 300 mV. For the A V = +2 curves, with slight peaking occurs. This peaking at high frequency (>100 MHz) is caused by a large input signal at high enough frequency that exceeds the am- plifier’s slew rate. The peaking in frequency response does not limit the pulse response in time domain, and the LM7171 is stable with noise gain of ≥+2. Layout Consideration PRINTED CIRCUIT BOARDS AND HIGH SPEED OP AMPS There are many things to consider when designing PC boards for high speed op amps. Without proper caution, it is very easy to have excessive ringing, oscillation and other de- graded AC performance in high speed circuits. As a rule, the signal traces should be short and wide to provide low induc- tance and low impedance paths. Any unused board space needs to be grounded to reduce stray signal pickup. Critical components should also be grounded at a common point to eliminate voltage drop. Sockets add capacitance to the board and can affect high frequency performance. It is better to solder the amplifier directly into the PC board without us- ing any socket. USING PROBES Active (FET) probes are ideal for taking high frequency mea- surements because they have wide bandwidth, high input impedance and low input capacitance. However, the probe ground leads provide a long ground loop that will produce er- rors in measurement. Instead, the probes can be grounded directly by removing the ground leads and probe jackets and using scope probe jacks. COMPONENT SELECTION AND FEEDBACK RESISTOR It is important in high speed applications to keep all compo- nent leads short. For discrete components, choose carbon composition-type resistors and mica-type capacitors. Sur- face mount components are preferred over discrete compo- nents for minimum inductive effect. Large values of feedback resistors can couple with parasitic capacitance and cause undesirable effects such as ringing or oscillation in high speed amplifiers. For LM7171, a feed- back resistor of 510 Ω gives optimal performance. Compensation for Input Capacitance The combination of an amplifier’s input capacitance with the gain setting resistors adds a pole that can cause peaking or oscillation. To solve this problem, a feedback capacitor with a value C F > (RG xCIN)/RF can be used to cancel that pole. For LM7171, a feedback ca- pacitor of 2 pF is recommended. Figure 1 illustrates the com- pensation circuit. Power Supply Bypassing Bypassing the power supply is necessary to maintain low power supply impedance across frequency. Both positive and negative power supplies should be bypassed individu- ally by placing 0.01 µF ceramic capacitors directly to power supply pins and 2.2 µF tantalum capacitors close to the power supply pins. Termination In high frequency applications, reflections occur if signals are not properly terminated. Figure 3 shows a properly termi- nated signal while Figure 4 shows an improperly terminated signal. DS012385-10 FIGURE 1. Compensating for Input Capacitance DS012385-11 FIGURE 2. Power Supply Bypassing www.national.com 13 |
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