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APPLICATION NOTE
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LVDS Data Outputs for High-Speed Analog-to-Digital Converters
by Cindy Bloomingdale and Gary Hendrickson
Analog-to-digital converter (ADC) sample rates have
been increasing steadily for years to accommodate
newer bandwidth-hungry applications in communication,
instrumentation, and consumer markets. Coupled with
the need to digitize signals early in the signal chain to
take advantage of digital signal processing techniques,
this has motivated the development of high-speed ADC
cores that can digitize at greater than 100 MHz to
200 MHz clock rates with 8- to 12-bit resolution.
In standalone converters, the ADC needs to be able to
drive receiving logic and accompanying PCB trace capacitance. Current switching transients due to driving the
load can couple back to the ADC analog front end,
adversely affecting performance. One approach to minimize this effect has been to provide the output data at
one-half the clock rate by multiplexing two output ports,
reducing required edge rates, and increasing available
settling time between switching instants. The
AD9054A, AD9884, AD9410, and AD9430 are
recent examples.
A new approach to providing high-speed data outputs
while minimizing performance limitations in ADC applications is the use of LVDS (low voltage differential
signaling). ADI is incorporating LVDS output capability
in a new 170 MSPS, 12-bit ADC—the AD9430—and will
include LVDS in some of its future high-speed ADCs and
DACs (digital-to-analog converters).
LVDS is, as the name says, a low voltage differential
signaling scheme. The operative words here are
voltage
have developed specifications that will be discussed
later in this note. Lower voltage signal swings have the
intrinsic advantage of shorter switching times as well as
reduced EMI concerns (adjacent differential traces tend
to cancel each others’ EMI).
(~350 mV) and
Figure 1. LVDS Output Levels
differential
. Standards bodies
350mV~1.2V
low
Differential signaling also has the well-known commonmode rejection benefit. Noise that is coupled to the
signals tends to be common to both signal paths and
cancelled out by a well designed differential receiver.
LVDS outputs are current output stages requiring a 100 W
terminating resistor at the receiver, differing from
CMOS outputs that generally do not require termination. The current output results in a fixed dc load current
on the output supplies—avoiding current spikes on the
supply that can couple to the sensitive analog front end.
OUT+
LVDS
DRIVER
Figure 2. LVDS Requires Far End Termination
STANDARDS
Two standards have been written to define LVDS. One is
the ANSI/TIA/EIA-644 which is titled, ”Electrical Characteristics of Low Voltage Differential Signaling (LVDS)
Interface Circuits.” The other is IEEE Standard 1596.3
which is titled, ”IEEE Standard for Low-Voltage Differential Signals (LVDS) for Scalable Coherent Interface
(SCI).” A brief summary of these two standards is
provided below.
ANSI/TIA/EIA-644
The ANSI/TIA/EIA Standard was developed under the
Telecommunications Industry Association (TIA) Subcommittee TR-30.2 and contains only generic electrical
specifications for LVDS. Its purpose was to create a
general high-speed interface standard for use in pointto-point connections between data communications
equipment. The maximum data signaling rate is 655
Mbps. The TIA Subcommittee intended other
standards bodies to reference ANSI/TIA/EIA-644 in more
complete interface specifications between transmitters
and receivers.
OUT–
100
REQUIRED
TERMINATION
RESISTOR
LVDS
RECEIVER
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© Analog Devices, Inc., 2002
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IEEE Standard 1596.3
The IEEE Standard 1596.3 was developed as an extension to the 1992 SCI protocol (IEEE Standard 1596-1992).
The original SCI protocol was suitable for high-speed
packet transmissions in high-end computing and used
ECL levels. However, for low-end and power-sensitive
applications, a new standard was needed. LVDS signals
were chosen because the voltage swing is smaller than
that of ECL outputs, allowing for lower power supplies
in power-sensitive designs.
AD9430 LVDS Specifications
As mentioned above, the AD9430 is the first in a series
of high-speed A/D converters designed with an LVDS
output option (CMOS outputs are also available). It is
a 12-bit, 170 MSPS ADC optimized for outstanding
dynamic performance in wideband carrier systems. A
simplified equivalent circuit for the AD9430 LVDS
outputs is shown in Figure 3.
V
DD
IS T
Q1 Q2
A– A+
OUT+
operation can be explained as follows (also see Figure 4).
A current source (I
T) is established on-chip from VDD and
S
is steered through Q2. In this example, a logic 1 is being
transmitted (V+ > V–). The 100 W receiver termination
resistor provides a current path for the current to return
back to the driver to the lower current sink (I
B) to
S
ground through Q3. The nominal current source/sink is
set to approximately 3.5 mA, resulting in a 350 mV swing
for an external termination resistor of 100 W.
V
DD
IS T
Q1 Q2
100 R
A– A+
Q3 Q4
A+ A–
B
I
S
Z0 = 50
Z0 = 50
TERM
V+
V–
LVDS
RECEIVER
Figure 4. LVDS Output Current
OUT–
Q3 Q4
A+ A–
mately 1.2 V (common-mode control circuitry not shown),
the output resistor can be modeled as two 50 W resis-
tors in series with their center-tap sitting at 1.2 V. This
provides a match to a typical PCB trace characteristic
Assuming an output common-mode voltage of approxi-
B
I
S
impedance (Z
) of 50 W and minimizes reflections.
O
The AD9430 LVDS outputs are more closely aligned with
Figure 3. LVDS Data Outputs
the ANSI/TIA/EIA-644 specification. Table I compares
the ANSI/TIA/EIA-644 and AD9430-170 specifications.
The differential outputs are indicated in Figure 3 by
OUT+, the positive or true data output, and OUT–, the
complement data output of the differential signal. Circuit
Table I. Summary of ANSI/TIA/EIA-644 and AD9430 Specifications
ANSI/TIA/EIA-644 AD9430-170
Specification Min Max Typ
Output Current 2.47 mA 4.54 mA Determined by RSET (nominally 3.5 mA)
Differential Output Voltage Magnitude 247 mV 454 mV 350 mV
Output Offset Voltage (Common Mode) 1.125 V 1.375 V 1.2 V
Transition Time: Rise Time (t
) and £0.3 tUI = 0.3 5.88 ns 0.5 ns
R
Fall Time (tF); 20% to 80% = 1.76 ns
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T
WTS
T
W
DTWTST
W
DIFF PAIR 1 DIFF PAIR 2
T
G
GROUND
Output Current
Unlike CMOS, which is typically a voltage output, LVDS is a
current output technology. The AD9430 output current is
set by a resistor with a typical value of 3.7 kW. This 3.7 kW
resistor sets the output current to 3.5 mA. Note that the
nominal output current (I
) of 3.5 mA can be varied in the
S
AD9430, allowing some system design flexibility. See
the AD9430 data sheet for more information.
Differential Output Voltage and Offset Voltage
The ANSI specification defines the differential output
voltage swing to be between 247 mV and 454 mV.
Assuming that the ADC outputs are set for a 3.5 mA
output current and the receiver input impedance is
100 W, the nominal differential output voltage is 350 mV.
The output drivers of the AD9430 were designed for a
nominal 1.2 V common mode, which is well within the
ANSI specification. The common mode was specified in
the range 1.1 V to 1.375 V to account for ground shifts of
as much as ±1 V between the driver and receiver grounds.
For best performance, the ADC outputs should be located
very close to the receiver inputs on the same PCB, so
ground shifts should not be a concern in this application.
Transition Time
In Table I, the ANSI/TIA/EIA-644 specification defines the
signal transition times for data signal rates less than or
equal to 200 MHz as less than or equal to 0.3 ¥ t
value of t
is defined as the inverse of the data signaling
UI
. The
UI
rate. In the case of the AD9430, the maximum signaling
rate is 170 MHz, so t
5.88 ns for t
in the specification equation results in 0.3
UI
5.88 ns = 1.76 ns. Therefore, the rise time (t
(t
) for the sum of the differential signals should be less
F
is equal to 5.88 ns. Substituting
UI
) and fall time
R
than 1.76 ns. For the AD9430, the rise and fall time values
are nominally 0.5 ns. The definitions of the rise and fall
times for the LVDS differential output voltage are shown
in Figure 5. LVDS rise and fall times are defined as the
time for the signal to change from 20% to 80% of the
maximum magnitude of the signal, as opposed to CMOS
outputs, which are usually defined as the change from
10% to 90%.
t
UI
t
R
Figure 5. Timing Diagram
t
F
+350 mV
80% OF SIGNAL
0V DIFFERENTIAL
20% OF SIGNAL
–350mV
LVDS PCB DESIGN CONSIDERATIONS
LVDS outputs for high-performance ADCs should be treated
differently than standard LVDS outputs used in digital
logic. While standard LVDS can drive 1 to 10 meters in
high-speed digital applications (dependent on data rate),
it is
not
recommended to let a high-performance ADC
drive that distance. It is recommended to keep the output
trace lengths short (< 2 in.), minimizing the opportunity
for any noise coupling onto the outputs from the adjacent circuitry, which may get back to the analog inputs.
The differential output traces should be routed close
together, maximizing common-mode rejection with the
100 W termination resistor close to the receiver. Users
should pay attention to PCB trace lengths to minimize
any delay skew.
A typical differential microstrip PCB trace cross section
is shown in Figure 6.
Figure 6. PCB Trace Spacing
Layout Guidelines
∑ Keep T
∑ Keep T
∑ Avoid use of vias where possible
∑ Keep D > 2 T
∑ Avoid 90C bends if possible
∑ Design T
, TS, and D constant over the trace length
W
~ < 2 T
S
W
S
and TG for ~50 W
W
Power supply decoupling is very important with these fast
(< 0.5 ns) edge rates. A low inductance, surface-mount
capacitor should be placed at every power supply and
ground pin as close to the ADC as possible. Placing the
decoupling caps on the other side of the PCB is not recommended, since the via inductance will reduce the effective
decoupling. The differential Z
lower than twice the single-ended Z
due to proximity effects—the Z
will tend to be slightly
O
of each conductor
O
of each line should be
O
designed to be slightly higher than 50 W. Simulation can
be used in critical applications to verify impedance matching. In short runs, this should not be critical.
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OTHER CONSIDERATIONS
LVDS also offers some benefit in reduced EMI. The EMI
fields generated by the opposing currents will tend to
cancel each other (for matched edge rates). Trace
length, skew, and discontinuities will reduce this benefit
and should be avoided.
LVDS also offers simpler timing constraints compared
to a demuxed CMOS solution at similar data rates. A
demuxed databus requires a synchronization signal that
is not required in LVDS. In demuxed CMOS buses, a
clock equal to one-half the ADC sample rate is needed,
adding cost and complexity, that is not required in
LVDS. In general, the LVDS is more forgiving and can
lead to a simpler, cleaner design by the customer.
SUMMARY
The advantages LVDS offers in high-speed signaling
can provide performance and system benefits in
converter applications. ADI’s portfolio of this
technology within the ADC and TxDAC
response to the ever increasing high-speed converter
performance requirements.
REFERENCES
1. ANSI/TIA/EIA-644,
Electrical Characteristics of Low
®
products is in
Voltage Differential Signaling (LVDS) Interface Circuits
March 1996.
2. IEEE Std 1596.3-1996,
IEEE Standard for Low-Voltage
Differential Signals (LVDS) for Scalable Coherent
Interface (SCI)
, March 1996.
,
E02796–0–5/02(0)
TxDAC is a registered trademark of Analog Devices, Inc.
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PRINTED IN U.S.A.
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