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Analog Circuit Design
Techniques at 0.5 V
Shouribrata Chatterjee
Department of Electrical Engineering,
Indian Institute of Technology Delhi
Fully integrated 0.5 V analog circuits
1.2 mm
1 mm
Biasing circuits
Master Slave
1.2 mm
PLL
1 mm
Filter
OTAs
Biasing circuits
•
•
•
•
Track-and-holds
Gate and body-input OTAs
PLL-tuned filter and track-and-hold circuits
Operation from 0.45 V to 0.6 V
True low-voltage circuits for the nano-scale era
2
Deep-sub-1 V
for nano-scale CMOS devices
Thin Oxide Vdd
Volts
Thick Oxide Vdd
Threshold
Technology node [nm]
[ITRS'04]
3
Latch-up
• Positive feedback structure if Q2 switches ON
• Requires VDD to be more than at least one diode drop
4
Body effect
5
Body effect
6
0.5 V OTA design
OTA design challenges
0.5 V
0.15 V
-0.15 V
0.3 - 0.35 V
0.8 V
0.15 V
0.65 V
0.15 V
Assuming |VGS - VT | ≈ 0.15 V, |VT | = 0.5 V
8
Basic body-input OTA stage
0.25 V
0.25 V
0.5 V
0.25 V
0.1 V
0.15 - 0.35 V
[S. Chatterjee, Y. Tsividis, P. Kinget, ESSCIRC 2004]
9
Two-stage fully-differential bodyinput OTA
Pole splitting using Miller capacitor
10
Chip prototype
Layout
Micro-photograph
• 0.18 µm CMOS mixed-signal process:
– Standard nMOS and pMOS devices,
– High resistivity poly resistors,
– MIM capacitors.
• Die Area: 0.026 sq. mm
11
Basic gate-input OTA
0.1 V
0.5 V
0.1 V
0.15 - 0.35 V
0.4 V
0.25 V
0.4 V
0.4 V
Assuming |VGS - VT | ≈ 0.15 V, |VT | = 0.5 V
12
0.5 V gate-input OTA gain stage
[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC 2005]
13
Two stage gate-input OTA
0.4 V
0.4 V
0.25 V
0.4 V
• Common-mode output of first stage is 0.4 V
• 55 dB gain, 15 MHz GBW, 60º PM for diff 10pF load
14
Two-stage gate-input fully differential 0.5 V
OTA with Miller compensation
15
Setting common-mode voltages
for the gate-input OTA
0.5 V
Rb = 2/3 • Ri||Rf
0.4 V
0.25 V
0.25 V
0.4 V
0.5 V
16
Gate-input OTA automatic biasing
circuits
Error amplifier for biasing
Vin
Vout [V]
• 20 kHz GBW for 1 pF load
• 2 µA current
• Controlled body voltage
sets the amplifier threshold
Vout
[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC 2005]
Vin [V]
18
On-chip biasing circuits
Level shift biasing circuit
(Simplified OTA)
Vbn generating circuit
19
OTA dc transfer characteristics
and VNR generation
VNR generating circuit
Increasing VNR
Replica of OTA
stage 1
Input differential voltage [mV]
20
0.5 V body and gate-input OTA
measurements
Body-input OTA open-loop
frequency response
DC gain: 52 dB
GBW: 2.5 MHz
Measurement
Simulation
Phase Margin: 450
Frequency [Hz]
22
Gate-input OTA open-loop
frequency response
Gain [dB]
350 mV (Automatic gain-boosting)
Increasing gainboosting bias
50 mV
GBW:
10 MHz
Frequency [Hz]
23
0.5 V measured performance summary
Parameter
Body OTA
Gate OTA
110
75
0.026
0.017
Load capacitance [pF] (single-ended)
20
20
Offset standard deviation (20 samples) [mV]
3
2
Open-loop DC gain [dB] (diff.)
52
62/42
Open-loop unity-gain BW [MHz] (diff.)
2.5
10.0
Slew rate [V/µsec] (diff.)
2.9
2.0
Closed-loop unity-gain BW [MHz] (diff.)
2.2
5.0
CMRR @ 5 kHz [dB]
78
74
PSRR @ 5 kHz [dB]
76
81
Input ref. noise @ 10kHz [nV/sqrt-Hz] (diff.)
280
225
Input ref. noise @ 1MHz [nV/sqrt-Hz] (diff.)
80
70
Output amp. for 1% THD [mV p-p] (diff.)
400
712
Power dissipation [µW]
Closed loop
Open loop
Area [mm2]
0.5 V weak-inversion varactor for
frequency tuning
Filter tuning challenges at 0.5 V
•
•
•
•
Gm-C
MOSFET-C
Switching banks of R’s and C’s
Varactor-R techniques
Body (Vtune)
Gate
(0.4 V)
Source
Drain
(0.25 V)
Cgs/Cox
[S. Chatterjee, Y. Tsividis, P.
Kinget, VLSI 2005]
VGS = 0.25 V
VGS = 0.20 V
VGS = 0.15 V
Vgate-Vtune or VGB [V]
26
Capacitance characteristics
Charge Sheet Model
Cgs/Cox
Channel doping =
3.5e17/cm3
VFB = -1 V
Increasing VGB
from -0.1V to 0.4V
Region of interest
for use as weakinversion varactor
VGS [V]
27
5th order elliptic low-pass filter
using tunable integrators
Gain [dB]
135 kHz
[S. Chatterjee, Y. Tsividis,
P. Kinget, ISSCC 2005]
280 kHz
Frequency [Hz]
28
Block diagram
29
Chip micrograph
1 mm
PLL
1 mm
Filter
• 0.18 µm CMOS
• MIM capacitors
• High-res resistors
• Standard VT
• Triple well devices
OTAs
Biasing circuits
30
Gain [dB]
Measured filter response for
different supply voltages
Frequency [Hz]
31
Gain [dB]
Filter tuning through the varactor
VB
[V]
Notch at
120kHz, -42dB
Notch Notch
depth depth
(sim.) (meas.)
[dB]
[dB]
0.5
-45
-42
0.3
-47
-44
0.0
-53
-50
200kHz, -50dB
Frequency [Hz]
32
Filter performance summary at 27C
VDD [V]
0.45
0.50
0.55
0.60
-3 dB cut-off frequency [kHz]
135.0
135.0
135.0
135.0
Total current [mA]
1.5
2.2
3.3
4.3
Noise [µV rms]
87
74
68
65
Input [mV rms] (100kHz / 1% THD)
50
50
50
50
In-band IIP3 [dBV]
-5
-3
-3
-3
Out-of-band IIP3 [dBV]
3
5
3
5
Dynamic range [dB]
55
57
57
58
96.5
153.0
88.0
154.5
84.5
148.0
69.0
150.5
104
85
72
72
Tuning range [kHz]
Vtune = VDD
Vtune = 0.0 V
VCO feed-thru @280kHz [µV rms]
• Measured CMRR (10 kHz common mode tone): 65 dB
• Measured PSRR (10 kHz tone on power supply): 43 dB
Functionality tested from 5C to 85C at 0.5 V
33
0.5 V fully-differential track-andhold circuit
Sampling challenges at 0.5 V
Large VDD
Small VDD
Enough
headroom
No
headroom
35
Basic track-and-hold architecture
vin
vout
[Ishikawa, JSSC Dec 89]
• Voltages on both sides of the switches are
signal independent.
• Signal-independent charge injection.
• Does this work at a 0.5V power supply?
36
Differential implementation at 0.5V
• Gate-input OTA used.
• Track phase during 1,
hold phase during 2.
• During track phase, pole
and zero cancel out to
enable fast response.
• pMOS switches have VT
of about 0.5V.
37
Track mode operation
• Resistors to 0.5V
maintain required
OTA input CM
voltage of 0.4V.
• To enable better
switching, both gate
and body of the
switch are used.
• No voltage swing on
either side of the
switches.
38
Hold mode operation
• Gate and body of the
switch used for
better switching.
• No signal swing on
both sides of the
switches.
• OTA input voltages
held constant.
39
0.5 V fully-differential OTA
[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC’05, JSSC Dec’05]
40
0.5 V fully-differential OTA
[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC’05, JSSC Dec’05]
41
0.5 V fully-differential OTA
[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC’05, JSSC Dec’05]
42
0.5 V fully-differential OTA
[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC’05, JSSC Dec’05]
43
Design targets
• 1MHz of sampling rate.
• 60 dB of signal to noise-distortion range.
•
•
•
•
•
OTA worst case gain-bandwidth of 20MHz.
Worst case slew rate of 6V/µs.
Sampling capacitor of 1pF.
To be designed using devices with VT 0.5 - 0.6V
Switches sized to optimize resistance for
settling, minimize noise, feedthrough.
44
Test plan: two track-and-holds in
cascade
CLKA
[Vorenkamp, JSSC Jul 92]
CLKB
45
Prototype-chip block diagram
46
Track-and-hold chip micrograph
1.2mm
Biasing circuits
1.2mm
Master
Slave
Track-and-holds
• 0.25µm CMOS
• |Vt|= 0.6V
• MIM capacitors
• Triple-well
devices
• High-resistivity
resistors
• Chip fabrication
supported by
Philips.
47
Input at
FS/2 x
127/128
Output transient [V]
Some simulated results at 0.5V,
1M-sample/sec
Output at
FS/2 x
1/128
62dB
simulated
dynamic
range
SNDR [dB]
Time [sec]
Input amplitude [dBV]
48
Output differential voltage [mV]
Typical time-domain output
waveform
Time [µsec]
Re-sampled 25kHz output for a 200mVpp input at 475kHz
49
SNDR [dB]
Measured SNDR
Input differential rms [dBV]
50
T/H noise analysis and
measurements
Integrated rms differential
input-refd noise:
kT
1
v 10

C
1 6/(  gb RC)
2
n

Simulated
Noise:
OTA GBW:
200µVRMS
20MHz
Measured
Noise:
OTA GBW:
188µVRMS
15MHz
51
Measured performance
Power supply
0.5V
Current consumption
600µm
Sampling rate
1Msps
Diff. input refd. integrated noise
188µVRMS
Peak SNDR fIN=50kHz; Vin,diff=178mVRMS
60dB
Peak SNDR fIN=495kHz; Vin,diff=100mVRMS
57dB
Hold mode droop rate on diff. output
7.6µV/µV
Pedestal on diff. output
0.8mV
Track mode bandwidth
3.9MHz
52
Conclusions
• Developed true low voltage design
techniques for 0.5 V analog circuits.
• 0.5 V gate and body-input OTAs designed can be used as building blocks.
• Robust automatic biasing techniques
developed.
• Weak-inversion MOS varactor developed.
• PLL-tuned 5th-order LPF demonstrated.
• 0.5 V track-and-hold circuit proposed.
• Step towards nano-scale circuits.
53
Further reading
• S. Chatterjee, Y. Tsividis, P. Kinget, “0.5-V Analog
Circuit Techniques and Their Application to OTA and
Filter Design”, IEEE Journal of Solid State Circuits,
Dec. 2005, vol. 40, no. 12, pp. 2373-2387.
• S.Chatterjee, P.Kinget, “A 0.5-V 1-Msps Track-andHold Circuit with 60-dB SNR”, IEEE Journal of Solid
State Circuits, Apr. 2007, vol. 42, no. 4, pp. 722-729.
• K. Pun, S. Chatterjee, P. Kinget, “A 0.5-V 74-dB
SNDR 25-kHz Continuous-Time Delta-Sigma
Modulator with a Return-to-Open DAC”, IEEE Journal
of Solid State Circuits, Mar. 2007, vol. 42, no. 3, pp.
496-507.
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