Low-power Sigma-Delta AD Converters

Transcription

1 Low-power Sigma-Delta AD Converters Willy Sansen KULeuven, ESAT-MICAS Leuven, Belgium Willy Sansen

2 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Ref. Norsworthy, Delta-Sigma Converters, Wiley 1996 Ref. Op t Eynde, Peluso, Geerts, Marquez, Geerts, Yao, Kluwer/Springer Willy Sansen

3 Sigma-Delta ADC Sigma-Delta ADC exchanges resolution with speed by means of : Oversampling Noise shaping Willy Sansen

4 Quantization noise (4 bit) Number bits Quant. noise Quantization noise Step = V ref 2 B B = 8 bits SNR = 50 db B = 4 B =16 bits SNR = 98 db SNR 2 + 6B Willy Sansen

5 Sigma-Delta modulator Signal transfer function: Noise transfer function: H H x e ( z) ( z) H ( z) = 1 + H ( z) 1 = 1 + H ( z) Willy Sansen

6 Noise filtering x + - Σ k 2 y v H k 1 LPF k 2 e - + y v = H (x - k 2 y) y = k 1 v + e Noise shaping } k 1 H y = x + e x + e 1 + k 1 k 2 H 1 + k 1 k 2 H k 1 k 2 H k 2 Willy Sansen

7 Feedback loop with low-pass filter H 1 1 y x + e k 1 k 2 H k 2 y f m f x k 2 SNR e k 1 k 2 H Noise shaping f m f Willy Sansen

8 Higher-order Sigma-delta converters Willy Sansen

9 Mash Sigma-delta topologies (2-1-1) Willy Sansen

10 SNR vs OSR for single-bit Σ SNR (db) Stability Power Oversampling ratio OSR 100 Willy Sansen

11 Multibit versus Single-bit Cascaded Single loop 4th Cascaded 2-1 Multibit (4 bit) Ref. Marquez CAS Sept 98, Willy Sansen

12 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Willy Sansen

13 Low Voltage SC: problem Φ 1 Switch: V in V out Φ 1 C 1 nmos: V in < pmos: V in > V DD -V GSn V DD -0.8 V V GSp 0.8 V Limit : V DD -V GSn = V GSp V DDmin > 1.6 V Willy Sansen

14 Low Voltage switch : g DS versus input voltage g DS 5 V g DS 1 V nmos pmos nmos? pmos V Tp 0 V DD -V Tn V DD 0 V DD -V Tn V Tp V DD Willy Sansen

15 Low Voltage switch : ON- resistance Ω 4.0 k 3.0 k V DD = 5 V 2.0 k 60 k 40 k 20 k 0 k VIN VIN V DD = 1.8 V Willy Sansen

16 Low Voltage SC: solutions Low V T techology special technology : cost switch-off leakage On-Chip voltage multipliers poor power efficiency applicability in submicron technologies? Switched Opamp Ref.Crols, ESSCIRC 93, JSSC Aug.94 Willy Sansen

17 Smaller V DD require smaller V T Smaller V T is not possible because 1. Leakage : wi curve crosses axis! Minimum value : 0.3 V 2. Temperature variations : V log i DS leakage 3. Mismatch : V 0 V T V GS >>> V T cannot be smaller than V Willy Sansen

18 On-chip voltage multipliers Dickson,.., JSSC June 76, pp Willy Sansen

19 Voltage multipliers : power efficiency P loss R eq I 2 out P VDD I out V DD η 1 - R eq I out V DD 50 % R eq n fc 1 tan (2f R on,sw C) Willy Sansen

20 Drawbacks of voltage multipliers High voltage technology: In deep submicron : V DD < 1.8 V in 0.18 µm CMOS Oxide cannot take more!! 800 V/µm or 0.8 V/nm Requires high-speed clock drivers Injection in substrate : coupling to Analog Low power-efficiency Willy Sansen

21 Voltage multiplier for rail-to-rail opamp Vddx - Vdd 1 V 60 µa 10 MHz ripple I out 2C o f c 1 g m5 r o5 5 mv Duisters,.., JSSC July 98,pp Willy Sansen

22 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Principle : Switched-opamp filter Improved switching 0.9 V - 40 µw 12 bit CMOS SO Σ Other low-power Delta-sigma converters Willy Sansen

23 Conventional SC Integrator C v IN Φ1 ac Φ2 - V DD v OUT Φ2 Φ1 + V REF A v = a Willy Sansen

24 Switched Opamp Critical input switch is replaced by a switched opamp φ2 φ2 φ1 φ2 φ1 φ2 φ1 φ2 V ref V ref V ref V ref V ref V ref Crols,.., JSSC Aug.94, Willy Sansen

25 Switched-opamp schematic Vdd M10 M8 M7 M5 φ Ibias Vin- M1 M2 Vin+ Cc Vout M6 Vss M3 M4 M9 φ Crols,.., JSSC Aug.94, Willy Sansen

26 Switched-Opamp response 400mV s 0.5u s 1.0u s 1.5u s 2.0u s 2.5u s Time 3.0u s s Willy Sansen

27 Switched-opamp low-pass biquad One extra opamp per biquad V DD > V GS + V signal 1.2 V V = 1.5 V (0.6 V ptp ) Standard 2.4 µm CMOS (V Tn V Tp 0.9 V) Crols,.., JSSC Aug.94, Willy Sansen

28 Measured transfer characteristic BW = 1.5 khz Willy Sansen

29 THD versus input signal swing -60 db for < 0.6 V ptp Input noise 140 µv RMS : DR > 70 db Crols,.., JSSC Aug.94, Willy Sansen

30 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Principle : Switched opamp filter Improved switching 0.9 V - 40 µw 12 bit CMOS SO Σ Other low-power Delta-sigma converters Willy Sansen

31 1 Volt OTA 1 V (min: V T +2V DSsat ) Fully differential : 75 db 30 MHz 1 pf 80 µa < 100 ns 4 Switches 2n : Only 2nd stage switched off! Baschirotto,.. JSSC Dec.97,pp Willy Sansen

32 SO SC integrator CDC for level shift : CDC= CIN V O,DC = V DD /2 Baschirotto,.. JSSC Dec.97, pp Willy Sansen

33 SO SC integrator : Φ1 closed CDC for level shift : CDC= CIN V O,DC = V DD /2 Q TOT = V DD C IN Baschirotto,.. JSSC Dec.97,pp Willy Sansen

34 SO SC integrator : Φ2 closed CDC for level shift : CDC= CIN V O,DC = V DD /2 Q TOT = V DD C DC + V O C IN if C DC = C IN V DD /2 = V O Baschirotto,.. JSSC Dec.97,pp Willy Sansen

35 CMFB with level shifting C M = C P = 0.1 pf C CM = 0.1 pf C CMFB = 2 pf C FF = 0.1 pf provides zero V OUT,DC = V DD /2 Willy Sansen

36 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Principle : Switched opamp filter Improved switching 0.9 V - 40 µw 12 bit CMOS SO Σ Other low-power Delta-sigma converters Willy Sansen

37 Differential SO integrator C S Sampling C INT Integrat. C L Load C CM Level shift C CMS,eq CMFB V REF,hI = V DD V REF,lo = 0 Peluso,..., JSSC, Dec.98, Peluso etal Design of low-voltage low-power CMOS Delta-Sigma ADC s, Kluwer 1999 Willy Sansen

38 Σ topology with half-delay integrators - 3rd order single-loop implementation - coefficients a 1 = 0.2 ; a 2 = 0.5 ; a 3 = 0.5-1/2 phase delays in feedback path Willy Sansen

39 Class AB differential Voltage amplifier 1 V V T = 0.6 V M3 V in2 M4 V GS -V T = 0.2 V V GS = 0.8 V V DSsat = 0.2 V V in1 M1 M2 V in2 i out M2 is source follower V GS1 = V in1 -V in2 I B1 I B2 i out ~ (V in1 -V in2 ) 2 >>> Class AB Peluso,..., JSSC, Dec.98,pp Willy Sansen

40 Differential class AB OTA Willy Sansen

41 Class AB characteristic Larger input W/L Larger current source W/L Willy Sansen

42 CMFB and level-shift out2 out1 Willy Sansen

43 Low voltage comparator (level shift omitted) Two switches Input at V SS Willy Sansen

44 The input integrator V REF,hi = V DD V REF,lo = 0 Willy Sansen

45 Spectrum for maximum input signal (470 mv ptp ) BW 16 khz Clock freq. 1.5 MHz Peak SNR 76 db Peak SNDR 62 db Peluso,..., JSSC Dec.98,pp Willy Sansen

46 SNDR versus input signal level Peak SNR = 76 db DR = 77 db SNDR = 62 db Willy Sansen

47 SO 12 bit 0.9 V 40 µw CMOS Σ INT1 INT2 INT3 COMP 0.5 µm CMOS V Tn = 0.62 V V Tp = 0.55 V V DD = 0.9 V 40 µw Peluso,... JSSC Dec.98, pp Willy Sansen

48 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Unity-gain-reset Optimized input switching Switched input resistor Full feedforward Willy Sansen

49 Reset-opamp integrator / 2 / 2 V out = 0 1 V V out,av 0.5 V V in 0 V previous stage V dd /2 0.5 V On Φ 1 : Q 1 = C 1 V in Q 2 V out = 0 On Φ 2 : Q 1 = 0 Q 2 + C 1 V in V out Level shift needed to avoid forward biased junctions! Keskin,.., JSSC July 02, Willy Sansen

50 Pseudo-differential opamp 170 MHz 100 V/µs 3.5 pf 1 V 200 µa 0.35 µm CMOS V Tn 0.52 V V Tp 0.45 V Ref.Keskin, JSSC July 2002, Willy Sansen

51 1-Volt 2nd-order 13 -bit Σ modulator Willy Sansen

52 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Unity-gain-reset Optimized input switching Switched input resistor Full feedforward Willy Sansen

53 Σ Modulator on 1 Volt in 90 nm CMOS x z 1 z 1 z y Single-loop third-order single-bit topology Simple and robust Tolerance to building block non-idealities Coefficients selected not sensitive to capacitance mismatches Yao,..., JSSC Nov.04, Yao. etal. Low-Power Low-Voltage Σ modulators in Nanometer CMOS, Springer 06 Willy Sansen

54 Gain enhancement 2I 1 inp M1 inn outn outn K*I 1 1:B M2 M3 A = (1 k )( V GS 2 0 V T ) 1 λ 3 = A 1 k Willy Sansen

55 Stability I 1 I 2 The non-dominate pole must be > 3GBW for sufficient phase margin K*I 1 M1 M2 inn 1:B Cc M3 CL P P nd GBW = nd = gm2 2π C c = B gm 2π C 1 L > 3GBW k 2(1 k) I1 2π C ( V V = c L GS 2B I 2π C ( V C < 1 3B C c L 1 GS T ) 2 V T ) 1 Willy Sansen

56 Full OTA circuit VDDA Bp CMFB 1:2 outp inp inn outn 1:K : db 57 MHz 6 pf 80 µa (1 V) VSSA Yao,..., JSSC Nov.04, Willy Sansen

57 Full modulator circuits refp refn refp refn cmo cmi refp refn cmo cmi cmi Lp Ln cmi Ln Lp c2 c1 Ln Lp c2 c1 Ci1p Ci2p Ci3p d d c1 inp c1 d c2 d c1 Cs1p c2 + _ c1 d c1 d Cs2p Cs2p c2 + _ c1 d c1 d Cs3p Cs3p c2 + _ c2 + _ Lp OTA1 cmo OTA2 cmo OTA3 QTZ inn c1 d c2 d Cs1n c1 cmi c2 _ + c1 d c1 d Cs2n Cs2n c2 _ + c1 d c1 d Cs3n Cs3n c2 _ + c2 _ + Ln Ln refp Lp refn Ci1n Lp Ln c2 c1 Ci2n Lp Ln c2 c1 Ci3n d d refp refn cmo cmi refp refn cmo cmi cmi Willy Sansen

58 Measurement Output spectrum Output spectrum of a 5 khz input signal Willy Sansen

59 Measured SNR and SNDR vs input amplitude Yao,..., JSSC Nov.04, Willy Sansen

60 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Unity-gain-reset Optimized input switching Switched input resistor Full feedforward Willy Sansen

61 Switched-resistor integrator Input switch replaced by resistor R Larger resistor for better linearity Smaller resistor for higher speed Ahn,.. ISSCC 05, Willy Sansen

62 Input sampling : maintain constant V INCM C Ls added Baschirotto, JSSC Dec.1997, R & C S doubled Ahn,.. ISSCC 05, Willy Sansen

63 Mash 2-2 Σ Audio ADC Low-distortion : - switched resistor - loop filter processes only quantization error OSR = khz 3 MHz clock SNDR = 78 db 0.35 µm CMOS 0.6 V 1 mw Ahn,.. ISSCC 05, Silva, Electronic Letters, June 01, Willy Sansen

64 4-th order Σ converter with switched-resistors Mash 2-2 OSR = µm CMOS Two-stage opamp with folded cascode 0.6 V 1 mw 24 khz BW Willy Sansen

65 Measured SNDR Low-distortion : Vref = 0.6 V SNR SNDR = 78 db at 1 khz Willy Sansen

66 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Unity-gain-reset Optimized input switching Switched input resistor Full feedforward Willy Sansen

67 Full Feedforward Topology Convent. Sigma-Delta topology Full feedforward topology H H x e ( z) ( z) ai 1 = 1 + ai 1 1 = 1 + ai 1 H H x e ( z) = 1 ( z) = aci 1 1 Silva, Electronic Letters, June 01, Willy Sansen

68 4th-Order single-bit 1 Ms/s Σ modulator Single feedback loop : processes quantization noise only Full feedforward : unity-gain transfer 4th order - single bit Optimization coefficients or equal swing Yao,.., VLSI Circuits 05 Yao. etal. Low-Power Low- Voltage Σ modulators in Nanometer CMOS, Springer 06 Willy Sansen

69 Performance comparison 4th-order conventional topology Behavioral simulation with: a 1 =-0.1, a 2 =-0.1, A 0 =40 db 4th-order full feedforward topology Behavioral simulation with: a 1 =-0.1, a 2 =-0.1, A 0 =40 db Willy Sansen

70 Output swing of integrators Output swings of each integrator in the conventional topology Output swings of each integrator in the full-feedforward topology Willy Sansen

71 Single-stage OTA for fast settling (0.13 µm) Yao,.., VLSI Circ.05 Yao. etal. Low-Power Low-Voltage Σ modulators in Nanometer CMOS, Springer 06 Willy Sansen

72 Circuit Realization OSR = 64 Clock of 64 MHz 1 V 6.1 ma ma Yao,.., VLSI Circuits, 05 Yao. etal. Low-Power Low-Voltage Σ modulators in Nanometer CMOS, Springer 06 Willy Sansen

73 Measured output spectrum Willy Sansen

74 Measured SNR versus Input voltage Willy Sansen

75 Comparison of Low-power Σ converters Ref. Type V DD DR BW P FOM V db khz µw x 10-6 Ahn 05 SwR Sauerbrey 02 SO,LV Peluso 98 SO Dessouky 01 LV Keskin 02 ResetOp Yao 04 LV Rabii 96 SC, VM Yin k 346 Geerts k 144 Vleugels k 700 Gaggl k 400 Yao k 706 Doerrer 05 Track k 280 Hezar k 737 FOM = 4kT DR BW / P Willy Sansen

76 Low-voltage low-vt comparison Sauerbrey Ahn Sauerbrey,.., JSSC Dec Willy Sansen

77 Table of contents Delta-sigma modulation The switch problem The switched-opamp solution Other low-power Delta-sigma converters Willy Sansen