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A Stable System Clock
Generator Using Reference
Clock Sampling
Aatmesh Shrivastava
Robust Low Power VLSI Group
University of Virginia
23 May 2017
Alicia Klinefelter
Robust Low Power VLSI Group
University of Virginia
1
Outline
• Motivation
 Crystal oscillator design and power consumption
• Novel Circuit Architecture
 Three phases of operation viz calibration, conversion and retention phase
• Circuit Design: Calibration Phase




F to V converter
Op-Amp
VCO
PTAT
• Conversion and Retention Phase
 ADC
 Capacitive DAC
 R-2R DAC
• Summary
• References
23 May 2017
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Motivation : Crystal Oscillator
CRYSTAL
• Almost all microprocessors, micro-controllers,
PICs, and CPUs operate using a crystal oscillator.
 Crystal oscillator provides the reference clock.
• Why??
 Highest accuracy and frequency stability compared to any
known oscillator.
• Usually Fed to PLL to generate system clock,
Sometimes can be used as is.
• Frequency range from 10Khz-50Mhz
CRYSTAL SYMBOL
Board Hook-up
23 May 2017
Equivalent circuit
Circuit Diagram of Xtal oscillator
3
Motivation : Crystal Oscillator Power
• Crystal with Char. frequency of 32Khz to 50Mhz designed to obtain power.
 Most optimal point for oscillation.
 Crystal parameter obtained through vendor data-sheets.
• Power dissipated ranges from 2.6uW @ 32Khz to 70mW @ 50Mhz.
 At 200Khz power ranges from 5uW to 30uW.
• Crystal Oscillator consumes significant amount of power, critically impacts the
power consumption of a system designed for lower power.
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Motivation : Proposed Circuit technique
We propose a fully integrated reference/system clock circuit in CMOS.
 The circuit achieves a high frequency stability of +/-250ppm.
 We reduce the power consumption to a very low level < 2u Watts @ 200Khz
 Unlike a crystal oscillator the power consumption of proposed circuit does not scale up with the frequency
because of the architecture.
done
Counter
clock
ADC
Reg
Block
DAC
done
Clock coming
from crystal or
RF source
F to V
Converter
1
_
PTAT
VCO
0
Op. Amp.
Analog
Mux
+
F to V
Converter
control
out
Proposed Circuit
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Circuit Operate in 3 Phases.
•
Calibration Phase.



(done=0)
A stable voltage is obtained at A, corresponding to
reference frequency through crystal oscillator
The amplifier controls the VCO. The VCO oscillation
frequency is converted back to voltage using F to V
converter.
Feedback structure makes sure that A=E. (Need very
done
Counter
Register
Block
done
Clock coming
from crystal or
RF source
F to_V
Converter
high gain amplifier to insure this)

Calibration is completed once voltage at E becomes
equal to A.
DAC
ADC
1
A
PTAT
Op. Amp.
clock
E
+
•
Conversion Phase.



•
VCO
0
Analog Mux
(done=0)
Once E settles to value of A, Analog to Digital converter
(ADC) and Digital to Analog Converter is enabled.
These two blocks generates C which is equal to B with
<1mV quantization error.
Done signal goes high after conversion is done which
causes MUX to select the DAC output.
F to V
Converter
control
out
Retention Phase.(done=1)




After done goes high, DAC is calibrated with desired o/p voltage for VCO. Clock can be turned off.
The voltage at DAC can be maintained through bank of registers.
All other blocks except DAC, PTAT and VCO can be disabled and are disabled.
PTAT controls the temperature drift of VCO.
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Circuit Design: Frequency to Voltage converter[1]
VDD
1
Is
Clk
T1
Clk
MP1
MN1
2
T2
T1
C1
2
1
T2
1
MN2
C2
ClK
1
T1
2
T2
Circuit Diagram FVC
Time
• Figure shows Frequency to Voltage converter (FVC) schematic which is based on
switching capacitors and current source.
• F1 and F2 are pulse signals.
 F1 is generated after rising edge of Clock, while F2 is generated after falling edge of F1
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Functioning
ClK
1
T1
2
T2
Time
• When Clock is Low MP1 is on transmission
gate T1 is off and MN2 is off.
• C1 gets charged through Is.
• When F1 goes high MP1 is off and MN2 is OFF.
T1 is on and C1 and C2 Share charge as shown
Is
C1=C2
C1
C2
C2
• When F2 goes high T1 is off MP1 is off and
MN2 is ON which discharges C1 to ground.
• This process is repeated and eventually charges C2 to the maximum Voltage of C1.
• A voltage thus inversely proportional to frequency is obtained at C2.
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Output waveforms
Steady state internal net waveforms
A
OUT
Out build-Up
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Circuit Design: Folded Cascode Operational Amplifier
VDD
VDD
INN
MP2
MP1
+
_
OUT
INP
VDD
VM
Cp
C
OUT
VDD
M1
A
B
VM
MN1
MN2
•
•
•
•
Gain of the amplifier gets multiplied through each stage.
Very High gain.
Quiescent current = 200nA
Since Op-amp is used in feedback structure, stability of
the system is achieved Cp. Compensating Cap
VM=VDD/2
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Stability of the system
Feedback path
Proposed Circuit
Because of the feedback, stability sims were done.
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Insuring Stability of the system
Pole to replace VCO and F to V for
stability analysis
REF
REF
_
+
A
VCO
F to V
OUT
_
A
OUT
+
=
Feedback portion of the system
open-loop ac analysis reduction for the
system.
• As voltage at A changes, frequency of VCO changes and because frequency changes
voltage at OUT will changes. In other words voltage at A changes voltage at OUT.
• This means there is a phase difference b/w A and OUT or there is a pole b/w A and OUT.
• OUT being feedback net so this pole impacts the stability.
• We obtain the delay from A to OUT. Using the delay number we approximate the pole by
RC and perform open loop AC analysis.
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Stability Analysis
Phase Margin= 80
DC Gain=
100 dB
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Voltage Controlled Oscillator
VP
VP
=
MP
_
IN
VN
OUT1
OUT5
INV
BUF
+
VN
OPAMP
MN
Current Mirror
Delay Element in VCO
Functioning
• Current sources MP and MN determine the delay and hence frequency of VCO.
• As Output of OPAMP increases, VN increases and VP decreases.
• Current sources MP and MN drive increases, which increases frequency.
• Five such delay elements are used.
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Voltage Controlled Oscillator
Voltage Controlled Oscillator
VCO
VCOoutput
outputatatVN=700mV
VN=883mV f=200Khz
Effect of Temperature variation on VCO
• After the calibration phase, VCO input would remain at constant voltage.
• Temperature will cause current source to vary.
• This will change the frequency.
 Frequency changes from 220Khz to 200 Khz from 0 to 100oC
Sweeping temperature for IN.
• With increasing temperature IDRIVE of NMOS drops
Removing effect of Temperature: PTAT[2]
• Idrive reduces with temp. Use PTAT to control temperature drift.
• Add current of MOS and PTAT to get zero temperature coefficient. (ZTC)
MP1
Start-up
IB
• As Temperature increases, the Threshold voltage of
transistor decreases.
MP2
1
1
IB
• Current in the circuit is given by
MN1
MN2
1
M>1
• With temperature VT of MN1 drops hence VGS,
consequently VB increases increasing IB.
VGS
RB
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VB
17
PTAT Output
Sweeping temperature for IPTAT.
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Getting ZTC currents for VCO
Variation of Current sources in VCO.
• Current in VCO’s current source vary by 3nA over ~400nA over a temperature variation
of 0 to 100oC
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VCO output at 0, 50 and 100 Degrees
• With change is temperature frequency of oscillation changes from 200 Khz to 201 Khz.
Frequency stability = +/-250ppm
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Clock coming
from crystal or
RF source
F to V
Converter
FV1
_
VCO_in
Op. Amp.
Putting it together:
Calibration Phase
PTAT
VCO
+
FV2
F to V
Converter
out
control
VCO_in
FV1
FV2
FV2=FV1
Clock coming
from crystal or
RF source
F to V
Converter
_
Op. Amp.
Putting it together:
Calibration Phase
CLKIN
PTAT
VCO
+
VCO_OUT
F to V
Converter
out
control
ADC Architectures
Sigma Delta
Successive
Approximation
High Accuracy
Determined by DAC and
Comparator
Higher
Lower
Conversion Time
Fast
(due to oversampling)
N*(cycle time)
No pipeline delay
Typical Bit Range
> 10 bits
< 15 bits
High
Low
Accuracy
Power
Complexity
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Successive Approximation (SAR) ADC
• Begins by making
assumptions about bit
values starting with the
MSB and forcing 1000…00
(Vref/2)onto the DAC.
• If this voltage is above the
analog input, the assumed
bit goes to 0, else it remains
1.
• Then assumptions are made
about all bits till the LSB and
checked so the system
“zeros in” on the result.
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SAR ADC Block Diagram [4]
• Comparator: Compares assumption coming from DAC and input
analog voltage.
• DAC: Takes current assumption bits and turns them into analog
voltage for comparison in next stage.
• SAR Logic: Takes output of comparator (0 or 1) to determine the
next bit value and set the next bit assumption.
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Comparator
• Three op amps used (two P, one N)
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The Charge Scaling 10-bit DAC [3]
Charge Scaling DAC Output
0.6
0.5
voltage (V)
0.4
0.3
0.2
0.1
0
-0.1
0
0.002
0.004
0.006
time (ms)
0.008
0.01
0.012
• Capacitor DAC acts as both DAC and sample-hold circuit.
• Design consideration: base cap size, C.
• Needs to be reset before use (discharge caps).
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ADC Converging with Comp. Output
Analog Ref.
Comp. Out
DAC Out
1
voltage (V)
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
time (ms)
0.8
1
1.2
-3
x 10
• DAC makes series of guesses.
– If (guess voltage) > (analog input)
over-approximation. This bit is 0.
– Else the guess is below the expected value. This bit is 1.
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The R-2R 10-bit DAC [3]
R-2R Ladder DAC
0.5
0.45
0.4
voltage (V)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
•
•
•
0.002
0.004
0.006
time (ms)
0.008
0.01
0.012
Simple voltage divider network.
Does not require initialization signal and does not need to be periodically refreshed.
Tradeoff: branch current vs. resistor size
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The ADC/DAC Network
• Active Power
consumption: 550nA
• Technology: IBM 130nm
• Supply Voltage: 1 V
• Sampling Rate: 20 kHz
• Future optimizations:
– Low-power
optimization for ADC
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Summary
• Novel low power, high stability clock circuit is proposed.
• The circuit achieves a frequency stability of +/-250ppm.
• Power in calibration-phase = 5uW, retention-phase = 2uW.
• Similar to crystal in stability (+/-100ppm) better than
crystal in terms of power consumption.
 Power consumption does not scale up with frequency unlike crystal oscillator
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Questions?
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References
[1] A Djemouai et al. “New Frequency Locked Loop based CMOS frequency to voltage
converter: Design and Implementation” IEEE Transactions on Circuits and systme II:
Analog and Digital Signal Processing. vol. 48 No-5, May 2001.
[2] D. Liu et al. “A simple voltage reference circuit using transistor with ZTC point and
PTAT current source” IEEE J Solid-State Circuits vol. 29 pp 663- 670, June 1994.
[3] Allen E. and Holberg D. “CMOS Analog Circuit Design” Oxford University Press, New
York, 2002.
[4] Simone Gambini and Jan Rabaey. “Low-Power Succesive Approximation Converter
with 0.5 V Supply in 90 nm CMOS” IEE Transactions of Solid-State Circuits. Vol. 42, no.
11, Nov. 2007.
23 May 2017
Aatmesh
[email protected]
33
BACK UP
23 May 2017
Aaatmesh
34
Crystal Oscillator: Design
RBIAS
CL
CL
23 May 2017
• During start-up phase RBIAS( extremely high value) keeps
the Inverter at VM.
• Oscillation builds up with noise.
• The crystal passes only the resonant frequency voltage,
which gets amplifier.
• The amplified value gets passed through crystal which
again gets amplified.
• This goes on till Oscillation saturates.
Aatmesh
[email protected]
35
Pole to replace VCO and F to V for
stability analysis
REF
_
+
A
VCO
F to V
REF
OUT
_
OUT
A
+
=
Feedback portion of the system
Configuration for open-loop ac analysis of
the system.
A to OUT delay = 100uS
So if R=10K C=100pF
We choose R=50K and
C=100pF for our ac
analysis
23 May 2017
Aaatmesh
36
Addressing variability: Global variation
• Global variation will cause the shift in the current in PTAT which can cause the
temperature compensation to drift either in CTAT or PTAT direction.
MP1
Start-up
1
MN2
1
MP2
1
MN1
M>1
IB
• Vary RB through bit control to affect a good ztc
point.
RB
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Addressing variability: Local Mismatch
• Local mismatch can cause an offset which can cause frequency of VCO to be different
from Clock.
done
Counter
clock
ADC
Reg
Block
DAC
done
Clock coming
from crystal or
RF source
F to V
Converter
FV1
1
_
PTAT
VCO
0
Op. Amp.
Analog
Mux
+
FV2
F to V
Converter
control
out
• If we bit control the current source of FV2, we can set f(Clock)=f(VCO).
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Solution for high frequency clocks
• In order to get higher frequency output divide the VCO output by K just like a PLL.
• This would have very little impact on over all power consumption of the system.
done
Counter
ADC
clock
Reg
Block
DAC
done
Clock coming
from crystal or
RF source
F to V
Converter
1
_
PTAT
Op. Amp.
Analog
Mux
+
Divide/K
control
23 May 2017
VCO
0
F to V
Converter
out
39