Decoupling Technique for
Reducing Sensitivity of
Differential Pairs to
Power-Supply-Induced Jitter
John McNeill
Vladimir Zlatkovic
David Bowler
Lawrence M. DeVito
ANALOG
DEVICES
Presentation Overview
• Application
– Ring Oscillator VCO
• Problem: Supply influence on VCO frequency
– Stage delay ⇐ Bias current ⇐ Supply noise
– Coupling mechanisms:
Low frequency vs. High Frequency
• Decoupling Technique
– Analysis
• Results
– Simulated
– Measured
• Summary
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Presentation Overview
• Application
– Ring Oscillator VCO
• Problem: Supply influence on VCO frequency
– Stage delay ⇐ Bias current ⇐ Supply noise
– Coupling mechanisms:
Low frequency vs. High Frequency
• Decoupling Technique
– Analysis
• Results
– Simulated
– Measured
• Summary
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
TRANSMIT END
Application
TDATA
TRANSMIT END
TCLK
• Serial data
transmission
clock recovery
TDATA
TCLK
FIBER
LINK
CLOCK
RECOVERY
RECEIVE
END
FIBER
LINK
CLOCK
RECOVERY
Vin
RECEIVE
END
RCLK
RDATA
Vin
Vin
RCLK
RDATA
RCLK
Vin
RDATA
RCLK
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Figure 1.1.
RDATA
Typical fiber optic serial data transmission system.
PLL Clock Recovery
Vin
D
PHASE
DETECTOR
RETIMED
DATA
LOOP
FILTER
Vin
VOLTAGE
CONTROLLED
OSCILLATOR
RECOVERED CLOCK
RCLK
RCLK
"LATE"
Q
RDATA
RCLK
"EARLY"
Figure 1.3. PLL used for clock and data recovery.
• VCO output is recovered clock
• Decision circuit samples Vin at clock transitions
• Low bit error rate requires low jitter VCO output
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
RCLK
Ring Oscillator VCO
1
=
2 " N " tD
fVCO
• VCO frequency expression:
EACH GATE:
VCC
• Low jitter:
NUMBER STAGE
Prevent undesired
CS1
CS2
R1
R2
OF STAGES DELAY
500!
500!
variation in delay tD
Q4
!
– Random noise (thermal
noise)Q3
– Interference: Power supply coupling
+
Vin
-
Q1
Q2
540
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
µA
+
Vout
200
µA
200
µA
Applications Requiring Low Jitter VCO
• Microprocessor Clock Synthesis
– Multiply lower off-chip frequency to
higher frequency clock on-chip
– Jitter reduces timing margin
• Oversampled data conversion
– Multiply sample-rate clock for
oversampling
– Jitter produces artifacts in frequency
domain
Require low jitter clock
in mixed signal IC environment
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Presentation Overview
• Application
– Ring Oscillator VCO
• Problem: Supply influence on VCO frequency
– Stage delay ⇐ Bias current ⇐ Supply noise
– Coupling mechanisms:
Low frequency vs. High Frequency
• Decoupling Technique
– Analysis
• Results
– Simulated
– Measured
• Summary
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Ring VCO: Delay Stage Examples
VCC
RL
VDD
RL
RL
RL
+
VO
-
Q2
+
VI
Q3
-
Q1
VB
EACH GATE:
+
+
VO
VI
-
-
M2 M3
IEE
M1
VB
VEE
ISS
VSS
VCC
CS1
Bipolar
CMOS
CS2
R1
R2
500!
500!
• Problem:
Differential
pair
delay
influenced
by
Q4
bias current IEE , ISS
Q3
Q1
Q2
McNeill et. al, “Decoupling Techniques
540
200
+
Vout
…,” NEWCAS2003
200
Delay Dependence on Bias Current
SUPPLY
+ NOISE
DELAY STAGE
VCC
RL
RL
fT
Vn
Q2
+
VI
Q3
-
Q1
VB
IEE
I BIAS
VEE
Δ Supply voltage (Vn) ⇒
Δ Bias current IEE ⇒
Δ Transistor fT ⇒
Δ Differential pair delay
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Mechanisms of Bias Current Variation
• Q1 collector:
derived from VCC
SUPPLY
+ NOISE
VCC
RL
• Q1 base, emitter:
referenced to VEE
PREVIOUS
STAGE
RL
Vn
Q2
• Bias device Q1
variation vn
resulting bias
Q3
IEE + in
sees all of supply
• Need to minimize
DELAY
STAGE
Q1
VEE
current variation in
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
VB
Bias Current Variation: Low Frequency
• Bipolar: Base with
modulation
VCE → IC
• CMOS: Channel
length modulation
VDS → ID
SUPPLY
+ NOISE
VCC
DELAY STAGE
RL
RL
Vn
+
VI
Q2
Q3
-
• Solution: Cascode
– Increases output
impedance
– Good supply
rejection at DC,
low frequencies
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
VCASC
Q4
VB
Q1
VEE
IEE
Bias Current Variation: High Frequency
• Capacitive
coupling:
– Bipolar:
Cjc, Cjs
SUPPLY
+ NOISE
VCC
RL
DELAY
STAGE
RL
Vn
– CMOS:
Cgd, Cdb
• Both see all
of supply
variation vn
PREVIOUS
STAGE
Q2
Q3
IEE + in
Cjc
Q1
VEE
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
VB
Cjs
Analysis: High Frequency
re
• Supply noise to bias variation
transfer function
" 1 %
in
= sCs$
'
vn
# sreCs +1 &
re
small signal resistance
"looking into" Q1, Q2 emitters
Q2
Q3
Q2
Q3
in Cjc
Vn
Cs
Q1
Cjs
Q1
Cjs
VB
Cjc
VB
in vn
! Cs = Cjs + Cjc
f
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Simulated Results: Current
Process:
5-GHz-fT D.I.
fOSC = 155 MHz
VCC = 5V
vn = 200mV p-p
IEE = 100µA
Maximum p-p
variation: 30µA
30% of bias IEE !
in [µA p-p]
10
1
10
100
vn FREQUENCY [MHz]
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
1000
Simulated Results: Jitter
P-P JITTER
(% UNIT
INTERVAL)
1%
ORIGINAL
WITH DECOUPLING
NETWORK
0.1 %
100MHz
1GHz
RIPPLE FREQUENCY
Figure 5.13.
Simulated supply-induced jitter, with/without decoupling network.
• Exceeds system specification: 1% U.I. jitter
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Bias Current Variation: High Frequency
• Not fixed by
cascode!
• Just moves
problem to
Cjc, Cjs of
cascoding
device
Full supply
noise vn
must appear
across some
capacitance!
SUPPLY
+ NOISE
VCC
PREVIOUS
STAGE
RL
DELAY
STAGE
RL
Vn
Q2
Q3
IEE + in
VCASC
VB
VEE
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Cjc
Q4
Q1
Cjs
Presentation Overview
• Application
– Ring Oscillator VCO
• Problem: Supply influence on VCO frequency
– Stage delay ⇐ Bias current ⇐ Supply noise
– Coupling mechanisms:
Low frequency vs. High Frequency
• Decoupling Technique
– Analysis
• Results
– Simulated
– Measured
• Summary
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Decoupling Technique
• Add RC network in
series with bias
current
• Provide path for in
around differential
pair
• Same voltage
headroom cost as
cascode
VCC
RL
Q2
+
VI
RL
Q3
-
IEE + in
RBP
Q1
VB
VEE
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
CBP
Analysis
• Supply noise vn to bias
variation in transfer function
in
CBP
"
%
in
1
= sCs$
'
vn
# s( re + RBP )(Cs + CBP ) +1 &
• Improved by factor
" re %" Cs %
$
'$
'
# re + RBP &# Cs + CBP &
!
!
• Can improve with RBP or CBP
– Allows optimization of
headroom, area tradeoff
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Vn
re
RBP
Cs
in vn
f
Presentation Overview
• Application
– Ring Oscillator VCO
• Problem: Supply influence on VCO frequency
– Stage delay ⇐ Bias current ⇐ Supply noise
– Coupling mechanisms:
Low frequency vs. High Frequency
• Decoupling Technique
– Analysis
• Results
– Simulated
– Measured
• Summary
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Simulated Results: Current
RBP = 10 kΩ
CBP = 2pF
IEE = 100µA
Maximum p-p
variation: 1.5µA
in [µA p-p]
10
Improved by
20X
1
10
100
vn FREQUENCY [MHz]
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
1000
Simulated Results: Jitter
P-P JITTER
(% UNIT
INTERVAL)
1%
ORIGINAL
WITH DECOUPLING
NETWORK
0.1 %
100MHz
1GHz
RIPPLE FREQUENCY
Figure 5.13.
Simulated supply-induced jitter, with/without decoupling network.
• Within system specification: 1% U.I. jitter
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Measured Results: Test Configuration
DATA
SOURCE
D.U.T.
TCLK
Vtrig
RCLK
Vin
TDATA
TRIG
VERT
RDATA
DATA
SOURCE
TEK 11801C
VCLOCK
VEE
CC RECOVERY
PLL
COMMUNICATIONS
SIGNAL ANALYZER
(D.U.T)
V
Figure 1.20. Measurement technique:
n
Time domain, closed loop, transmit clock referenced.
p(t)
• Inject supply noise on VEE
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
!x
Measured Results: Example
With Decoupling Network
σ = 18ps rms
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Decoupling Network Removed
σ = 119ps rms
Measured Results
σ [ps rms]
300
Decoupling
Network
Removed
200
100
0
1
With
Decoupling
Network
10
100
vn FREQUENCY [MHz]
fOSC = 155MHz
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
1000
Presentation Overview
• Application
– Ring Oscillator VCO
• Problem: Supply influence on VCO frequency
– Stage delay ⇐ Bias current ⇐ Supply noise
– Coupling mechanisms:
Low frequency vs. High Frequency
• Decoupling Technique
– Analysis
• Results
– Simulated
– Measured
• Summary
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Extension: gm-C Quadrature CCO
• Analogous requirement:
Supply immunity of oscillator phase
Tewksbury et. al., "A 480MHz variable rate QPSK demodulator …", ISSCC97, pp. 86-87 © IEEE
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Summary
• Decoupling network reduces sensitivity of
differential stage delay to supply variation
• Need to address both low frequency and
high frequency coupling paths
• RC network improves supply noise
immunity of bias current
• Useful in other mixed signal applications
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003
Acknowledgments
• Analog Devices
– Graduate Fellowship
– Evaluation support (Bob Surette)
• National Science Foundation awards
– MIP-9701408 (CAREER)
– CDA-9617333 (Instrumentation)
McNeill et. al, “Decoupling Techniques …,” NEWCAS2003