AN-2235 Datasheet, PDF (1/6 Pages) Texas Instruments – AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed IF/RF Feedback Amplifiers

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.
Contents
1 General Description ......................................................................................................... 1
2 Conclusion ................................................................................................................... 5
List of Figures
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 ................................................................................. 3
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
1 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.
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SNOA869A – April 2012 – Revised May 2013
AN-2235 Circuit Board Design for LMH6517/21/22 and Other High-Speed
1
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