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INFN - Laboratori Nazionali di Frascati
LVDS
links
LVDS links
[1]
Servizio Elettronico
Laboratori Frascati
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LVDS overview
What is LVDS
 Low Voltage Differential Signaling (LVDS)
 I/O signal levels are defined by ANSI/TIA/EIA-644
• Standard specifies LVDS as a way to transmit and receive hundreds of megabits
per second per channel over differential media
 Primarily a point to point interface
Why differential signaling
 Benefits
• Low noise
Smooth current mode outputs and low demand on power/ground
Differential fields tend to couple, reducing EMI
Noise picked up on differential lines is rejected by the receiver as a
common mode
 High speed
 Low power consumption
• Virtually flat versus frequency
• 10% of the power required for ECL
LVDS links
[2]
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
• A method to communicate data using a very low voltage swing (350 mV) over
differential Printed Circuit Board (PCB) traces or balanced cables
The ANSI/TIA/EIA-644 standard
1.0V VOL
LVDS links
[3]
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
G. Felici
ANSI/TIA/EIA-644 specs
LVDS driver consists of a current source output which drives a closelycoupled (closely-spaced) differential pairs of conductors
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LVDS driver
• The direction of current flow determines the logic level at the receiver
LVDS links
[4]
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
 Termination resistor at the receiver which matches the differential
impedance of the transmission line completes the current loop
Cable shielding and ground return wires help for EMC compliance
INFN - Laboratori Nazionali di Frascati
 The shield reduces the EMI
 The ground return wire provide a small return path for common-mode
currents
The network prevents DC
current flow in the shield
LVDS links
[5]
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Cable ground and shield connections
Servizio Elettronico
Laboratori Frascati
G. Felici
LVDS signal quality: the eye diagram method
Common methods to
measure LVDS signal quality
are:
INFN - Laboratori Nazionali di Frascati
 rise time measurement at the
load;
 Jitter measurement in an eye
pattern;
 Bit Error Rate (BER)
measurement
Eye pattern is used to
measure the effects of intersymbol interference on
random data and allows a
good estimation of the jitter
 A transition high after a long
series of low has a slower
rise time than the rise time of
a periodic waveform due to
the low pass filter effects of
the cable.
LVDS links
[6]
Servizio Elettronico
Laboratori Frascati
LVDS links
[7]
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
NRZ coding;
frequency increased until measured jitter equaled 20% with respect to
the unit interval for the particular cable length;
measurements @ 0 V and ± 100 mV differential voltage receiver
thresholds.
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LVDS signal quality: NRZ data eye pattern
ttcs = peak-to-peak threshold crossing jitter
LVDS links
[8]
1.22
1.33
1.40
2.30
6.43
ttcs
5.0
5.54
6.88
7.53
20.0
ttcs
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
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LVDS signal quality: Jitter measurements results (National)
 The common-mode range is +/-1V around the driver offset voltage (+1.25V
typical). This supports the input operating range of GND to +2.4V on the
receivers. M-LVDS supports a typically common mode of +/-2V.
Noise margin
 Noise margin is related to the +/-1V common mode range of LVDS. Since
noise will be coupled as common-mode, the receivers will reject it.
Input/Output protection network
LVDS links
[9]
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
The common mode range
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LVDS signal quality: noise margin & device protection
Maximum rating
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
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Comments
For 15 m cable length we can (probably) cope with threshold crossing
jitter
On-detector and off-detector ground difference must be minimized to
avoid noise margin reduction and device failure/damaging
Protection network must be added if not foreseen in the driver and/or
receiver side chip
LVDS links
[ 10 ]
Servizio Elettronico
Laboratori Frascati
INFN - Laboratori Nazionali di Frascati
G. Felici
LVDS signal quality: noise margin & device damage