Finally, because the circuit is based on a nonlinear principle, the internal network affects distortion and load drive ability, and these factors influence amplifier performance in video applications. Though the networks presence does not by any means make devices like the AD847 or AD817 unusable for video, it does not permit the very lowest levels of distortion and differential gain and phase which are achievable with otherwise comparable amplifiers(for example, the ad8 18) While the individual techniques for countering cap loading outlined above have various specific tradeoffs as noted all of the techniques have a serious common drawback of reducing speed(both bandwidth and slew rate). If these parameters cannot be sacrificed, then a matched transmission line system is the solution, and discussed in more detail later in the chapter. As for choosing among the cap load compensation schemes, it would seem on the surface that amplifiers using the internal form offer the best possible solution to the problem- just pick the right amplifier and forget about it. And indeed, that would seem the"panacea"solution for all cap load situations- if you use the "right" amplifier you never need to think about cap loading again Could there be more to it? Yes! The"gotcha"of internal cap load compensation is subtle, and lies in the fact that the dynamic adaptive nature of the compensation mechanism actually can produce higher levels of distortion, vis-a-vis an otherwise similar amplifier, without the Ce-resistor network like the old saying about no free lunches, if you care about attaining top- notch levels of high frequency AC performance, you should give the issue of whether to use an internally compensated cap load amplifier more serious thought than simply picking a trendy device On the other hand, if you have no requirements for the lowest levels of distortion then such an amplifier could be a good choice. Such amplifiers are certainly easier to use,and relatively forgiving about loading issues. Some applications of this chapt lustrate the distortion point specifically, quoting performance in a driver circuit with/without the use of an internal cap load compensated amplifiers With increased gain bandwidths of greater than or equal to 100MHz available in today s ICs, layout, grounding and the control of parasitics become much, much more important. In fact, with the fastest available ICs such as the XFCB types, these issues simply cannot be ignored, they are critical and must be addressed for stable performance. All high frequency designs can profit from the use of low parasitic construction techniques, such as described in Chapter 9. In the circuit discussions which follow, similar methods should be used for best results, and in the very high frequency circuits(greater than 100MHz) it is mandatory. Some common pitfalls are covered before getting into specific circuit examples As with all wide bandwidth components, good PC board layout is critical to obtain the best dynamic performance with these high speed amplifiers. The ground plane in the area of the op amp and its associated components should cover as much of the component side of the board as possible(or first interior ground layer of a multilayer
1 1 Finally, because the circuit is based on a nonlinear principle, the internal network affects distortion and load drive ability, and these factors influence amplifier performance in video applications. Though the network’s presence does not by any means make devices like the AD847 or AD817 unusable for video, it does not permit the very lowest levels of distortion and differential gain and phase which are achievable with otherwise comparable amplifiers (for example, the AD818). While the individual techniques for countering cap loading outlined above have various specific tradeoffs as noted, all of the techniques have a serious common drawback of reducing speed (both bandwidth and slew rate). If these parameters cannot be sacrificed, then a matched transmission line system is the solution, and is discussed in more detail later in the chapter. As for choosing among the cap load compensation schemes, it would seem on the surface that amplifiers using the internal form offer the best possible solution to the problem- just pick the right amplifier and forget about it. And indeed, that would seem the “panacea” solution for all cap load situations - if you use the “right” amplifier you never need to think about cap loading again. Could there be more to it? Yes! The “gotcha” of internal cap load compensation is subtle, and lies in the fact that the dynamic adaptive nature of the compensation mechanism actually can produce higher levels of distortion, vis-à-vis an otherwise similar amplifier, without the CF-resistor network. Like the old saying about no free lunches, if you care about attaining top-notch levels of high frequency AC performance, you should give the issue of whether to use an internally compensated cap load amplifier more serious thought than simply picking a trendy device. On the other hand, if you have no requirements for the lowest levels of distortion, then such an amplifier could be a good choice. Such amplifiers are certainly easier to use, and relatively forgiving about loading issues. Some applications of this chapter illustrate the distortion point specifically, quoting performance in a driver circuit with/without the use of an internal cap load compensated amplifiers. With increased gain bandwidths of greater than or equal to 100MHz available in today’s ICs, layout, grounding and the control of parasitics become much, much more important. In fact, with the fastest available ICs such as the XFCB types, these issues simply cannot be ignored, they are critical and must be addressed for stable performance. All high frequency designs can profit from the use of low parasitic construction techniques, such as described in Chapter 9. In the circuit discussions which follow, similar methods should be used for best results, and in the very high frequency circuits (greater than 100MHz) it is mandatory. Some common pitfalls are covered before getting into specific circuit examples. As with all wide bandwidth components, good PC board layout is critical to obtain the best dynamic performance with these high speed amplifiers. The ground plane in the area of the op amp and its associated components should cover as much of the component side of the board as possible (or first interior ground layer of a multilayer board)
The ground plane should be removed in the area of the amplifier inputs and the feedback and gain set resistors to minimize stray capacitance at the input. Each power supply trace should be decoupled close to the package with a minimum of 0. luF ceramic (preferably surface mount), plus a 6.8uF or larger tantalum capacitor within 0.5", as a charge storage reservoir when delivering high peak currents (line drivers, for example). Optionally, larger value conventional electrolytic can be used in place of the tantalum types, if they have a low ESr. All lead lengths for input, output, and feedback resistor should be kept as short as possible. All gain setting resistors should be chosen for low values of parasitic capacitance and inductance, i.e., microwave resistors(buffed metal film rather than laser-trimmed spiral-wound) and/or carbon resistors Microstrip techniques should be used for all input and output lead lengths in excess of one inch(Reference 1) Sockets should be avoided if at all possible because of their parasitic capacitance and inductance If sockets are necessary, individual pin sockets such as amp p/n 6-330808-3 should be used. These contribute far less strap ' sockets capacitance and inductance than molded socket assemblies The effects of inadequate decoupling on harmonic distortion performance are dramatically illustrated in Figure 2.8. The left photo shows the spectral output of the AD9631 op amp driving a 100ohm load with proper decoupling(output signal is 20MHz, 2V p-p). Notice that the second harmonic distortion at 40MHz is approximately -70dBc If the decoupling is removed, the distortion is increased, as shown in the right photo of the same figure Figure 2.8(right-hand photo)also shows stray rf pickup in the wiring connecting the power supply to the op amp test fixture. Unlike lower frequency amplifiers, the power supply rejection ratio of many high frequency amplifiers is generally fairly poor at high frequencies. For example at 20MHz, the power supply rejection ratio of the AD9631 is less than 25dB. This is the primary reason for the degradation in performance with inadequate decoupling The change in output signal produces a corresponding signal-dependent load current change The corresponding change in power supply voltage due to inadequate decoupling produces a signal-dependent error in the output which manifests itself as an increase in distortion
1 2 The ground plane should be removed in the area of the amplifier inputs and the feedback and gain set resistors to minimize stray capacitance at the input. Each power supply trace should be decoupled close to the package with a minimum of 0.1µF ceramic (preferably surface mount), plus a 6.8µF or larger tantalum capacitor within 0.5", as a charge storage reservoir when delivering high peak currents (line drivers, for example). Optionally, larger value conventional electrolytic can be used in place of the tantalum types, if they have a low ESR. All lead lengths for input, output, and feedback resistor should be kept as short as possible. All gain setting resistors should be chosen for low values of parasitic capacitance and inductance, i.e., microwave resistors (buffed metal film rather than laser-trimmed spiral-wound) and/or carbon resistors. Microstrip techniques should be used for all input and output lead lengths in excess of one inch (Reference 1). Sockets should be avoided if at all possible because of their parasitic capacitance and inductance. If sockets are necessary, individual pin sockets such as AMP p/n 6-330808-3 should be used. These contribute far less stray capacitance and inductance than molded socket assemblies. The effects of inadequate decoupling on harmonic distortion performance are dramatically illustrated in Figure 2.8. The left photo shows the spectral output of the AD9631 op amp driving a 100ohm load with proper decoupling (output signal is 20MHz, 2V p-p). Notice that the second harmonic distortion at 40MHz is approximately –70dBc. If the decoupling is removed, the distortion is increased, as shown in the right photo of the same figure. Figure 2.8 (right-hand photo) also shows stray RF pickup in the wiring connecting the power supply to the op amp test fixture. Unlike lower frequency amplifiers, the power supply rejection ratio of many high frequency amplifiers is generally fairly poor at high frequencies. For example, at 20MHz, the power supply rejection ratio of the AD9631 is less than 25dB. This is the primary reason for the degradation in performance with inadequate decoupling. The change in output signal produces a corresponding signal-dependent load current change. The corresponding change in power supply voltage due to inadequate decoupling produces a signal-dependent error in the output which manifests itself as an increase in distortion
EFFECTS OF INADEQUATE DECOUPLING ON HARMONIC DISTORTION PERFORMANCE OF AD9631 OP AMP PROPER DECOUPLING NO DECOUPLING VERTICAL SCALES: 10dB/div, HORIZONTAL SCALES: 10MHz/div igure 2.8 Inadequate decoupling can also severely affect the pulse response of high speed amplifiers such as the AD9631. Figure 2.9 shows normal operation and the effects of removing all decoupling capacitors on the aD9631 in its evaluation board. Notice the severe ringing on the pulse response for the poorly decoupled condition, in the right photo. A Tektronix 644A, 500MHz digitizing oscilloscope was used to make the measurement(as well as the pulse responses in Figure 2.10, 2.14, 2.15, 2.16, and 2.17) EFFECT OF INADEQUATE DECOUPLING ON PULSE RESPONSE OF AD9631 OP AMP PROPER DECOUPLING NO DECOUPLING VERTICAL SCALE: 100mVldiv HORIZONTAL SCALE: 10ns/div Figure 2.9 The effects of stray parasitic capacitance on the inverting input of such high speed op amps as the ad8001 is shown in Figure 2.10. In this example, 10pF was connected to the inverting input, and the overshoot and ringing increased significantly. (The AD8001 was configured in the inverting mode with a gain of -1 and the feedback and feedforward resistors were equal to 649ohms) In some cases
1 3 EFFECTS OF INADEQUATE DECOUPLING ON HARMONIC DISTORTION PERFORMANCE OF AD9631 OP AMP Figure 2.8 Inadequate decoupling can also severely affect the pulse response of high speed amplifiers such as the AD9631. Figure 2.9 shows normal operation and the effects of removing all decoupling capacitors on the AD9631 in its evaluation board. Notice the severe ringing on the pulse response for the poorly decoupled condition, in the right photo. A Tektronix 644A, 500MHz digitizing oscilloscope was used to make the measurement (as well as the pulse responses in Figure 2.10, 2.14, 2.15, 2.16, and 2.17). EFFECT OF INADEQUATE DECOUPLING ON PULSE RESPONSE OF AD9631 OP AMP Figure 2.9 The effects of stray parasitic capacitance on the inverting input of such high speed op amps as the AD8001 is shown in Figure 2.10. In this example, 10pF was connected to the inverting input, and the overshoot and ringing increased significantly. (The AD8001 was configured in the inverting mode with a gain of –1, and the feedback and feedforward resistors were equal to 649ohms). In some cases
low-amplitude oscillation may occur at frequencies of several hundred megahertz when there is significant stray capacitance on the inverting input. Unfortunately, you may never actually observe it unless you have a scope or spectrum analyzer which has sufficient bandwidth. Unwanted oscillations at rF frequencies will probably be rectified and averaged by devices to which the oscillating signal is applied. This is referred to as RF rectification and will create small unexplained dc offsets which may even be a function of moving your hand over the PC board. It is absolutely essential when building circuits using high frequency components to have high bandwidth test equipment and use it to check for oscillation at frequencies well beyond the signals of interest EFFECT OF 10pF STRAY INVERTING INPUT CAPACITANCE ON PULSE RESPONSE OF AD8001 OP AMP NO CAPACITOR WITH CAPACITOR VERTICAL SCALE: 100mVldiv HORIZONTAL SCALE: 10ns/div Figure 2.10 Many of these problems occur in the prototype phase due to a disregard for high frequency layout and decoupling techniques. The solutions to them lie in rigorous attention to such details as above and those described in Chapter 9 CABLE DRIVING For a number of good reasons, wide bandwidth amplifier systems traditionally use transmission line interconnections, such as that shown in the basic diagram of Figure 2.11. This system uses a drive amplifier A, matched in terms of output impedance by the 75ohm source termination RT to the transmission line connecting stages A and B In this particular case the line is a 75ohm coax, but in general it is a wideband line matched at both ends, and can alternately be of twisted pair stripline construction. It is followed immediately by the differential receiver circui B, which terminates the line with a load RTERM, equal to its 75ohm impedance. The receiver stage recovers a noise-free 1V signal which is referenced to system ground B
1 4 low-amplitude oscillation may occur at frequencies of several hundred megahertz when there is significant stray capacitance on the inverting input. Unfortunately, you may never actually observe it unless you have a scope or spectrum analyzer which has sufficient bandwidth. Unwanted oscillations at RF frequencies will probably be rectified and averaged by devices to which the oscillating signal is applied. This is referred to as RF rectification and will create small unexplained dc offsets which may even be a function of moving your hand over the PC board. It is absolutely essential when building circuits using high frequency components to have high bandwidth test equipment and use it to check for oscillation at frequencies well beyond the signals of interest. EFFECT OF 10pF STRAY INVERTING INPUT CAPACITANCE ON PULSE RESPONSE OF AD8001 OP AMP Figure 2.10 Many of these problems occur in the prototype phase due to a disregard for high frequency layout and decoupling techniques. The solutions to them lie in rigorous attention to such details as above, and those described in Chapter 9. CABLE DRIVING For a number of good reasons, wide bandwidth amplifier systems traditionally use transmission line interconnections, such as that shown in the basic diagram of Figure 2.11. This system uses a drive amplifier A, matched in terms of output impedance by the 75ohm source termination RT to the transmission line connecting stages A and B. In this particular case the line is a 75ohm coax, but in general it is a wideband line matched at both ends, and can alternately be of twisted pair or stripline construction. It is followed immediately by the differential receiver circuit, B, which terminates the line with a load RTERM , equal to its 75ohm impedance. The receiver stage recovers a noise-free 1V signal which is referenced to system ground B