AN1: Board Layout for High
Analog Application Note
AAN-1
Board Layout for High-Speed Amplifiers
Building a circuit with bandwidths in the megaHertz (MHz) range makes board layout very important, becoming more
so as the bandwidth increases. Note that signal bandwidth is not the only important factor; bandwidth of the active
components that exceed signal bandwidth can be just as important. Parasitic and nonlinear effects beyond signal
bandwidth can cause excess noise, overdriven stages, higher than expected distortion and even DC offsets due to asymmetric slew rates.
Parasitic Capacitance on Signal Traces
Parasitic capacitance can cause amplifiers to peak and in extreme cases oscillate. Fractions of a picofarad can control
details of gain flatness and bandwidth. Whole picofarads can cause more gross effects. In general terms, capacitance
is minimized by increasing distance between wires (reducing trace to trace capacitance) and minimizing trace length
(reducing trace to ground capacitance).
One of the most critical nodes to minimize parasitic capacitance on is the inverting input of an op amp. This is due to
the interaction of parasitic capacitance with the feedback and gain set resistors, Rf and Rg. Many amplifiers are used at
low gain, with relatively high Rf and Rg. Capacitance from the inverting input to ground is in parallel with Rg, and directly
impacts the op amp closed loop transfer function. In addition, this capacitance can add a pole to the loop transmission,
making the feedback unstable. It doesn’t take much; here is what happens when one extra picoFarad is added (Rf, Rg
= 510 Ohms):
REV 0.0.1
CLC2600 AC Response vs. Inverting Input Capacitance
2
INVCAP = 1pF
Normalized Gain (dB)
1
0
-1
INVCAP = 0
-2
-3
-4
Rf = Rg = 510 Ohms
RL = 100 Ohms
-5
-6
0.1
1
10
100
1000
Frequency (MHz)
Exar Corporation
48720 Kato Road, Fremont CA 94538, USA
AAN-1 Board Layout for High-Speed Amplifiers
Introduction
www.exar.com
Tel. +1 510 668-7000 - Fax. +1 510 668-7001
Analog Application Note
The non-inverting input of a high speed amplifier is often driven
from a lower impedance than the inverting input (whose driving impedance is Rf||Rg). The non-inverting input is typically
driven by another, relatively low impedance amplifier or by a
terminated transmission line (50 or 75 Ohms/2). This makes the
non-inverting input less sensitive to parasitic capacitance.
Another pin which can be sensitive to capacitance is the output.
This sensitivity is caused by the (internal) output impedance
of the amplifier interacting with a load capacitance, causing an
additional pole in the feedback loop transmission (See “AN-2:
Driving Capacitive Loads” application note). In general, amplifiers become more sensitive to parasitic effects as the bandwidth increases.
Given how sensitive high speed amplifiers are to capacitance,
you can imagine the huge effect most sockets have on AC
response. Sockets with small, short pin receptacles may sometimes be used, but are not recommended. The type with a locking arm and spring contacts are definitely not recommended for
high speed amplifiers.
Power Supply Bypassing
Correct bypassing can greatly improve the AC response of
an amplifier circuit. For each supply, there should be a high
frequency bypass capacitor very close to each amplifier (usually
0.1uF or 0.01uF). A second, larger value capacitor can serve a
section of the printed circuit (PC) board, spaced several centimeters apart. The small, high frequency capacitor has a large
impact on the response; total lead and trace length should be a
few millimeters or less, including both amplifier to capacitor and
capacitor to ground plane trace length.
©2007-2013 Exar Corporation As mentioned earlier, ground and power planes should be
removed from the vicinity of the inverting input pin. For very
high speed amplifiers (>50MHz), removing ground plane from
around the output pin is also a good idea.
There are other issues to consider in component grounding.
Ground planes are not ideal conductors; they are made up of
a mesh of parasitic resistance and inductance. A current in the
ground plane will flow in multiple paths, distributed relative to
the conductance of each path. As the current flows through the
ground plane impedance, it causes varying signal voltages to
appear throughout the plane. The signal levels are quite small,
but can have an impact on sensitive circuits.
Harmonic distortion (mostly the even harmonics) can be very
sensitive to ground current routing, especially at high gains.
Current leaving the output of an amplifier flows to the load
and returns to the power supply bypass capacitors according
to polarity. Since multiple supply pins (+VS and -VS) are not
usually located next to each other, the ground current return
path varies depending on polarity. Some of this current will
route past the input grounds, with one supply having a stronger
effect due to ground current routing. This causes one polarity of the input signal to be altered, but not the other polarity,
leading to additional even harmonic distortion (2nd, 4th, 6th,
etc.). Minimize this by rotating the bypass capacitors so that
their ground connections are towards the output rather than
the inputs. A very good approach for triple and quad bypassing
is to ground the positive and negative bypass capacitors at a
common point, forcing all polarities of return currents to flow by
the same paths.
REV 0.0.1
Trace to trace capacitance can cause undesired coupling between sections of a circuit. High impedance traces (at the input
pins) are much more affected by coupling than low impedance
traces (outputs). Focus on possible coupling to high impedance
traces, minimizing it by adding distance between traces where
possible. Another approach is to put a ground trace between
two signal traces—this causes the parasitic to be a capacitance
to ground rather than to another signal carrying trace.
Parasitic Issues in Grounding
Input and Output Connections
In addition to capacitance affecting the amplifier, long input and
output traces or wires can corrupt signals due to self inductance
and capacitance. This can cause the trace itself to have an AC
response that varies with frequency from one end of the trace
to the other. The best solution for this, where the length is
unavoidable, is to use a controlled impedance connection. This
consists of termination resistors at both input and output, with
a length of transmission line between the terminating resistors.
The transmission line can be made from microstrip if the signal
remains on the same PC board or coaxial cable if the signal
leaves the board. Typically, traces longer than a few centimeters
benefit from using a controlled impedance connection.
Breadboarding
A breadboard (no PC board, just components wired together
through the air above a ground plane) is especially important
in the early phases of constructing a circuit. It can often be
made much more quickly than a PC board. In addition, a carefully constructed breadboard will have the smallest parasitics
2/3
AAN-1 Board Layout for High-Speed Amplifiers
A small amount of parasitic capacitance cannot be avoided.
Cadeka parts are measured in an application board with a realistic layout, having 0.5 to 1 pF of parasitic capacitance on the
inverting input. This amount of parasitic is present for specifications and plots in Cadeka data sheets. A layout that adds
additional picoFarads is what hurts. Minimize extra parasitic
capacitance by putting components that are connected to the
inverting input as close to it as is reasonable. Don’t run any
traces connected to the inverting input pin over a distance—it
doesn’t take much to add a picoFarad or two. Remove ground
and power planes from the vicinity of the inverting input to
reduce ground capacitance (see Cadeka evaluation board layouts). Rev 0.0.1
Analog Application Note
AAN-1 Board Layout for High-Speed Amplifiers
possible, and will show the maximum performance achievable
for a given circuit. Wires running through the air generally have
much lower capacitance than traces on a board. Building critical
sections of the circuit by hand can speed both PC board design
(minimize iterations) and provide a performance goal (such as
gain flatness and distortion) for the PC board.
A good start for a breadboard is to glue the amplifier package
to a piece of copper clad board upside down (dead bug) using
cyanoacrylate glue. Next, add high frequency bypass capacitors and other pin to ground components from the amplifier
pins to the ground plane. Pin to pin components (such as Rf)
come next, followed by input and output connections. Be careful attaching cables to connectors—it is very easy to put a crack
REV 0.0.1
in surface mount components due to mechanical stress on the
connection. The cracks are nearly invisible.
Here is an example breadboard:
Leads are kept short in any parts of the circuit conducting
signal currents. A hand built breadboard with good performance
makes first pass PC board success much more likely.
Cadeka evaluation boards for common configurations are available to speed customer evaluation, but they don’t fully replace
breadboards and prototyping of your actual circuit.
For Further Assistance:
Exar Corporation Headquarters and Sales Offices
48720 Kato Road
Tel.: +1 (510) 668-7000
Fremont, CA 94538 - USA
Fax: +1 (510) 668-7001
www.exar.com
NOTICE
EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any
circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration
purposes and may vary depending upon a user’s specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies.
EXAR Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or
to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage
has been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances.
Reproduction, in part or whole, without the prior written consent of EXAR Corporation is prohibited.
©2007-2013 Exar Corporation 3/3
Rev 0.0.1
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