High-speed Serial Interfacetera.yonsei.ac.kr/class/2016_1_2/lecture/Lect 8 TX_Driver and...
Transcript of High-speed Serial Interfacetera.yonsei.ac.kr/class/2016_1_2/lecture/Lect 8 TX_Driver and...
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High-speed Serial Interface
Lect. 8 – TX Driver and Signaling
(Ref.: Razavi, Prof. Palermo's Lecture Notes for "High-Speed Links Circuits and Systems", Texas A&M)
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Block diagram
Serializer Sampler
Clock
Recovery
Deserializer
PLL
Channel
Tx Rx
Rx
Equalizer
Tx
Driver
What is the best way to send data from Tx to Rx?
- Modulation Format? NRZ for most serial interface applications
- NRZ is not bandwidth efficient but simplest allowing higher-speed operation
- Tx driver (output buffer) structure
- Single or differential signaling, current or voltage mode, termination
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Simplest Tx Driver
Channel
- Operation speed is limited by full-swing (rail-to-rail)
- Impedance matching is difficult
- CMOS inverter
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Open-Drain Tx Driver
- Current mode
- Output voltage swing:
- RL = Zo required for impedance matching
IssRL
- Impedance matching on Tx side ?
- Noise immunity?
Single-ended signaling
- Reduced switching time since only the current path is steered
- Larger power consumption because of static power
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Differential Open-Drain
- Current mode
- Output voltage swing: 2 IssRL
- Much better noise immunity
- Differential signaling
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Single-ended vs Differential
Signaling
Channel
Threshold
generated
in RX
Positive channel
Negative channel
Common-mode
noise
- Common-mode noise rejection with differential signaling
- Self-referencing for receiver threshold for differential signaling
- Supply and ground levels can be different for TX and RX
Differential signaling much preferred for high-speed serial interface
High-speed circuits may use differential signaling for their logic circuits
- Larger output voltage swing for differential
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Differential Open-Drain
- Current mode
- Output voltage swing: 2 IssRL
- Much better noise immunity
- Differential signaling
- Input impedance matching ?
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Current-Mode Logic (CML) Driver
- Current mode
- Output voltage swing: IssRL
- Double termination
- Differential signaling
Most popular for high-speed serial interface
- Guarantee for R1=R2=RL=Z0 ?
- Actively adjustable termination required
for demanding applications
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Single-Ended Termination vs Differential
Termination
- Impedance matching with differential termination: 2 Z0
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50-Ω termination
• Why 50Ω?– Historical reason
• It was found for coax cables for early microwave systems (1930s),
– 33Ω gives best performance in max power handling capacity
– 75Ω gives the minimum loss
– 50 Ω, a value close to the medium (54 Ω) was selected as a
compromise
• Nowadays, almost all high-speed instruments, interconnects,
circuits are 50Ω-based
– CATV systems still use 75-Ω coax cables
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DC/AC Coupling
- Rx common-mode? IR/2
Cannot be independently controlled
VCM
VCM
- Rx common-mode? VCM
- Low frequency cut-off
- Data must be coded
- 8B/10B: Map 8-bit symbol into 10-bit
so that counts for 1's and 0's are balanced
and not more than five 1's or 0's
Several standards use this but there is overhead for data rate
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Tx Driver Power Reduction
Current source + Parallel Tx termination
d,0
Voltage source + Serial Tx termination
Voltage mode driver
Factor of 2 Tx power reduction
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Tx Driver Power Reduction
Single-ended Rx termination
d,0
Differential Rx termination
d,0
Further power reduction of factor 2
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Voltage Mode Driver
Z0
Z0
2Z0
TXP
TXN
DP
DN
VZN
VZP
VDRV
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Tx Termination
Replica circuits fix the voltages so that the desired Tx output impedance is achieved
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Tx Output Swing Control
Reference voltage determines the bias voltage required for desirable swing level
VMDRVReplica DRV
VDD
VDRV
REF_VDRV
Design exercise