analog circuit design techniques at 0.5 v

Analog Circuit Design Techniques at 0.5 V

Shouribrata Chatterjee

Department of Electrical Engineering,

Indian Institute of Technology Delhi

2

Fully integrated 0.5 V analog circuits

• 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

Track-and-holds

Biasing circuits

Master Slave

1.2 mm1.2 m

mFilter PLL

OTAs

1 mm

1 mm

Biasing circuits

3[ITRS'04]

Thick Oxide Vdd

Thin Oxide Vdd

Threshold

Technology node [nm]

Vol

ts

Deep-sub-1 V for nano-scale CMOS devices

4

Latch-up

• Positive feedback structure if Q2 switches ON• Requires VDD to be more than at least one diode drop

5

Body effect

6

Body effect

0.5 V OTA design

8Assuming |VGS - VT | ≈ 0.15 V, |VT | = 0.5 V

OTA design challenges

0.15 V

0.5 V

0.65 V

0.8 V

-0.15 V

0.15 V

0.15 V

0.3 - 0.35 V

9

Basic body-input OTA stage

[S. Chatterjee, Y. Tsividis, P. Kinget, ESSCIRC 2004]

0.25 V 0.5 V 0.25 V

0.25 V 0.15 - 0.35 V0.1 V

10

Two-stage fully-differential body-input OTA

Pole splitting using Miller capacitor

11

Micro-photograph

Chip prototype

• 0.18 µm CMOS mixed-signal process:– Standard nMOS and pMOS devices,– High resistivity poly resistors, – MIM capacitors.

• Die Area: 0.026 sq. mm

Layout

12

Assuming |VGS - VT | ≈ 0.15 V, |VT | = 0.5 V

Basic gate-input OTA

0.5 V

0.4 V

0.4 V

0.15 - 0.35 V

0.4 V

0.25 V

0.1 V

0.1 V

13

0.5 V gate-input OTA gain stage

[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC 2005]

14

Two stage gate-input OTA

• Common-mode output of first stage is 0.4 V

• 55 dB gain, 15 MHz GBW, 60º PM for diff 10pF load

0.25 V

0.4 V

0.4 V

0.4 V

15

Two-stage gate-input fully differential 0.5 V OTA with Miller compensation

16

Setting common-mode voltages for the gate-input OTA

0.4 V

0.4 V

Rb = 2/3 • Ri||Rf

0.25 V 0.25 V

0.5 V

0.5 V

Gate-input OTA automatic biasing circuits

18

Error amplifier for biasing

• 20 kHz GBW for 1 pF load• 2 µA current• Controlled body voltage sets the amplifier threshold

Vin Vout

Vo

ut [

V]

Vin [V][S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC 2005]

19

On-chip biasing circuits

Vbn generating circuit

Level shift biasing circuit

(Simplified OTA)

20

OTA dc transfer characteristics and VNR generation

VNR generating circuit

Replica of OTA stage 1

Input differential voltage [mV]

Out

put

diff

vol

t age

[V

]

Increasing VNR

0.5 V body and gate-input OTA measurements

22

Body-input OTA open-loop frequency response

DC gain: 52 dB

GBW: 2.5 MHz

Phase Margin: 450

Simulation

Measurement

Frequency [Hz]

23Frequency [Hz]

Gai

n [d

B]

350 mV (Automatic gain-boosting)

50 mV

Increasing gain-boosting bias

GBW: 10 MHz

Gate-input OTA open-loop frequency response

0.5 V measured performance summaryParameter Body OTA Gate OTA

Power dissipation [µW] 110 75

Area [mm2] 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

Clo

sed

lo

op

Op

en l

oo

p

0.5 V weak-inversion varactor for frequency tuning

26

Filter tuning challenges at 0.5 V• Gm-C• MOSFET-C• Switching banks of R’s and C’s• Varactor-R techniques

Gate (0.4 V) Source

Drain (0.25 V)

Body (Vtune)

Vgate-Vtune or VGB [V]

Cg

s/C

ox

VGS = 0.15 V

VGS = 0.20 V

VGS = 0.25 V

[S. Chatterjee, Y. Tsividis, P. Kinget, VLSI 2005]

27

Capacitance characteristics

VGS [V]

Cg

s/C

ox

Region of interest for use as weak-

inversion varactor

Charge Sheet ModelChannel doping =

3.5e17/cm3 VFB = -1 V

Increasing VGB from -0.1V to 0.4V

28

5th order elliptic low-pass filter using tunable integrators

Frequency [Hz]

Gai

n [d

B]

280 kHz

135 kHz

[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC 2005]

29

Block diagram

30

Chip micrograph

• 0.18 µm CMOS• MIM capacitors• High-res resistors• Standard VT

• Triple well devices

Filter PLL

Biasing circuits

OTAs

1 mm

1 mm

31Frequency [Hz]

Gai

n [d

B]

Measured filter response for different supply voltages

32

Filter tuning through the varactor

Frequency [Hz]

Gai

n [d

B]

Notch at 120kHz, -42dB

200kHz, -50dB

VB [V]

Notch depth (sim.) [dB]

Notch depth

(meas.) [dB]

0.5 -45 -42

0.3 -47 -44

0.0 -53 -50

33

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

Tuning range [kHz] Vtune = VDD

Vtune = 0.0 V

96.5

153.0

88.0

154.5

84.5

148.0

69.0

150.5

VCO feed-thru @280kHz [µV rms] 104 85 72 72

Functionality tested from 5C to 85C at 0.5 V

• Measured CMRR (10 kHz common mode tone): 65 dB• Measured PSRR (10 kHz tone on power supply): 43 dB

0.5 V fully-differential track-and-hold circuit

35

Sampling challenges at 0.5 V

Large VDD

Small VDD

Enough headroom

No headroom

36

Basic track-and-hold architecture

•Voltages on both sides of the switches are signal independent.

•Signal-independent charge injection.

•Does this work at a 0.5V power supply?

voutvin

[Ishikawa, JSSC Dec 89]

37

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.

38

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.

39

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.

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

0.5 V fully-differential OTA

[S. Chatterjee, Y. Tsividis, P. Kinget, ISSCC’05, JSSC Dec’05]

44

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.

45

Test plan: two track-and-holds in cascade

CLKA

CLKB

[Vorenkamp, JSSC Jul 92]

46

Prototype-chip block diagram

47

Track-and-hold chip micrograph

•0.25µm CMOS

•|Vt|= 0.6V

•MIM capacitors•Triple-well

devices•High-resistivity

resistors•Chip fabrication

supported by

Philips.

Track-and-holds

Biasing circuits

Master Slave

1.2mm

1.2mm

48

Some simulated results at 0.5V, 1M-sample/sec

Time [sec]

Ou

tpu

t tra

nsi

ent

[V]

Input at FS/2 x

127/128

Output at FS/2 x 1/128

Input amplitude [dBV]

SN

DR

[dB

]62dB simulated dynamic

range

49

Typical time-domain output waveform

Time [µsec]

Out

put

diff

eren

tial v

olta

ge [

mV

]

Re-sampled 25kHz output for a 200mVpp input at 475kHz

50

Measured SNDR

Input differential rms [dBV]

SN

DR

[dB

]

51

T/H noise analysis and measurements

€

vn2 =10 kT

C⋅

1

1+6/(ωgbRC)

Integrated rms differential input-refd noise:

Simulated Noise: 200µVRMS

OTA GBW: 20MHz

MeasuredNoise: 188µVRMS

OTA GBW: 15MHz

52

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

53

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.

54

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-and-Hold 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.