High Speed & RF Design and Layout: RFI/EMI Considerations (Design Conference 2013)

High Speed & RF Design and Layout:
RFI/EMI Considerations
Advanced Techniques of Higher Performance Signal Processing

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2

Todays Agenda
PCB Layout Overview
Schematic
Critical Component Location and Signal Routing
Power Supply Bypassing
Parasitics, Vias and Placement
Ground Plane
Layout Review
Summary
3

Overview
What is high speed?
 The frequency above which a PCB can significantly degrade circuit
performance. 50MHz and above can be considered high speed.
PCB layout is one of the final steps in the design process and often
not given the attention it deserves. High Speed circuit performance
is heavily dependant on board layout.
Today we will address
 Practical layout guidelines that:
 Improve the layout process
 Help ensure expected circuit performance
 Reduce design time
 Lower design cost
4

Schematics
A good layout starts with good Schematics!
Basic Function of Schematics
 Represent actual circuit connections
 Generate NetList for layout.
Can it be made more effective?
 Can it represent functionality more clearly?
 Others can understand circuit
 Can it show signal path?
 Aid layout
 Aid troubleshooting, debug
 Represent functionality
Can it be made more attractive?
 Can increase perceived value
More effective schematics decrease time to market
6

Schematics
7
 A perfectly good schematic.
 What are these?
 What is this?
 Too much unnecessary text
 Text orientation
 Alignment
 Lines cross. Is it necessary?
 Contradicting text
 No systematic approach
 Components are scattered
 No indication of functionality
 Difficult to read
 A good layout starts with a
good Schematic!

Schematics – Example. Is this better?
8
 Functionality evident at first site.
 Recognizable signal path.
 Components grouped by function.
 Auxiliary functions separated.
 No clutter
 No crossed lines
 No excessive text
 All additional hidden information
is carried to layout automatically.
 Occupies less paper space, yet
symbol sizes are larger.
 Color can add to overall
appearance.
 Separators can aid in recognizing
functional blocks.
 Is this Better?

Schematics – A circuit with more complexity
9

Component Placement
and Signal Routing

Component Placement and Signal Routing
Just as in real estate location is everything!
Input/output and power connections on a board are
typically defined
Component placement and Signal routing require
deliberate thought and planning
11

Use of Plane Layers
Plane LayerPrepregCopper Signal TraceSolder MaskSignal Current
Return Current
follows the path of
least inductance
12

Plane layer cutouts
Plane LayerPrepregCopper Signal TraceSolder MaskSignal Current
Return Current
Not so good.
Minimize Voids in
plane layers
13

Signal Routing
Placement not optimized – Minimize crossings
Connector
Digital ADC
RF
Power
Conditioning
Analog
Temp
Sensor
Connector
ADC
Driver
Placement optimized – Idealized
14

Return Path Routing
Clock
Circuitry
Analog
Circuitry
Resistor
Digital
Circuitry
Sensitive Analog
Circuitry Disrupted by
Digital Supply Noise
Not so good
ID
Voltage Drop
A better way
Sensitive Analog
Circuitry Safe from
Digital Supply Noise
 Use GND and PWR planes to
reduce return path R and L.
 Use separate AGND and DGND
planes to minimize digital
coupling into AGND plane.
 Compartmentalize functions
 Group components associated
with functions.
 Place functions to coincide with
signal path.
 Route functions first with input
and output along signal path.
 Route connections between
functions next.
Voltage Drop
More Voltage
Drop
ANALOG
CIRCUITS
DIGITAL
CIRCUITSVD VA
+ +
ID
IA
IA + ID
VIN
GND
REF
15

Example
 Two Inputs. Carbon copies to
ensure balance.
 Gain and feedback. Carbon
copies to ensure symmetry.
 Outputs. Carbon copies to
ensure symmetry.
 Level shifting tapped into signal
path. Carbon copies to ensure
symmetry.
 Auxiliary function.
 Critical Signal path as short as
possible.
 Critical signal paths are carbon
copies to maintain balance.
16

Packaging and Pinout choices
Packaging plays a large role in high-speed applications
Smaller packages
 Improved high frequency response
 Compact layout
 Lower package parasitics
Low Distortion Pinout (dedicated feedback)
 Compact layout
 Streamline signal flow
 Lower distortion
1
2
3
4
8
7
6
5
FB
INP
INN
VOUT
+
-
Low Distortion
1
2
3
4
8
7
6
5
VOUT
+
-
Standard
INP
INN
17

Bottom Silk
 Carries assembly and/or
component ID information.
 Informative only. Does not affect
performance. Not essential.
 Information contains text, lines,
shapes.
 Information can become useless if
not placed carefully.
 Min. line width = 5 mils (0.127 mm)
 Text height-to-line width ratio
should be > 12 to retain readability.
 Avoid placing text over vias, holes,
landing pads.
 Maintain a minimum distance
between landing pads.
 Quality varies between
manufacturers, ranging from sharp
to smudged edges.
Bottom Mask
 Protects copper from environmental
effects.
 Minimizes solder bridging. Can
prevent bridging if designed with
care.
 Affects PCB performance
somewhat.
 Not required. Essential to maintain
PCB longevity. Greatly increases
PCB assembly yield.
 Normally green. Other popular
colors are black, blue red, white.
Bottom Copper
 Can be a signal layer or a plane
layer.
 Normally a 1.4 mils (0.04 mm) thick
copper plate. Can be thicker.
 Etched to form signal traces and
landing pads.
 Minimum trace width is 4 mils (0.1
mm).
 Minimum space requirement
between two objects is 4 mils (0.1
mm).
 Forms a capacitor with other
nearby copper plates.
 Has inductance.
PrePreg
 Separates two copper layers.
 Is a woven glass epoxy base
material with glue.
 Has relative permittivity between
about 4.7 and 2.2.
 Weave density determines high
frequency performance.
 Comes in a range of thickness. A
1080 laminate is 3.2 mils (0.08 mm)
thick.
 Material determines maximum
soldering temperature.
Core
 Two copper foils already attached
to a woven glass material.
 Same as Prepreg but already
glued.
 Same characteristics as PrePreg.
 Can provide “built-in” or “inter-
planar” capacitance if one or both
copper foils are used as GND or
PWR planes.
Another PrePreg and Core
 Can act as a spacer to ensure
specified finished PCB thickness
 If made thick, it minimizes
interplanar capacitance.
One more PrePreg
 Can form controlled impedance
lines when combined with copper
traces above and GND plane
below.
 Impedance depends on trace width
above and thickness and
permittivity of PrePreg.
 Impedance accuracy depends on
the weave density.
Top Copper
 Usually a signal layer.
 Normally a 1.4 mils (0.04 mm) thick
copper plate. Can be thicker.
 Etched to form signal traces and
landing pads.
 Minimum trace width is 4 mils (0.1
mm).
 Minimum space requirement
between two objects is 4 mils (0.1
mm).
 Forms a capacitor with other
nearby copper plates.
 Traces have inductance.
Top Mask
 Same as bottom mask
 Protects copper from environmental
effects.
 Minimizes solder bridging. Can
prevent bridging if designed with
care.
 Affects PCB performance
somewhat.
 Not required. Essential to maintain
PCB longevity. Greatly increases
PCB assembly yield.
 Normally green. Other popular
colors are black, blue red, white.
Top Silk
 Same as bottom silk
 Carries assembly and/or
component ID information.
 Informative only. Does not affect
performance. Not essential.
 Information contains text, lines,
shapes.
 Information can become useless if
not placed carefully.
 Min. line width = 5 mils (0.127 mm)
 Text height-to-line width ratio
should be > 12 to retain readability.
 Avoid placing text over vias, holes,
landing pads.
 Maintain a minimum distance
between landing pads.
 Quality varies between
manufacturers, ranging from sharp
to smudged edges.
PCB
A typical 62 mils (1.6mm) 6 layer PCB stackup
19

PCB
PCB Material selection examples
Isola – FR4 types
 Common general purpose material.
 High temperature versions for leadfree solder exist
 High permittivity 4.7-4.2. Generates high parasitic capacitances
 Specified to 1 GHz
 Controlled impedance trace consistency OK but not great.
Rogers – PTFE types
 Good high frequency, high temperature material
 Low permittivity. 2.2 and up. Can reduce parasitic capacitances
 .Expensive
 Good impedance consistency.
 Specified to 10 GHz
Numerous other manufacturers. Some with performance
specifications similar to above.
20

PCB
Component Landing pad design
 Landing pad size
 Traditionally oversized by ≈ 30% from component pad.
 Can fit soldering iron on it
 Can allow visual inspection of solder joint
 Can accommodate component with larger placement errors.
 Increases parasitic capacitance – lowers effective useful frequency
 Increases chances for solder bridging
 Requires more board space
 Minimum oversizing: 0-5% from component pad.
 Retains mechanical strength
 Contact area between component and PCB
remains the same
 Reduces parasitic capacitance – retains
higher useful frequency
 Reduces required board space
 Pad shape
 Traditionally rectangular with sharp corners
 Rounded corners allow tighter pad-to-trace
spacing. Reduces board size.
21
This vs. This
ThisOr This

Signal Routing
Use GND and PWR Planes
 Connect pads to planes using “Via-in-pad” method to minimize parasitics
Place components of a functional block as close as possible
 0.5 mm component-to-component spacing is sufficient for manual placement
Minimize vias in signal traces. The less the better.
 Keep traces within a functional block on the same layer.
Use interplanar capacitance for bypassing
Keep plane layers as contiguous as possible
 Avoid unnecessary vias perforating plane layers.
 Avoid cutouts in plane layers
Keep traces as straight as possible
 Minimize bends and turns
23

Examples
 A perfectly good high frequency board
 BUT:
 Excessive number of unnecessary vias
 Plane layer compromised with a large
cutout
 Unnecessarily long signal traces
 Landing pads are too large
 No internal plane layers
 Same circuit with added provisions for
an auxiliary function
 A better alternative?
 More components yet smaller board size
 Vias are minimized
 Several internal plane layers
 “Properly” sized Landing pads
24

Examples - Performance vs PCB
 6 layer PCB
 No bypass caps
 No GND plane on top
 No plane cut outs
 No “stitching” vias

Performance vs. Component Location
26

AD8099 Harmonic Distortion Vs. Frequency
CSP and SOIC Packages
27
HARMONICDISTORTION(dBc)
0.1
–120
–100
–110
–80
–90
–60
–70
–50
1 10 50
04511-0-085
SOLID LINES – SECOND HARMONICS
DOTTED LINES – THIRD HARMONICS
G = +5
VOUT = 2V p-p
VS = ±5V
RL = 100Ω
FREQUENCY (MHz)
SOIC
CSP
Improvement
10dB at 1MHz 14dB
at 10MHz
00:09:52

High Speed PCB Boards
 Small signal BW:
 New: 1.41 GHz
 Existing: 976 MHz
 This is nearly a 50%
improvement
28

Large signal (10 dm) BW:
5V suply:
New: 1.07 GHz
Existing: 948 MHz
10V supply:
New: 1.25 GHz
Existing: 891 MHz
29

Pin4 - VN
Pin8 - VP
VOUT2
R5
1
VOUT1
R13
IN1+
R7
2
3
IN1-
R1
R3
RS1
R11
R15
R9
7
5
6
R14
IN2+
R8
IN2-
R6
R2
R16
RS2
R12
R4
R10
C3 C2C1
VPVN
GND
C4 C5
30

Crosstalk and Coupling
Capacitive Crosstalk or Coupling
 This results from traces running on top of each other, which forms a parasitic
capacitor
 Solutions run traces orthogonal, to minimize trace coupling and lower area
profile
Inductive Crosstalk
 Inductive crosstalk exists due to the magnetic field interaction between long
traces parallel traces
 There are two types of inductive crosstalk; forward and backward
 Backward is the noise observed nearest the driver on the victim trace
 Forward is the noise observed farthest from the driver on the driven line
Minimize crosstalk by
 Increasing trace separation (improving isolation)
 Using guard traces
 Using differential signals
31

Power Supply Bypassing
Bypassing is essential to high speed circuit performance
33

Capacitors right at power supply pins
34

 Capacitors provide low impedance AC return
 Provide local charge storage for fast rising/falling edges
35

 Provide local charge storage for fast
rising/falling edges
Keep trace lengths short
EQUIVALENT DECOUPLED POWER
LINE CIRCUIT RESONATES AT:
f =
1
2π LC√
IC
+VS
C1
L1
0.1µF
1nH
f = 16MHz
36

37

Close to load return
 Helps minimize transient
currents in the ground plane
38

Optimized Load and Bypass Capacitor
Placement and Ground Return
Tantalum
Tantalum
C
C
RL
AD80XX
RT
RG
RF
00
39

Board Capacitance
40
4 layer stack up Component/signal side
Ground plane
Power plane
Circuit side
d
K = relative dielectric constant
A = area in cm2
d = spacing between plates in cm
A
kA
11.3d
C=

Power Plane Capacitance
*Courtesy of Lee Ritchey
*
41

Power Supply Bypassing Capacitor Model
ESR (Equivalent Series Resistance)
 Rs
Capacitance
 XC = 1/2πfC
ESL (Equivalent Series
Inductance)
 XL=2πfL
Effective Impedance
At Series resonance
 XL=XC
 Z = R
2)(2 XCXLRsZ −+=
*
42

Capacitor Choices
0603 0612
*
43

Multiple Parallel Capacitors
1 x 330µF T520, 1 x 1.0µF 0603, 2 x 0.1µF 0603, and 6 x 0.01µF 0603
*
2 x (1 x 330µF T520, 1 x 1.0µF 0603, 2 x 0.1µF 0603, and 6 x 0.01µF 0603)
1µF
330µF
0.1µF
0.01µF
45

Parasitics
47
PCB parasitcs take the
form of hidden
capacitors, inductors
and resistors in the PCB
Parasitics degrade and
distort performance

Trace/Pad Capacitance and Inductance
48
113
kXY
C pF
Z
=
X = Copper Length (mm)
Y = Copper Width (mm)
Z = Distance to nearest Plane (mm)
Example1: SOIC landing pad
X = 0.51 mm Y = 1.27mm
Z = 0.16mm: C = 0.17 pF; L=0.08 nH
Z = 0.13mm: C = 0.21 pF; L=0.08 nH
2
0 2 0 5 2235
X Y Z
L X nH
Y Z X
. . ln .
 +   
= + +    +    
Example2: 3x3mm LFCSP landing pad
X = 0.3 mm Y = 0.6 mm
Z = 0.16mm: C = 0.05 pF; L=0.05 nH
Z = 0.13mm: C = 0.05 pF; L=0.05 nH
FR4 PCB with 1 oz Cu on top, 50Ω
controlled impedance for 10 mils and 0.2mm
wide traces
K= 4.7, Z=0.16mm and 0.13mm
Minimize Capacitance
1) Increase board thickness
2) Reduce trace/pad area
3) Remove ground plane
Minimize Inductance
1) Use Ground plane
2) Keep length short (halving the length
reduces inductance by 44%)
3) Doubling width only reduces
inductance by 11%

Trace/Pad Capacitance and Inductance
49
113
kXY
C pF
Z
=
X = Copper Length (mm)
Y = Copper Width (mm)
Z = Distance to nearest Plane (mm)
2
0 2 0 5 2235
X Y Z
L X nH
Y Z X
. . ln .
 +   
= + +    +    
Z
 An internal or Bottom Plane Layer
 Forms an Interplanar capacitance with a
Power Plane layer (not shown) under it.
 Spacer
 Large distance to eliminate interaction with
Controlled Impedance Layer above it.
 Controlled Impedance Plane Layer
 Traces on the top signal layer, the spacer
between and this plane forms transmission
lines with a characteristic impedance.
 Top (Signal) layer
 Has signal traces and component landing
pads.
 Traces are transmission lines with
characteristic impedance
 Top Solder mask
 Has effect on characteristic impedance

Via Parasitics
50






+





= 1
4
ln2
d
h
hL
L = inductance of the via, nH
H = length of via, cm
D = diameter of via, cm
H= 0.157 cm thick board,
D= 0.041 cm
 Via Inductance Via Capacitance






+





= 1
041.0
)157.0(4
ln)157.0(2L
L = 1.2nh
12
155.0
DD
TD
C r
−
=
ε
D2 = diameter of clearance hole in the
ground plane, cm
D1 = diameter of pad surrounding via, cm
T = thickness of printed circuit board, cm
= relative electric permeability of circuit
board material
C = parasitic via capacitance, pF
T = 0.157cm,
D1=0.071cm
D2 = 0.127
C = 0.51pf
rε
nH

Via Placement*
0603
and 0402
51

Capacitor Parasitic Model
C = Capacitor
RP = insulation resistance
RS = equivalent series resistance (ESR) inductance of the leads
and plates
RDA = dielectric absorption
CDA = dielectric absorption
52
L
r
RP
C
RDA CDA
RS

Resistor Parasitic Model
R = Resistor
CP = Parallel capacitance
L= equivalent series inductance (ESL)
53
CP
R
L

Low Frequency Op Amp Schematic
54

High Speed Op Amp Schematic
55

High Frequency Op Amp Schematic
56
Stray Capacitance

Stray Capacitance Simulation Schematic
57

Frequency Response with 1.5pF Stray
Capacitance
1.5dB peaking
58

Stray Inductance
Stray Inductance
59

Parasitic Inductance Simulation Schematic
24.5mm x .25mm” =29nH
60

Pulse Response With and Without Ground
Plane
0.6dB overshoot
61

Ground and Power Planes Provide
A common reference point
Shielding
Lowers noise
Reduces parasitics
Heat sink
Power distribution
High value capacitance
63

Ground Plane Recommendations
 There is no single grounding method which is guaranteed to work 100% of
the time!
 At least one layer on each PC board MUST be dedicated to ground plane!
 Provide as much ground plane as possible especially under traces that
operate at high frequency
 Use thickest metal as feasible (reduces resistance and provides improved
thermal transfer)
 Use multiple vias to connect same ground planes together
 Do initial layout with dedicated plane for analog and digital ground planes,
split only if required
 Follow recommendations on mixed signal device data sheet.
 Keep bypass capacitors and load returns close to reduce distortion
 Provide jumper options for joining analog and digital ground planes
together
64

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What we covered
 High speed PCB design requires deliberate thought and attention to detail!
 Load the schematic with as much information as possible
 Where you put components on the board is just as important as to where
you put entire circuits
 Take the lead when laying out your board, don’t leave anything to chance
 Use multiple capacitors for power supply bypassing
 Parasitics must be considered and dealt with
 Ground and Power planes play a key role in reducing noise and parasitics
 New packaging and pinouts allow for improved performance and more
compact layouts
 There are many options for signal distribution, make sure you choose the
right one for your application
 Check the layout very carefully
65

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Electromagnetic compatibility (EMC)
There are two aspects of EMC:
 It describes the ability of electronic systems to operate without interfering with
other systems
 It also describes the ability of such systems to operate as intended within a
specified electromagnetic environment
Primary specifications are IEC-60050 and IEC1000
Extensive reviews in tutorial MT-095 and Analog Dialog 30-4 on
Analog Devices website (www.analog.com)
Inability to meet these requirements will compromise your
equipment
Inability to meet these requirements will severely limit the ability to
sell the equipment to customers
66

High Speed & RF Design and Layout: RFI/EMI Considerations (Design Conference 2013)

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