Application Report SNOA869A – April 2012 – Revised May 2013 AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers ..................................................................................................................................................... ABSTRACT High-bandwidth, high-performance amplifiers require care in board layout techniques in order to ensure optimum performance. This paper will address the key components of PCB design for high-performance amplifiers, including the new LMH6521 and LMH6522 DVGAs. 1 2 Contents General Description ......................................................................................................... 1 Conclusion ................................................................................................................... 5 List of Figures 1 1 Supply Bypass Capacitors - Double Vias Used for Lower Inductance............................................... 2 2 Illustration of Metal Removed From Beneath Amplifier Input and Output Pins ..................................... 2 3 Filter Design ................................................................................................................. 3 4 Amplifier With No Common Mode Load 5 Amplifier With Common Mode Load ...................................................................................... 4 6 Amplifier With High-Frequency Common Mode Load .................................................................. 4 7 CMRR of the LMH6522 DVGA ............................................................................................ 4 8 Filters With and Without Common Mode Termination ................................................................. 5 9 Filters With and Without Common Mode High-Frequency Bypassing ............................................... 5 ................................................................................. 3 General Description When designing a printed circuit board (PCB) there are a few key considerations. High-speed amplifiers require good power supply bypassing, low parasitic capacitance on the input and output pins, and good termination of the signal paths. Providing power to a high-speed amplifier is difficult because the amplifier will draw both DC as well as AC power. The DC power is usually the easiest. The LMH6522, for example, contains four amplifiers and requires only 370 mA of current at a nominal 5 V. The LMH6522 has a bandwidth of 1.4GHz. This means that the power supplies must be low impedance not only at DC, but also at 1.4GHz. This condition requires some special care. When selecting power supply bypass capacitors, the capacitors should offer a low impedance up to the amplifier 3dB bandwidth. That means that they should have a low series resistance and low parasitic inductance. The power supply bypass capacitors should be located within 4 mm of the amplifier supply pins, and any vias used to connect the capacitors to the power supply or ground planes should be designed for low inductance. Using multiple vias can help reduce via inductance. An example of supply bypass capacitor layout is shown in Figure 1. There are a few things to note in the figure. One is that there are two capacitors used in parallel. For a given capacitor, using devices in parallel reduces the equivalent series resistance (ESR) as well as the parasitic inductance. This can offer a significantly higher series resonant frequency (SRF) than could be achieved by one capacitor. Another common technique with parallel capacitors is to use different value capacitors in order to stagger the resonant frequency of the capacitors over a larger frequency range. It is common practice to use values a decade apart, such as 0.1 µF and 0.01 µF. All trademarks are the property of their respective owners. SNOA869A – April 2012 – Revised May 2013 Submit Documentation Feedback AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers Copyright © 2012–2013, Texas Instruments Incorporated 1 General Description www.ti.com While they are fairly new to the market, at least one company is now offering capacitors with reversed aspect ratios. The conductive contacts are placed on the long edge of the capacitor to provide lower parasitic resistance and inductance. The author has not yet tested this new capacitor configuration, but the datasheet specifications look very good. Figure 1. Supply Bypass Capacitors - Double Vias Used for Lower Inductance Figure 2. Illustration of Metal Removed From Beneath Amplifier Input and Output Pins While capacitance to ground is desirable for power supply pins, the output pins of high-speed amplifiers are very sensitive to capacitive loading. With high-impedance amplifiers output capacitance will cause a dramatic loss of bandwidth. With low-impedance feedback amplifiers, capacitive loading will cause a loss of phase margin and can cause gain peaking or instability. One way to reduce capacitive loading on the amplifier output pins is to remove portions of any metal layers beneath the pins. This is shown in Figure 2 . 2 AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers Copyright © 2012–2013, Texas Instruments Incorporated SNOA869A – April 2012 – Revised May 2013 Submit Documentation Feedback General Description www.ti.com Figure 3. Filter Design One very important, but often overlooked, consideration for board layout is proper loading and termination of the signal path. This is especially important when only a small amount of the amplifier bandwidth is being used. For instance, the example shown in Figure 3 is designed for a 100 MHz-wide filter centered at 250 MHz. The differential filters provide fifth order selectivity and are designed for 100 Ω differential impedance. While the filters are impedance matched at 250 MHz, the LMH6521 amplifier has a bandwidth of 1.2 GHz. The filters will present reactive load conditions outside of the filter pass-band. At higher frequencies the filter reactance, as well as any transmission line impedance transformation, should be considered. For instance, if a filter is designed to present a short circuit at very high frequencies, a one quarter wavelength of transmission line can transform this into an open circuit at the amplifier input. At 1GHz approximately 4cm of trace on an FR4 substrate equals one quarter wavelength. With low-impedance output amplifiers like the LMH6517/21/22 the output pins normally require resistors to match the impedance of following stages. Just as with power supply bypass capacitors, it is important to keep the termination resistors close to the DVGA output pins. Common Mode Floating 1 µH RT + LMH6521 RT . 1 µH Figure 4. Amplifier With No Common Mode Load SNOA869A – April 2012 – Revised May 2013 Submit Documentation Feedback AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers Copyright © 2012–2013, Texas Instruments Incorporated 3 General Description www.ti.com Common Mode Terminated at all Frequencies 1 PH RCM + LMH6521 RT . RT 1 PH RCM Figure 5. Amplifier With Common Mode Load Common Mode Shorted to Ground at High Frequencies 1 PH RT + LMH6521 RT . 1 PH Figure 6. Amplifier With High-Frequency Common Mode Load Differential signal paths are chosen due to their immunity to many forms of noise. This is because most distant noise sources will couple into a differential signal path as common-mode noise rather than differential noise. In theory, common-mode noise on a differential signal path is of little concern since many elements in the signal path will “reject” the common-mode noise. This is correct in many cases. However, care must still be taken to keep the signal free from excessive noise. One reason is that at very high frequencies many active elements do not have very good common-mode rejection (CMRR). Baluns are one circuit element that have poor CMRR at very high frequencies. Many amplifiers also have trouble suppressing common-mode noise at very high frequencies. Many high-speed analog-to-digital converters (ADC) do not even specify CMRR at all. -10 Maximum Gain 16dB Attenuation -20 CMRR (dBc) -30 -40 -50 -60 -70 -80 1 10 100 FREQUENCY (MHz) 1000 Figure 7. CMRR of the LMH6522 DVGA 4 AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers Copyright © 2012–2013, Texas Instruments Incorporated SNOA869A – April 2012 – Revised May 2013 Submit Documentation Feedback Conclusion www.ti.com As shown in Figure 7 the LMH6522 DVGA has excellent CMRR at modest frequencies, but as frequencies increase, the CMRR gets worse. Fortunately, the board layout can help here as well. As shown in the design of the filter. Creative use of the filter elements and the termination resistors can help reduce common mode noise with very small changes in component configurations. Using capacitors to ground or providing resistive termination to ground directs the common-mode energy to ground instead of conducting it to the next circuit element. Common Mode Floating Common Mode Terminated to Ground at all Frequencies BETTER BAD Figure 8. Filters With and Without Common Mode Termination Common Mode Floating BAD Common Mode Terminated to Ground at High Frequencies GOOD Figure 9. Filters With and Without Common Mode High-Frequency Bypassing 2 Conclusion High-performance circuit elements require support from a carefully planned printed circuit-board layout. By following the following key guidelines, high-speed, high-performance circuits will perform to their highest potential. • Place bypass capacitors close to supply pins • Locate termination resistors close to output pins • Remove metal from beneath input and output pins • Terminate transmission lines • Keep filters close to the amplifier • Provide both differential and common mode termination SNOA869A – April 2012 – Revised May 2013 Submit Documentation Feedback AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers Copyright © 2012–2013, Texas Instruments Incorporated 5 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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