1. [ 1 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS links

2. [ 2 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS overview What is LVDS  Low Voltage Differential Signaling (LVDS) A method to communicate data using a very low voltage swing (350 mV) over differential Printed Circuit Board (PCB) traces or balanced cables  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

3. [ 3 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici ANSI/TIA/EIA-644 specs The ANSI/TIA/EIA-644 standard 1.0V V OL

4. [ 4 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS driver LVDS driver consists of a current source output which drives a closely- coupled (closely-spaced) differential pairs of conductors  Termination resistor at the receiver which matches the differential impedance of the transmission line completes the current loop The direction of current flow determines the logic level at the receiver

5. [ 5 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici Cable ground and shield connections Cable shielding and ground return wires help for EMC compliance  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

6. [ 6 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS signal quality: the eye diagram method Common methods to measure LVDS signal quality are:  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 inter- symbol 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.

7. [ 7 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS signal quality: NRZ data eye pattern 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.

8. [ 8 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS signal quality: Jitter measurements results (National) t tcs 1.22 1.33 1.40 2.30 6.43 t tcs 5.0 5.54 6.88 7.53 20.0 t tcs = peak-to-peak threshold crossing jitter

9. [ 9 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS signal quality: noise margin & device protection The common mode range  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

10. [ 10 ] LVDS links Servizio Elettronico Laboratori Frascati INFN - Laboratori Nazionali di Frascati G. Felici LVDS signal quality: noise margin & device damage Maximum rating 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