ecen620: network theory broadband circuit design fall 2020 · 2020. 10. 30. · lecture 13:...
TRANSCRIPT
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Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN620: Network TheoryBroadband Circuit Design
Fall 2020
Lecture 13: Transimpedance Amplifiers (TIAs)
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Announcements• Project descriptions posted on website
• Preliminary report due Nov 3 (HW4)• Final report due Nov 24• Presentation on Dec 4 (2PM-4:30PM)
2
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
3
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Optical Receiver Technology• Photodetectors convert optical
power into current• p-i-n photodiodes• Waveguide Ge photodetectors
• Electrical amplifiers then convert the photocurrent into a voltage signal• Transimpedance amplifiers• Limiting amplifiers• Integrating optical receiver
4
TIA Output
LA Output
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p-i-n Photodetector• A p-i-n photodetector has an
intrinsic layer (undoped or lightly doped) sandwiched between p-and n-doped material
• This p-n junction operates in reverse bias to create a strong electric field in the intrinsic region
5
[Sackinger]
• Normally incident photons create electron-hole pairs in the intrinsic region which are separated by the electric drift field, and photocurrent appears at the terminals
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p-i-n Photodetector Tradeoffs• There is a tradeoff between
efficiency and speed set by the intrinsic layer width W
• The quantum efficiency is the fraction of photons that create electron-hole pairs and is determined by W and detector’s absorption coefficient
6
[Sackinger]
• A wider W allows for higher , but also results in longer carrier transit times which reduces the detector bandwidth
We 1
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Waveguide p-i-n Photodetector
• A waveguide p-i-n photodetector structure allows this efficiency-speed trade-off to be broken
• The light travels horizontally down the intrinsic region and the electric field is formed orthogonal
• Allows for both a thin i-region for short transit times and a sufficiently long i-region for high quantum efficiency
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15m detector length
[Vivien OptExp 2009]
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p-i-n Photodetector Responsivity• The efficiency at which a photodetector converts
optical power to electrical current is called responsivity R
• As each photon has energy of hc/
8
A/W 108R where 5
hcq
PRPhcqIPIN
• Example: A photodetector with =0.6 operating at 1310nm has R=0.63 A/W
• There is the potential for R>1, with =1 and 1550nm operation R=1.24
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p-i-n Photodetector Equivalent AC Circuits
• The photodetectors main parasitics are the junction capacitance CPD and contact/spreading resistance RPD
• Additional LC parasitics are present in packaged devices due to wirebonds, etc…
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Bare Photodetector Photodetector + Packaging Parasitics
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p-i-n Photodetector Bandwidth
• Two time constants set the bare photodetector bandwidth
10
PDPDn
PDPDRC
nn
TR
CRvWBW
CR
vvW
121
by edapproximat becan Hzin bandwidth The
:Constant Time C-R
loictycarrier ve theis where :TimeTransit
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• Key design objectives• High transimpedance gain• Low input resistance for high bandwidth and efficient gain
• For large input currents, the TIA gain can compress and pulse-width distortion/jitter can result
Transimpedance Amplifier (TIA)
11
Ti
oT
ZivZ
log20by dB of unitsin expressed Also
anceTransimped
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• Input Overload Current• The maximum peak-to-peak input current for which we can
achieve the desired BER• Assuming high extinction ratio
• Maximum Input Current for Linear Operation• Often quantified by the current level for a certain gain
compression (1dB)
Maximum Currents
12
ovlppovl PRi 2
ppovl
pplin ii
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Resistive Front-End
• Direct trade-offs between transimpedance, bandwidth, and noise performance
13
DLDinpdB
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[Razavi]
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
14
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Common-Gate TIA
• Input resistance (input bandwidth) and transimpedance are decoupled
15
mombmDo
in
DT
grggRrR
RR1
1
m i o mb i D
out
i
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[Razavi]
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Common-Gate TIA Frequency Response
• Often the input pole may dominate due to large photodiode capacitance (100 – 500fF)
16
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r transistor Neglecting o
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[Razavi]
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Common-Gate TIA Noise• Both the bias current source
and RD contribute to the input noise current
• RD can be increased to reduce noise, but voltage headroom can limit this
• Common-gate TIAs are generally not for low-noise applications
• However, they are relatively simple to design with high stability
17
HzA
HzV
r transistor Neglecting
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o
Dminn
DD
mDRDnMnoutn
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[Razavi]
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Regulated Cascode (RGC) TIA
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[Park ESSCIRC 2000]• Input transistor gm is boosted by common-source amplifier gain, resulting in reduced input resistance
• Requires additional voltage headroom
• Increased input-referred noise from the common-source stage
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CMOS 20GHz TIA
19333222 RgARgA mm
• An additional common-gate stage in the feedback provides further gm-boosting and even lower input resistance
• Shunt-peaking inductors provide bandwidth extension at zero power cost, but very large area cost
[Kromer JSSC 2004]
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
20
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Feedback TIA w/ Ideal Amplifier
• Input bandwidth is extended by the factor A+1• Transimpedance is approximately RF• Can make RF large without worrying about voltage
headroom considerations 21
IDFTinp
FT
Fin
pTT
CCRA
CR
RA
AR
ARR
sRsZ
11
1
1
11
:AmplifierBandwidth InfiniteWith
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Feedback TIA w/ Finite Bandwidth Amplifier
22
1
1
11
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22
ARR
TCRTCRA
Q
TCRA
RA
AR
sQsRsZ
sTA
sAsA
Fin
ATF
ATF
ATFo
FT
ooTT
A
A
: AmplifierBandwidth Finite With
• Finite bandwidth amplifier modifies the transimpedancetransfer function to a second-order low-pass function
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Feedback TIA w/ Finite Bandwidth Amplifier
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• Non-zero amplifier time constant can actually increase TIA bandwidth!!
• However, can result in peaking in frequency domain and overshoot/ringing in time domain
• Often either a Butterworth (Q=1/sqrt(2)) or Bessel response (Q=1/sqrt(3)) is used• Butterworth gives maximally flat
frequency response• Bessel gives maximally flat group-
delay Butterworth
Bessel
[Sackinger]
-A(s)
CDiin
RF
voutCI
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Feedback TIA Transimpedance Limit
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• Maximum RT proportional to amp gain-bandwidth product• If amp GBW is limited by technology fT, then in order to
increase bandwidth, RT must decrease quadratically!
3dB
03dB
11
bandwidthgiven afor possible maximum theyields expression above into 1
Plugging
1 of case 0n larger tha times2211
:responseh Butterwort aFor
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TT
A
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A
TFAA
CRA
AA
RRA
AR
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CRA
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TQ
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AT C
AR
Maximum [Mohan JSSC 2000]
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Feedback TIA
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Dmmout
Dm
Fin
FDm
DmT
Dm
RggR
RgRR
RRg
RgR
RgA
12
1
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1 of gain ideal an hasfollower source thethat Assuming
F
D
A
in
D
out
• As power supply voltages drop, there is not much headroom left for RD and the amplifier gain degrades
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• CMOS inverter-based TIAs allow for reduced voltage headroom operation
• Cascaded inverter-gm + TIA stage provide additional voltage gain
• Low-bandwidth feedback loop sets the amplifier output common-mode level
CMOS Inverter-Based Feedback TIA
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[Li JSSC 2014]
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• TIA noise is modeled with an input-referred noise current source that reproduces the output TIA output noise when passed through an ideal noiseless TIA
• This noise source will depend on the source impedance, which is determined mostly by the photodetector capacitance
Input-Referred Noise Current
27
Hz
pA2
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• Input-referred noise current spectrum typically consists of uniform, high-frequency f2, & low-frequency 1/f components
• To compare TIAs, we need to see this noise graph out to ~2X the TIA bandwidth• Refer to noise bandwidth tables
Input-Referred Noise Current Spectrum
28
Hz
pA2
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• The input-referred rms noise current can be calculated by dividing the rms output noise voltage by the TIA’s midbandtransimpedance value
Input-Referred RMS Noise Current
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BW
TIAnTT
rmsTIAn dffIfZR
i 20
2,
2,
1
• If we integrate the output noise, the upper bound isn’t too critical. Often this is infinity for derivations, or 2X the TIA bandwidth in simulation
• This rms current sets the TIA’s electrical sensitivityrms
TIAnppsens Qii ,2
• To determine the total optical receiver sensitivity, we need to consider the detector noise and responsivity
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• TIA noise performance can also be quantified by the averaged input-referred noise current density
Averaged Input-Referred Noise Current Density
30
dB
rmsTIAnavg
TIAn BWi
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bandwidth.TIA over the spectrum, noise referred-input theaveragingthan different is thisNote,
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2, fI TIAn
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• The feedback resistor and amplifier front-end noise components determine the input-referred noise current spectrum
FET Feedback TIA Input-Referred Noise Current Spectrum
31
fIfIfI frontnresnTIAn 2,2,2,
• The feedback resistor component is uniform with frequency
F
resn RkTfI 42,
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FET Feedback TIA Input-Referred Noise Current Spectrum
32
• Gate current-induced shot noise
designs CMOSfor small typicallyis This
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• FET channel noise
process. on the depending 3-0.7 typicallyfactor, noise channel theis
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Input-Referring the FET Channel Noise
33
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Total Input-Referred FET Feedback TIA Noise
34
22
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241424 fgCkT
RgkTqI
RkTfI
m
T
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FTIAn
Feedback Resistor Gate Shot Noise FET Channel Noise
• Note that the TIA input-referred noise current spectrum begins to rise at a frequency lower than the TIA bandwidth
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
35
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Common-Gate & Feedback TIA
36
• Recall that the feedback TIA stability depends on the ratio of the input pole (set by CD) and the amplifier pole• Large variation in CD can degrade amplifier stability
• Common-gate input stage isolates CD from input amplifier capacitance, allowing for a stable response with a variety of different photodetectors
• Transimpedance is still approximately RFA/(1+A)
Common-Gate
Feedback TIA
[Mohan JSSC 2000]
RF RF
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BJT Common-Base & Feedback TIA
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• Transformer-based negative feedback boosts gm with low power and noise overhead
• Input series peaking inductor isolates the photodetectorcapacitance from the TIA input capacitance
• High frequency techniques allow for 26GHz bandwidth with group delay variation less than 19ps
[Li JSSC 2013]
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
38
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Differential TIAs
39
• Differential circuits have superior immunity to power supply/substrate noise
• A differential TIA output allows easy use of common differential main/limiting amplifiers• This comes at the cost of higher
noise and power• How to get a differential output
with a single-ended photocurrent input?• Two common approaches, based on
the amount of capacitance applied at the negative input
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Balanced TIA
40
• A balanced TIA design attempts to match the capacitance of the two differential inputs
DX CC
• This provides the best power supply/substrate noise immunity, as the noise transfer functions are similar
• Due to double the circuitry, the input-referred rms noise current is increased by sqrt(2)
design ended-single theasfunction transfer Same
11
1
andamplifierBW high an Assuming
ARsC
RA
A
ivvsZ
CCC
FT
F
i
ONOPT
IDT
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Pseudo-Differential TIA
41
• A pseudo-differential TIA design uses a very large capacitor at the negative input, such that it can be approximated as an AC ground XC
• While not good to reject power supply/substrate noise, it does provide significant filtering of the RF’ noise
• The differential transimpedanceis approximately doubled relative to the single-ended case
12
1
22
andamplifierBW high an Assuming
ARsC
RA
A
ivvsZ
CCC
FT
F
i
ONOPT
IDT
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Offset Control
42
• Due to the single-ended photodetector signal, the differential output signal swings from 0 to Vppd, which can limit the dynamic range
• Adding offset control circuitry can allow for an output swing of ±Vppd/2
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Differential Shunt Feedback TIA
43
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
44
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45
Optical RX Scaling Issues
Traditionally, TIA has high RT and low Rin
AARR FT 1
INFdB CR
A
13
Headrooom/Gain issues in 1V CMOS• A ≈ 2 – 3
Power/Area Costs 432 TIA dBINTD fCRI
23 LA dBD fI
VDDVVV GSGSA *8.021
VOD
VDDVOD
VVDDRgA ADm*2.0
1
-
46
Integrating Receiver Block Diagram
[Emami VLSI 2002]
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47
Demultiplexing Receiver
• Demultiplexing with multiple clock phases allows higher data rate̶ Data Rate = #Clock Phases x Clock Frequency̶ Gives sense-amp time to resolve data̶ Allows continuous data resolution
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48
1V Modified Integrating Receiver
Differential Buffer Fixes sense-amp common-mode input for improved
speed and offset performance Reduces kickback charge Cost of extra power and noise
Input Range = 0.6 – 1.1V
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49
Receiver Sensitivity Analysis
mVC
kT
sampsamp 92.0
2
mVbuffer 03.1 mVSA 45.0
Residual SA Offset = 1.15mV
Max Vin(IAVG) = 0.6mV
16Gb/sat 65.0 NoiseJitter Clock mVvT bb
jclk
mVclkSAbuffersamptot 59.1 NoiseInput Total2222
Vb for BER = 10-10 = 6.4tot + Offset = 11.9mV
b
inpdbavg T
CCVP
Gb/s Pavg (dBm)10 -9.816 -7.8
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50
Integrating Receiver Sensitivity
• Test Conditions̶ 8B/10B data patterns
(variance of 6 bits)̶ Long runlength data
(variance of 10 bits)
• BER < 10-10
[Palermo JSSC 2008]
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51
Integrating RX with Dynamic Threshold
• Dynamic threshold adjustment allows for un-coded data
[Nazari ISSCC 2012]
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Integrating RX with Dynamic Threshold
[Nazari ISSCC 2012]
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Agenda• Optical Receiver Overview• Transimpedance Amplifiers
• Common-Gate TIAs• Feedback TIAs• Common-Gate & Feedback TIA Combinations• Differential TIAs
• Integrating Optical Receivers• Equalization in Optical Front-Ends
53
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Low-BW TIA & CTLE Front-End
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• Improved sensitivity is possible by increasing the first stage feedback resistor, resulting in a high-gain low-bandwidth TIA
• The resultant ISI is cancelled by a subsequent CTLE
[Li JSSC 2014]
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Active CTLE Example
55
Din- Din+
Vo-Vo+
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Low-BW TIA & CTLE Front-End
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• Significant reduction in feedback resistor noise
• Low-frequency input and post amplifier noise is also reduced
[Li JSSC 2014]
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Low-BW TIA & CTLE Front-End
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[Li JSSC 2014]
25Gb/s Eye Diagram
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Low-BW TIA & DFE RX
58
• In a similar manner, a high-gain low-bandwidth TIA is utilized
• The resultant ISI is cancelled by a subsequent 1-tap loop-unrolled DFE
[Ozkaya JSSC 2017]
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DFE Example
59
• If only DFE equalization, DFE tap coefficients should equal the unequalized channel pulse response values [a1 a2 … an]
• With other equalization, DFE tap coefficients should equal the pre-DFE pulse response values
• DFE provides flexibility in the optimization of other equalizer circuits
• i.e., you can optimize a TX equalizer without caring about the ISI terms that the DFE will take care of
a1a2
[w1 w2]=[a1 a2]
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Low-BW TIA & DFE RX
60
• As RF is increased, the main cursor increases and the SNR improves is ISI is cancelled by a DFE
• Large performance benefit with a low-complexity 1-tap DFE
[Ozkaya JSSC 2017]
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Low-BW TIA & DFE RX
61
• Self-referenced TIA is used for differential generation
• Actual 64Gb/s pulse response has a significant pre-cursor ISI tap, which requires a 2-tap TX FFE
[Ozkaya JSSC 2017]
64Gb/s Pulse Response & Timing Margin
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Next Time• Main/Limiting Amplifiers
62