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Analysis of noise immunity at common circuits of
the front end parts of high-speed transceivers
S.V. Kondratenko
NRNU MEPhI
ICPPA-2016, 14.10.2016
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Plan
• The proposed system of parameters of drivers noise
immunity
• Comparing of different drivers types at noise immunity
• The simulation results of test examples
• Conclusions
ICPPA-2016, 14.10.2016
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Investigated objects
Alternative names of analog (A) blocks of the transceivers:
... interface (or front end) parts
... peripheral parts
... i/o buffers.
Next, use the term “analog”.
The focus is on the analog parts of the transmitters (drivers) due to large pulse
current in their output stages, and increased sensitivity to the parasitic inductance in
the common circuits (power and ground circuits).
ICPPA-2016, 14.10.2016
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The problem of noise immunity in the drivers
• Opened keys are connected
to the outputs of noisy power
and ground buses, leading to a
collapse of the eye.
•Internal noises Un.int are
correlated with the
transmitted signal unlike
interference received from the
digital part of the transmitter
Un.ext .
The stylized eye diagrams
for DTX = TXP-TXN output
considering the influence
of noises at common
circuits
VML driver
•The blur of eye diagrams
increases due to the influence
of parasitic RLC elements of
package pins and parasitic
capacitors CP on the outputs.
• Through current iTHR is grounded in different ways on HF, in
general, the noises on common circuits are not in phase and
are not suppressed completely in the load.
ICPPA-2016, 14.10.2016
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The proposed system of noise immunity parameters of the drivers
• Standard for OpAmp parameter PSRR (Power
Supply Rejection Ratio) is analogous to the K1,3-4
coefficient, but in general the proposed system of
parameters describes the noise immunity of drivers
for different values of parasitic elements more fully
and accurately .
•You can directly compare the parameters Zii, Kij
for different drivers to assess their general noise
immunity at common circuits.
• Model parameters are calculated and analyzed in the frequency domain (ac), which
simplifies findings and speeds up calculations.
Input
nodes
1
2
1
2
Z11
-
Input
impedance
neglecting
influence
Output nodes
3
4
3-4
K13
K14
K1,3-4
neglecting
influence
TC for a single
signal
TC for a single
signal
TC for a differ.
signal
Z22
K23
K24
K2,3-4
TC for a single
signal
TC for a differ.
signal
input
TC for a single
impedance
signal
TC –transfer coefficient
ICPPA-2016, 14.10.2016
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Compared types of drivers
Principle of operation: switching to the load reference...
voltage
current
VML
nVML
Conditions of the equivalence: general
Pseudo-LVDS
(simplified)
CML
 identical dimensions of the transistors at outputs (for LVDS-driver - after scaling)
 the same load resistance RL = 100 ohms
 the same own output impedance of 100 ohms
... and private (for numerical simulation)
 0.18 micron CMOS technology
 target 800 mV swing of differential output signal (for drivers operating in voltage mode sweep vdd/2)
 the same parasitic elements (CP = 5 pF, L = 2 ... 5 nH at common circuits).
ICPPA-2016, 14.10.2016
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Calculated values of the parameters of noise immunity for compared drivers
Pseudo-LVDS at vin={1,0}, changes
Input
nodes
1
2
nVML
Input
nodes
1
2
CML
Input
nodes
1
2
1
260 Ohm
-
1
4,2 kOhm
-
1
43 Ohm
-
at vin={0,1}
2
260 Ohm
Output nodes
3
0,49
0,70
4
0,30
0,51
3-4
0,19
0,21
2
213 Ohm
Output nodes
3
0,01
0,34
4
0,03
0,81
3-4
0,02
0,47
2
43 Ohm
Output nodes
3
0,79
0,19
4
0,45
0,54
3-4
0,34
0,35
2
200 Ohm
Output nodes
3
0,70
0,28
4
0,23
0,78
3-4
0,47
0,50
VML
Input
nodes
1
2
1
213 Ohm
-
ICPPA-2016, 14.10.2016
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Findings of the analysis and calculations of the drivers
 According circuitry - only LVDS-driver output signal levels are not adhered
to one or both of the common buses, which increases noise immunity.
 According output impedance - CML-driver has the lowest impedance (43
ohms in this case), which should increase the impact of noise on general
circuits due to the prevailing inductive reaction of the impedance of power
and ground pins.
 According transfer coefficients - the smallest differential noise transfer
coefficient has LVDS-driver, the greatest - VML-driver.
 The tabulated values of the asymptotic impedances and transfer
coefficients in fact depends on the frequency (decrease on HF), which may
change the comparison results.
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Simulated circuit of the TX-RX path and conditions of the simulation
Conditions of the simulation:
- at the input - a random 8b10b encoded signal at a rate of 1.25 Gbps (1X)
- normal conditions PVT
- for the CML, VML and nVML drivers the same type of receivers are used, for
the pseudo-LVDS driver - differed receiver with the power supply 2.5 V for input
stage.
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Eye diagrams: 1,25 Gbps, TQFP-208 package
RX inputs
All TX-RX pairs
are successfully
working, but
pseudo-LVDS
driver is better
RX outputs
ICPPA-2016, 14.10.2016
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Eye diagrams: 2,5 Gbps, TQFP-208 package
• Drivers sequence in
order of preference:
VML (worst)
nVML
CML
Pseudo-LVDS (best)
• This is consistent
with previous
findings on the
basis of parameters
of noise immunity.
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CONCLUSION
1. In the early stages of designing high-speed transceiver in serial channels it is
advisable to take into account the effect of parasitic constructive elements in order
to choose among alternative realizations of analog parts of the transmitter and
receiver.
2. To evaluate the noise immunity of the analog parts on common circuits (power and
ground) the system of parameters proposed including impedances of interested
nodes and transmission coefficients on their driver outputs. This system includes a
standard parameter PSRR, but in general it describes the noise immunity of drivers
for different values ​of parasitic elements more fully and accurately.
3. Comparison of parameters of noise immunity for the four types of drivers showed
the advantage of pseudo-LVDS drivers. This was confirmed in the simulation
transmitter-receiver path, designed in 0.18 micron CMOS technology with regard
parasitic package parameters such as TQFP-208 and running at speeds up to 2,5
Gbps.
4. Used system of noise immunity parameters forms a specialized macro model of
the drivers to analyze their noise immunity on common circuits. In contrast to the
IBIS-model parameters proposed linearized macro models are determined by
calculation or analysis in the frequency domain, less labor-intensive than
calculations or analysis in the time domain.
ICPPA-2016, 14.10.2016
Thank you for attention!
ICPPA-2016, 14.10.2016