MT-031
TUTORIAL
Grounding Data Converters and Solving the Mystery of
"AGND" and "DGND"
by Walt Kester, James Bryant, and Mike Byrne
INTRODUCTION
Today's signal processing systems generally require mixed-signal devices such as analog-todigital converters (ADCs) and digital-to-analog converters (DACs) as well as fast digital signal
processors (DSPs). Requirements for processing analog signals having wide dynamic ranges
increases the importance of high performance ADCs and DACs. Maintaining wide dynamic
range with low noise in hostile digital environments is dependent upon using good high-speed
circuit design techniques including proper signal routing, decoupling, and grounding.
In the past, "high precision, low-speed" circuits have generally been viewed differently than socalled "high-speed" circuits. With respect to ADCs and DACs, the sampling (or update)
frequency has generally been used as the distinguishing speed criteria. However, the following
two examples show that in practice, most of today's signal processing ICs are really "highspeed," and must therefore be treated as such in order to maintain high performance. This is
certainly true of DSPs, and also true of ADCs and DACs.
All sampling ADCs (ADCs with an internal sample-and-hold circuit) suitable for signal
processing applications operate with relatively high speed clocks with fast rise and fall times
(generally a few nanoseconds) and must be treated as high speed devices, even though
throughput rates may appear low. For example, a medium-speed 12-bit successive approximation
(SAR) ADC may operate on a 10-MHz internal clock, while the sampling rate is only 500 kSPS.
Sigma-delta (Σ-Δ) ADCs also require high speed clocks because of their high oversampling
ratios. Even high resolution, so-called "low frequency" Σ-Δ industrial measurement ADCs
(having throughputs of 10 Hz to 7.5 kHz) operate on 5-MHz or higher clocks and offer
resolution to 24-bits (for example, the Analog Devices AD77xx-series).
To further complicate the issue, mixed-signal ICs have both analog and digital ports, and because
of this, much confusion has resulted with respect to proper grounding techniques. In addition,
some mixed-signal ICs have relatively low digital currents, while others have high digital
currents. In many cases, these two types must be treated differently with respect to optimum
grounding.
Digital and analog design engineers tend to view mixed-signal devices from different
perspectives, and the purpose of this tutorial is to develop a general grounding philosophy that
will work for most mixed signal devices, without having to know the specific details of their
internal circuits.
Rev.A, 10/08, WK
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MT-031
GROUND AND POWER PLANES
The importance of maintaining a low impedance large area ground plane is critical to all analog
and digital circuits today. The ground plane not only acts as a low impedance return path for
decoupling high frequency currents (caused by fast digital logic) but also minimizes EMI/RFI
emissions. Because of the shielding action of the ground plane, the circuit's susceptibility to
external EMI/RFI is also reduced.
Ground planes also allow the transmission of high speed digital or analog signals using
transmission line techniques (microstrip or stripline) where controlled impedances are required.
The use of "buss wire" is totally unacceptable as a "ground" because of its impedance at the
equivalent frequency of most logic transitions. For instance, #22 gauge wire has about 20
nH/inch inductance. A transient current having a slew rate of 10 mA/ns created by a logic signal
would develop an unwanted voltage drop of 200 mV at this frequency flowing through 1 inch of
this wire:
Δv = L
10 mA
Δi
= 20 nH ×
= 200 mV.
Δt
ns
Eq. 1
For a signal having a 2-V peak-to-peak range, this translates into an error of about
10% (approximately 3.5-bit accuracy). Even in all-digital circuits, this error would result in
considerable degradation of logic noise margins.
Figure 1 shows the classic illustration of a situation where the digital return current modulates
the analog return current (top figure). The ground return wire inductance and resistance is shared
between the analog and digital circuits, and this is what causes the interaction and resulting error.
A possible solution is to make the digital return current path flow directly to the GND REF as
shown in the bottom figure. This is the fundamental concept of a "star," or single-point ground
system. Implementing the true single-point ground in a system which contains multiple high
frequency return paths is difficult because the physical length of the individual return current
wires will introduce parasitic resistance and inductance which can make obtaining a low
impedance high frequency ground difficult. In practice, the current returns must consist of large
area ground planes for low impedance to high frequency currents. Without a low-impedance
ground plane, it is therefore almost impossible to avoid these shared impedances, especially at
high frequencies.
Page 2 of 17
MT-031
ID
IA
+
VD
INCORRECT
+
VA
ANALOG
CIRCUITS
VIN
GND
REF
IA + ID
DIGITAL
CIRCUITS
ID
ID
IA
+
VD
CORRECT
+
VA
VIN
ANALOG
CIRCUITS
GND
REF
DIGITAL
CIRCUITS
IA
ID
Figure 1: Digital Currents Flowing in Analog Return Path Create Error Voltages
All integrated circuit ground pins should be soldered directly to the low-impedance ground plane
to minimize series inductance and resistance. The use of traditional IC sockets is not
recommended with high-speed devices. The extra inductance and capacitance of even "low
profile" sockets may corrupt the device performance by introducing unwanted shared paths. If
sockets must be used with DIP packages, as in prototyping, individual "pin sockets" or "cage
jacks" may be acceptable. Both capped and uncapped versions of these pin sockets are available
(AMP part numbers 5-330808-3, and 5-330808-6). They have spring-loaded gold contacts which
make good electrical and mechanical connection to the IC pins. Multiple insertions, however,
may degrade their performance.
LOW AND HIGH FREQUENCY DECOUPLING
Each power supply should be decoupled to the low-impedance ground plane with a high quality
electrolytic capacitor at the point it enters the PC board. This minimizes low frequency noise on
the supply runs. At each individual analog stage, further local, high-frequency-only filtering is
required at the individual IC package power pins.
Figure 2 shows this technique, in both correct (left) as well as incorrect example
implementations (right). In the left example, a typical 0.1-μF chip ceramic capacitor goes
directly to the opposite PCB side ground plane, by virtue of the via, and on to the IC's GND pin
by a second via. In contrast, the less desirable setup at the right adds additional PCB trace
inductance in the ground path of the decoupling cap, reducing effectiveness.
Page 3 of 17
MT-031
CORRECT
INCORRECT
OPTIONAL
FERRITE BEADS
POWER
SUPPLY
TRACE
DECOUPLING
CAPACITOR
DECOUPLING
CAPACITOR
POWER
SUPPLY
TRACE
V+
V+
PCB
TRACE
IC
VIAS TO
GROUND
PLANE
GND
IC
VIA TO
GROUND
PLANE
GND
Figure 2: Localized High Frequency Supply Filter(s) Provides Optimum Filtering
and Decoupling Via Short Low-Inductance Path (Ground Plane)
All high frequency (i.e., ≥10 MHz) ICs should use a bypassing scheme similar to Figure 2 for
best performance. The ferrite beads aren't 100% necessary, but they will add extra high
frequency noise isolation and decoupling, which is often desirable. Possible caveats here would
be to verify that the beads never saturate, when the ICs are handling high currents.
Note that with some ferrites, even before full saturation occurs, some beads can be non-linear, so
if a power stage is required to operate with a low distortion output, this should also be checked.
DOUBLE-SIDED VS. MULTILAYER PRINTED CIRCUIT BOARDS
Each PCB in the system should have at least one complete layer dedicated to the ground plane.
Ideally, a double-sided board should have one side completely dedicated to ground and the other
side for interconnections. In practice, this is not possible, since some of the ground plane will
certainly have to be removed to allow for signal and power crossovers, vias, and through-holes.
Nevertheless, as much area as possible should be preserved, and at least 75% should remain.
After completing an initial layout, the ground layer should be checked carefully to make sure
there are no isolated ground "islands," because IC ground pins located in a ground "island" have
no current return path to the ground plane. Also, the ground plane should be checked for
"skinny" connections between adjacent large areas which may significantly reduce the
effectiveness of the ground plane. Needless to say, auto-routing board layout techniques will
generally lead to a layout disaster on a mixed-signal board, so manual intervention is highly
recommended.
Systems that are densely packed with surface mount ICs will have a large number of
interconnections; therefore multilayer boards are mandatory. This allows at least one complete
layer to be dedicated to ground. A simple 4-layer board would have internal ground and power
plane layers with the outer two layers used for interconnections between the surface mount
components. Placing the power and ground planes adjacent to each other provides additional
Page 4 of 17
MT-031
inter-plane capacitance which helps high frequency decoupling of the power supply. In most
systems, 4-layers are not enough, and additional layers are required for routing signals as well as
power.
MULTICARD MIXED-SIGNAL SYSTEMS
The best way of minimizing ground impedance in a multicard system is to use a "motherboard"
PCB as a backplane for interconnections between cards, thus providing a continuous ground
plane to the backplane. The PCB connector should have at least 30-40% of its pins devoted to
ground, and these pins should be connected to the ground plane on the backplane motherboard.
To complete the overall system grounding scheme there are two possibilities:
1. The backplane ground plane can be connected to chassis ground at numerous points,
thereby diffusing the various ground current return paths. This is commonly referred to as
a "multipoint" grounding system and is shown in Figure 3.
2. The ground plane can be connected to a single system "star ground" point (generally at
the power supply).
The first approach is most often used in all-digital systems, but can be used in mixed-signal
systems provided the ground currents due to digital circuits are sufficiently low and diffused over
a large area. The low ground impedance is maintained all the way through the PC boards, the
backplane, and ultimately the chassis. However, it is critical that good electrical contact be made
where the grounds are connected to the sheet metal chassis. This requires self-tapping sheet
metal screws or "biting" washers. Special care must be taken where anodized aluminum is used
for the chassis material, since its surface acts as an insulator.
VA
PCB
VD
VA
GROUND PLANE
PCB
VD
GROUND PLANE
BACKPLANE
GROUND PLANE
CHASSIS
GROUND
POWER
SUPPLIES
Figure 3: Multipoint Ground Concept
Page 5 of 17
VA
VD
MT-031
The second approach ("star ground") is often used in high speed mixed-signal systems having
separate analog and digital ground systems and warrants further discussion.
SEPARATING ANALOG AND DIGITAL GROUND PLANES
In mixed-signal systems with large amounts of digital circuitry, it is highly desirable to
physically separate sensitive analog components from noisy digital components. It may also be
beneficial to use separate ground planes for the analog and the digital circuitry. These planes
should not overlap in order to minimize capacitive coupling between the two. The separate
analog and digital ground planes are continued on the backplane using either motherboard
ground planes or "ground screens" which are made up of a series of wired interconnections
between the connector ground pins. The arrangement shown in Figure 4 illustrates that the two
planes are kept separate all the way back to a common system "star" ground, generally located at
the power supplies. The connections between the ground planes, the power supplies, and the
"star" should be made up of multiple bus bars or wide copper braids for minimum resistance and
inductance. The back-to-back Schottky diodes on each PCB are inserted to prevent accidental dc
voltage from developing between the two ground systems when cards are plugged and
unplugged. This voltage should be kept less than 300 mV to prevent damage to ICs which have
connections to both the analog and digital ground planes. Schottky diodes are preferable because
of their low capacitance and low forward voltage drop. The low capacitance prevents ac coupling
between the analog and digital ground planes. Schottky diodes begin to conduct at about 300
mV, and several parallel diodes in parallel may be required if high currents are expected. In some
cases, ferrite beads can be used instead of Schottky diodes, however they introduce dc ground
loops which can be troublesome in precision systems.
VA
ANALOG
GROUND
PLANE
PCB
VD
VA
DIGITAL
GROUND
PLANE
ANALOG
GROUND
PLANE
D
A
PCB
VD
DIGITAL
GROUND
PLANE
D
A
DIGITAL GROUND PLANE
BACKPLANE
ANALOG GROUND PLANE
POWER
SUPPLIES
SYSTEM
STAR
GROUND
VA
VD
Figure 4: Separating Analog and Digital Ground Planes
Page 6 of 17
MT-031
It is mandatory that the impedance of the ground planes be kept as low as possible, all the way
back to the system star ground. DC or ac voltages of more than 300 mV between the two ground
planes can not only damage ICs but cause false triggering of logic gates and possible latchup.
GROUNDING AND DECOUPLING MIXED-SIGNAL ICs WITH LOW
DIGITALCURRENTS
Sensitive analog components such as amplifiers and voltage references are always referenced
and decoupled to the analog ground plane. The ADCs and DACs (and other mixed-signal ICs)
with low digital currents should generally be treated as analog components and also grounded
and decoupled to the analog ground plane. At first glance, this may seem somewhat
contradictory, since a converter has an analog and digital interface and usually has pins
designated as analog ground (AGND) and digital ground (DGND). The diagram shown in
Figure 5 will help to explain this seeming dilemma.
VA
VD
FERRITE BEAD
A
D
VD
VA
LP
LP
CSTRAY
RP
SEE
TEXT
RP
R
AIN/
OUT
ANALOG
CIRCUITS
DIGITAL
CIRCUITS DATA
A
RP
LP
B
CSTRAY
IA
ID
AGND
A
SHORT
CONNECTIONS
BUFFER
GATE OR
REGISTER
DATA
BUS
CIN ≈ 10pF
RP
LP
DGND
A
A = ANALOG GROUND PLANE
VNOISE
D
D = DIGITAL GROUND PLANE
Figure 5: Proper Grounding of Mixed-signal ICs With Low Internal Digital Currents
Inside an IC that has both analog and digital circuits, such as an ADC or a DAC, the grounds are
usually kept separate to avoid coupling digital signals into the analog circuits. Figure 5 shows a
simple model of a converter. There is nothing the IC designer can do about the wirebond
inductance and resistance associated with connecting the bond pads on the chip to the package
pins except to realize it's there. The rapidly changing digital currents produce a voltage at point B
which will inevitably couple into point A of the analog circuits through the stray capacitance,
CSTRAY. In addition, there is approximately 0.2-pF unavoidable stray capacitance between
every pin of the IC package! It's the IC designer's job to make the chip work in spite of this.
However, in order to prevent further coupling, the AGND and DGND pins should be joined
Page 7 of 17
MT-031
together externally to the analog ground plane with minimum lead lengths. Any extra impedance
in the DGND connection will cause more digital noise to be developed at point B; it will, in turn,
couple more digital noise into the analog circuit through the stray capacitance. Note that
connecting DGND to the digital ground plane applies VNOISE across the AGND and DGND
pins and invites disaster!
The name "DGND" on an IC tells us that this pin connects to the digital ground of the IC. This
does not imply that this pin must be connected to the digital ground of the system.
It is true that this arrangement may inject a small amount of digital noise onto the analog ground
plane. These currents should be quite small, and can be minimized by ensuring that the converter
output does not drive a large fanout (they normally can't, by design). Minimizing the fanout on
the converter's digital port will also keep the converter logic transitions relatively free from
ringing and minimize digital switching currents, and thereby reducing any potential coupling into
the analog port of the converter. The logic supply pin (VD) can be further isolated from the
analog supply by the insertion of a small lossy ferrite bead as shown in Figure 5. The internal
transient digital currents of the converter will flow in the small loop from VD through the
decoupling capacitor and to DGND (this path is shown with a heavy line on the diagram). The
transient digital currents will therefore not appear on the external analog ground plane, but are
confined to the loop. The VD pin decoupling capacitor should be mounted as close to the
converter as possible to minimize parasitic inductance. These decoupling capacitors should be
low inductance ceramic types, typically between 0.01 µF and 0.1 µF.
TREAT THE ADC DIGITAL OUTPUTS WITH CARE
It is always a good idea (as shown in Figure 5) to place a buffer register adjacent to the converter
to isolate the converter's digital lines from noise on the data bus. The register also serves to
minimize loading on the digital outputs of the converter and acts as a Faraday shield between the
digital outputs and the data bus. Even though many converters have three-state outputs/inputs,
this isolation register still represents good design practice. In some cases it may be desirable to
add an additional buffer register on the analog ground plane next to the converter output to
provide greater isolation.
The series resistors (labeled "R" in Figure 5) between the ADC output and the buffer register
input help to minimize the digital transient currents which may affect converter performance.
The resistors isolate the digital output drivers from the capacitance of the buffer register inputs.
In addition, the RC network formed by the series resistor and the buffer register input
capacitance acts as a lowpass filter to slow down the fast edges.
A typical CMOS gate combined with PCB trace and a through-hole will create a load of
approximately 10 pF. A logic output slew rate of 1 V/ns will produce 10 mA of dynamic current
if there is no isolation resistor:
ΔI = C
1V
Δv
= 10 pF ×
= 10 mA .
Δt
ns
Page 8 of 17
Eq. 2
MT-031
A 500-Ω series resistors will minimize this output current and result in a rise and fall time of
approximately 11 ns when driving the 10-pF input capacitance of the register:
t r = 2.2 × τ = 2.2 × R ⋅ C = 2.2 × 500 Ω × 10 pF = 11 ns.
Eq. 3
TTL registers should be avoided, since they can appreciably add to the dynamic switching
currents because of their higher input capacitance.
The buffer register and other digital circuits should be grounded and decoupled to the digital
ground plane of the PC board. Notice that any noise between the analog and digital ground plane
reduces the noise margin at the converter digital interface. Since digital noise immunity is of the
orders of hundreds or thousands of millivolts, this is unlikely to matter. The analog ground plane
will generally not be very noisy, but if the noise on the digital ground plane (relative to the
analog ground plane) exceeds a few hundred millivolts, then steps should be taken to reduce the
digital ground plane impedance, thereby maintaining the digital noise margins at an acceptable
level. Under no circumstances should the voltage between the two ground planes exceed 300
mV, or the ICs may be damaged.
Separate power supplies for analog and digital circuits are also highly desirable, even if the
voltages are the same. The analog supply should be used to power the converter. If the converter
has a pin designated as a digital supply pin (VD), it should either be powered from a separate
analog supply, or filtered as shown in the diagram. All converter power pins should be decoupled
to the analog ground plane, and all logic circuit power pins should be decoupled to the digital
ground plane as shown in Figure 6.
VA FERRITE
VA
VD
BEAD
SEE
TEXT
A
A
VA
VD
D
R
BUFFER
GATE
OR
REGISTER
ADC
OR
DAC
AMP
R
A
VA
AGND
A
DGND
A
VOLTAGE
REFERENCE
A
A
SAMPLING
CLOCK
GENERATOR
A
TO OTHER
DIGITAL
CIRCUITS
D
VA
A
A
ANALOG
GROUND PLANE
D
DIGITAL
GROUND PLANE
Figure 6: Grounding and Decoupling Points
Page 9 of 17
MT-031
In some cases it may not be possible to connect VD to the analog supply. Some of the newer,
high speed ICs may have their analog circuits powered by +5 V, but the digital interface powered
by +3 V to interface to 3 V logic. In this case, the +3 V pin of the IC should be decoupled
directly to the analog ground plane. It is also advisable to connect a ferrite bead in series with the
power trace that connects the pin to the +3 V digital logic supply.
The sampling clock generation circuitry should be treated like analog circuitry and also be
grounded and heavily-decoupled to the analog ground plane. Phase noise on the sampling clock
produces degradation in system SNR as will be discussed shortly.
SAMPLING CLOCK CONSIDERATIONS
In a high performance sampled data system a low phase-noise oscillator should be used to
generate the ADC (or DAC) sampling clock because sampling clock jitter modulates the analog
input/output signal and raises the noise and distortion floor. The sampling clock generator should
be isolated from noisy digital circuits and grounded and decoupled to the analog ground plane, as
is true for the op amp and the ADC.
The effect of sampling clock jitter on ADC signal-to-noise ratio (SNR) is given approximately
by the equation:
⎡ 1 ⎤
SNR = 20 log10 ⎢
⎥,
⎢⎣ 2πft j ⎥⎦
Eq. 4
where SNR is the SNR of a perfect ADC of infinite resolution where the only source of noise is
that caused by the rms sampling clock jitter, tj. Note that f in the above equation is the analog
input frequency. Just working through a simple example, if tj = 50 ps rms, f = 100 kHz, then
SNR = 90 dB, equivalent to about 15-bit dynamic range. This effect of clock jitter on SNR is
discussed in much more detail in Tutorial MT-007.
It should be noted that tj in the above example is the root-sum-square (rss) value of the external
clock jitter and the internal ADC clock jitter (called aperture jitter). However, in most high
performance ADCs, the internal aperture jitter is negligible compared to the jitter on the
sampling clock.
Ideally, the sampling clock oscillator should be referenced to the analog ground plane in a splitground system. However, this is not always possible because of system constraints. In many
cases, the sampling clock must be derived from a higher frequency multi-purpose system clock
which is generated on the digital ground plane. It must then pass from its origin on the digital
ground plane to the ADC on the analog ground plane. Ground noise between the two planes adds
directly to the clock signal and will produce excess jitter. The jitter can cause degradation in the
signal-to-noise ratio and also produce unwanted harmonics.
Page 10 of 17
MT-031
This can be remedied somewhat by transmitting the sampling clock signal as a differential signal
using either a small RF transformer as shown in Figure 7 or a high speed differential driver and
receiver IC. Many high-speed ADCs have differential sampling clock inputs to facilitate this
approach. If an active differential driver and receiver are used, they should be ECL, low-level
ECL, or LVDS to minimize phase jitter. In a single +5 V supply system, ECL logic can be
connected between ground and +5 V (PECL), and the outputs ac coupled into the ADC sampling
clock input. In either case, the original master system clock must be generated from a low phase
noise oscillator, and not the clock output of a DSP, microprocessor, or microcontroller.
In order to facilitate system clock management, a family clock generation and distribution
products is available from Analog Devices as well as a complete selection of phase-locked loops
(PLLs).
DIGITAL GROUND PLANE
VD
VD
LOW PHASE
NOISE
MASTER CLOCK
D
ANALOG GROUND PLANE
SAMPLING
CLOCK
SYSTEM CLOCK
GENERATORS
VD
METHOD 1
D
D
A
VD
DSP OR MICROPROCESSOR
VA
+
SAMPLING
CLOCK
_
METHOD 2
D
SNR = 20 log10
D
1
2π f tj
A
tj = Sampling Clock Jitter
f = Analog Input Frequency
Figure 7: Sampling Clock Distribution From Digital to Analog Ground Planes
THE ORIGINS OF THE CONFUSION ABOUT MIXED-SIGNAL GROUNDING:
APPLYING SINGLE-CARD GROUNDING CONCEPTS TO MULTICARD SYSTEMS
Most ADC, DAC, and other mixed-signal device data sheets discuss grounding relative to a
single PCB, usually the manufacturer's own evaluation board. This has been a source of
confusion when trying to apply these principles to multicard or multi-ADC/DAC systems. The
recommendation is usually to split the PCB ground plane into an analog plane and a digital
plane. It is then further recommended that the AGND and DGND pins of a converter be tied
together and that the analog ground plane and digital ground planes be connected at that same
point as shown in Figure 8. This essentially creates the system "star" ground at the mixed-signal
device.
Page 11 of 17
MT-031
All noisy digital currents flow through the digital power supply to the digital ground plane and
back to the digital supply; they are isolated from the sensitive analog portion of the board. The
system star ground occurs where the analog and digital ground planes are joined together at the
mixed signal device. While this approach will generally work in a simple system with a single
PCB and single ADC/DAC, it is not usually optimum for multicard mixed-signal systems. In
systems having several ADCs or DACs on different PCBs (or on the same PCB, for that matter),
the analog and digital ground planes become connected at several points, creating the possibility
of ground loops and making a single-point "star" ground system impossible. For these reasons,
this grounding approach is not recommended for multicard systems, and the approach previously
discussed should be used for mixed signal ICs with low digital currents.
VD
VA
VA
MIXED
SIGNAL
DEVICE
ANALOG
CIRCUITS
AGND
SYSTEM
STAR
GROUND
A
VD
DIGITAL
CIRCUITS
DGND
A
D
ANALOG
GROUND PLANE
D
DIGITAL
GROUND PLANE
D
A
ANALOG
SUPPLY
DIGITAL
SUPPLY
Figure 8: Grounding Mixed Signal ICs : Single PC Board (Typical Evaluation/Test
Board)
SUMMARY: GROUNDING MIXED SIGNAL DEVICES WITH LOW DIGITAL
CURRENTS IN A MULTICARD SYSTEM
Figure 9 summarizes the approach previously described for grounding a mixed signal device
which has low digital currents. The analog ground plane is not corrupted because the small
digital transient currents flow in the small loop between VD, the decoupling capacitor, and
DGND (shown as a heavy line). The mixed signal device is for all intents and purposes treated as
an analog component. The noise VN between the ground planes reduces the noise margin at the
digital interface but is generally not harmful if kept less than 300 mV by using a low impedance
digital ground plane all the way back to the system star ground.
Page 12 of 17
MT-031
However, mixed signal devices such as sigma-delta ADCs, codecs, and DSPs with on-chip
analog functions are becoming more and more digitally intensive. Along with the additional
digital circuitry come larger digital currents and noise. For example, a sigma-delta ADC or DAC
contains a complex digital filter which adds considerably to the digital current in the device. The
method previously discussed depends on the decoupling capacitor between VD and DGND to
keep the digital transient currents isolated in a small loop. However, if the digital currents are
significant enough and have components at dc or low frequencies, the decoupling capacitor may
have to be so large that it is impractical. Any digital current which flows outside the loop
between VD and DGND must flow through the analog ground plane. This may degrade
performance, especially in high resolution systems.
VN
VA
ANALOG
CIRCUITS
MIXED
SIGNAL
DEVICE
AGND
A
VD
FILTER
VA
VN = NOISE BETWEEN
GROUND PLANES
VD
R
BUS
BUFFER
LATCH
DGND
A
A
D
ANALOG
GROUND PLANE
A
DIGITAL
CIRCUITS
D
DIGITAL
GROUND PLANE
A
D
TO SYSTEM
ANALOG SUPPLY
D
TO SYSTEM
DIGITAL SUPPLY
TO SYSTEM STAR GROUND
Figure 9: Grounding Mixed Signal ICs with Low Internal Digital Currents:
Multiple PC Boards
It is difficult to predict what level of digital current flowing into the analog ground plane will
become unacceptable in a system. All we can do at this point is to suggest an alternative
grounding method which may yield better performance.
SUMMARY: GROUNDING MIXED SIGNAL DEVICES WITH HIGH DIGITAL
CURRENTS IN A MULTICARD SYSTEM (USE THIS METHOD WITH CAUTION!)
An alternative grounding method for a mixed signal device with high levels of digital currents is
shown in Figure 10. The AGND of the mixed signal device is connected to the analog ground
plane, and the DGND of the device is connected to the digital ground plane. The digital currents
are isolated from the analog ground plane, but the noise between the two ground planes is
applied directly between the AGND and DGND pins of the device. For this method to be
successful, the analog and digital circuits within the mixed signal device must be well isolated.
Page 13 of 17
MT-031
The noise between AGND and DGND pins must not be large enough to reduce internal noise
margins or cause corruption of the internal analog circuits.
Figure 10 shows optional Schottky diodes (back-to-back) or a ferrite bead connecting the analog
and digital ground planes. The Schottky diodes prevent large dc voltages or low frequency
voltage spikes from developing across the two planes. These voltages can potentially damage the
mixed signal IC if they exceed 300 mV because they appear directly between the AGND and
DGND pins. As an alternative to the back-to-back Schottky diodes, a ferrite bead provides a dc
connection between the two planes but isolates them at frequencies above a few MHz where the
ferrite bead becomes resistive. This protects the IC from dc voltages between AGND and
DGND, but the dc connection provided by the ferrite bead can introduce unwanted dc ground
loops and may not be suitable for high resolution systems.
VN = NOISE BETWEEN
GROUND PLANES
VN
VD
VA
BACK-TO-BACK
SCHOTTKY
DIODES
OR
FERRITE
BEAD
(SEE TEXT)
VA
ANALOG
CIRCUITS
VD
MIXED
SIGNAL
DEVICE
AGND
A
DIGITAL
CIRCUITS
DGND
A
D
D
ANALOG
GROUND PLANE
A
TO SYSTEM
ANALOG SUPPLY
A
DIGITAL
GROUND PLANE
D
TO SYSTEM STAR GROUND
D
TO SYSTEM
DIGITAL SUPPLY
Figure 10: Grounding Alternative for Mixed-Signal ICs with
High Digital Currents: Multiple PC Boards
Whenever AGND and DGND pins are separated in the special case of ICs with high digital
currents, provisions should be made to connect them together if necessary. Jumpers and/or strap
options allow both methods to be tried to verify which gives the best overall performance in the
system.
GROUNDING SUMMARY
There is no single grounding method which will guarantee optimum performance 100% of the
time! This section has presented a number of possible options depending upon the characteristics
of the particular mixed signal devices in question. It is helpful, however, to provide for as many
options as possible when laying out the initial PC board.
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MT-031
It is mandatory that at least one layer of the PC board be dedicated to ground plane! The initial
board layout should provide for non-overlapping analog and digital ground planes, but pads and
vias should be provided at several locations for the installation of back-to-back Schottky diodes
or ferrite beads, if required. It is also extremely important that pads and vias be provided so that
the analog and digital ground planes can be connected together with jumpers if required. It is
difficult to predict whether the "multi-point" (single ground plane) or the "star" ground (separate
analog and digital ground planes) method will give best overall system performance; therefore,
some experimentation with the final PC board using the jumpers may be required.
When in doubt, it is always better to start out with a split analog and digital ground plane and
later connect them with jumpers, rather than to start out with a single ground plane and try and
later try and split it!
SOME GENERAL PC BOARD LAYOUT GUIDELINES FOR MIXED-SIGNAL
SYSTEMS
It is evident that noise can be minimized by paying attention to the system layout and preventing
different signals from interfering with each other. High level analog signals should be separated
from low level analog signals, and both should be kept away from digital signals. We have seen
elsewhere that in waveform sampling and reconstruction systems the sampling clock (which is a
digital signal) is as vulnerable to noise as any analog signal, but is as liable to cause noise as any
digital signal, and so must be kept isolated from both analog and digital systems. If clock driver
packages are used in clock distribution, only one frequency clock should be passed through a
single package. Sharing drivers between clocks of different frequencies in the same package will
produce excess jitter and crosstalk and degrade performance.
The ground plane can act as a shield where sensitive signals cross. Figure 11 shows a good
layout for a data acquisition board where all sensitive areas are isolated from each other and
signal paths are kept as short as possible. While real life is rarely as tidy as this, the principle
remains a valid one.
There are a number of important points to be considered when making signal and power
connections. First of all a connector is one of the few places in the system where all signal
conductors must run in parallel—it is therefore imperative to separate them with ground pins
(creating a faraday shield) to reduce coupling between them.
Multiple ground pins are important for another reason: they keep down the ground impedance at
the junction between the board and the backplane. The contact resistance of a single pin of a
PCB connector is quite low (of the order of 10 mΩ) when the board is new—as the board gets
older the contact resistance is likely to rise, and the board's performance may be compromised. It
is therefore well worthwhile to allocate extra PCB connector pins so that there are many ground
connections (perhaps 30-40% of all the pins on the PCB connector should be ground pins). For
similar reasons there should be several pins for each power connection, although there is no need
to have as many as there are ground pins.
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MT-031
SAMPLING
CLOCK GENERATOR
REFERENCE
ANALOG
ADC
CONTROL
LOGIC
BUFFER
REGISTER
DEMULTIPLEXER
DIGITAL
FILTER
DSP
OR
µP
AMPLIFIER
POWER
TIMING
CIRCUITS
MULTIPLE
ANALOG GROUNDS
INPUT
DATA
BUS
ADDRESS
BUS
BUFFER
MEMORY
MULTIPLE
GROUNDS
Figure 11: Analog and Digital Circuits
Should be Partitioned on PCB Layout
Analog Devices and other manufacturers of high performance mixed-signal ICs offer evaluation
boards to assist customers in their initial evaluations and layout. ADC evaluation boards
generally contain an on-board low-jitter sampling clock oscillator, output registers, and
appropriate power and signal connectors. They also may have additional support circuitry such
as the ADC input buffer amplifier and external reference.
The layout of the evaluation board is optimized in terms of grounding, decoupling, and signal
routing and can be used as a model when laying out the ADC PC board in the system. The actual
evaluation board layout is usually available from the ADC manufacturer in the form of computer
CAD files (Gerber files). In many cases, the layout of the various layers appears on the data
sheet for the device.
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MT-031
REFERENCES
1.
Ralph Morrison, Grounding and Shielding Techniques, 4th Edition, John Wiley, Inc., 1998, ISBN: 0471245186.
2.
Henry W. Ott, Noise Reduction Techniques in Electronic Systems, 2nd Edition, John Wiley, Inc., 1988, ISBN: 0471-85068-3.
3.
Paul Brokaw, "An IC Amplifier User's Guide to Decoupling, Grounding and Making Things Go Right for a
Change", Analog Devices Application Note AN-202.
4.
Paul Brokaw and Jeff Barrow, "Grounding for Low- and High-Frequency Circuits," Analog Devices
Application Note AN-345.
5.
Howard W. Johnson and Martin Graham, High-Speed Digital Design, PTR Prentice Hall, 1993, ISBN:
0133957241.
6.
Ralph Morrison, Solving Interference Problems in Electronics, John Wiley, 1995.
7.
Crystal Oscillators: MF Electronics, 10 Commerce Drive, New Rochelle, NY, 10801, 914-576-6570.
8.
Mark Montrose, EMC and the Printed Circuit Board, IEEE Press, 1999 (IEEE Order Number PC5756).
9.
John Ardizzoni, "A Practical Guide to High-Speed Printed-Circuit-Board Layout," Analog Dialogue, Vol. 39,
Sept. 2005.
10. Grant, Doug and Scott Wurcer, “Avoiding Passive-Component Pitfalls,” Analog Devices Application Note AN348
11. Walt Kester, Analog-Digital Conversion, Analog Devices, 2004, ISBN 0-916550-27-3, Chapter 9.
available as The Data Conversion Handbook, Elsevier/Newnes, 2005, ISBN 0-7506-7841-0, Chapter 9.
Also
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Information furnished by Analog Devices applications and development tools engineers is believed to be accurate
and reliable, however no responsibility is assumed by Analog Devices regarding technical accuracy and topicality of
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