Transcript 9. DS Analog to Digital Converters

DS Analog to Digital
Converters
Alex Doboli, Ph.D.
Department of Electrical and Computer Engineering
State University of New York at Stony Brook
Email: [email protected]
©Alex Doboli 2006
DS ADC
The chapter introduces the following aspects:
• Basic concepts of DS ADC & 1sr and 2nd order ADCs
• ADC are main subsystems in any embedded system
 DS ADC offer high resolution through two mechanisms:
– Oversampling: reduces in-band quantization noise
– Noiseshaping: eliminates in-band quantization noise
• PSoC implementation of DS ADC: modulator, decimator, API
©Alex Doboli 2006
Nyquist ADCs
Embedded system front end:
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Sampling
• Collect sufficient data for correctly representing a
continuous-time signal
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Nyquist Sampling Theorem
• A bandlimited signal can be exactly reconstructed if the sampling
Frequency is greater than twice the signal bandwidth
• Nyquist frequency is twice the signal bandwidth
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Sampling
Xs(f) = X(f) + X(f+/-fs) + X(f+/-2fs) + X(f+/-3fs) + X(f+/-4fs) + …
aliasing
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Quantization
• Quantization is the process of converting the sampled continuousValued signals into discrete-valued data
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Quantization
• Discretization range:
• Quantization error:
D = 2 / (2B - 1)
er ε (-D/2, D/2)
White noise
xd = x s + e r
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Quantization Error
• Quantization error is white noise & is uncorrelated to the input
• Bennett’s conditions:
• Input does not overload quantizer
• B is large
• D is small
• Joint probability density function of the input at
various sampling moments is smooth
©Alex Doboli 2006
Quantization Error
• Power spectral density
• Quantization noise power
s2e = D2 / 12
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DS Analog to Digital Converter
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Oversampling
• Oversampling frequency
• Oversampling Ratio (OSR)
• Advantages of high OSR:
• simplifies elimination of the images
• reduced in-band noise power
Pin-band = s2e / OSR
©Alex Doboli 2006
Noiseshaping
Y(z) = H(z) / (1 + H(z)) X(z) + 1 / (1 + H(z)) E(z)
STF
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NTF
DS ADC Performance
• Signal-to-noise ratio (SNR):
SNR (dB) = 10 log (signal power) / (in band quantization noise power)
– For sinusoidal input:
SNR (dB) = 6.02 B + 1.76 (dB)
SNR (dB) = 6.02 B + 10 log OSR
• Dynamic range (DR):
– Ratio between the output power for a sinusoidal input with fullrange amplitude and the output power of the smallest input
signal that it can distinguish and quantize
DR (dB) = 10 log (D2 / 8) / (in band quantization noise power)
B (bits) = (DR (dB) – 1.76) / 6.02
©Alex Doboli 2006
First-order DS Modulator
yd(t) = z-1 x(t) + (1 – z-1) e(t)
STF = z-1
NTF = 1 – z-1
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First-order DS Modulator: STF & NTF
STF
NTF
High pass
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Power Spectrum Density
Frequency of the input signal
noise shaping
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DS Modulator Performance
• In-band quantization noise power:
Pin-band = p2 / (9 OSR3)
• Signal to noise ratio for sinusoidal input:
SNR = 10 log (9 A2 OSR3) / (2 p2)
©Alex Doboli 2006
Dynamic Range
34 dB (5 bits)
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Dynamic Range vs. OSR
DR=34db (OSR=32)
DR=38dB (OSR=64)
DR=42dB (OSR=128)
DR=50dB (OSR=256)
(8 bits)
©Alex Doboli 2006
PSoC Implementation
Vin  (-Vref, Vref)
Vref  {VDD/2, 1.6 Vbandgap, Vexternal}
OSR = 64
Vin = (n – 128) / 128 Vref
©Alex Doboli 2006
PSoC Implementation
 DS modulator
– Uses programmable SC blocks
• Decimator
– Low pass filtering (eliminates high frequency images)
– Downconversion by factor OSR
– Sinc2 filter
– Implementation: hardware (integration) – software
(differentiation)
– Downconversion: timer produces an interrupt after OSR clock
cycles & ISR implements differentiations
• API routines
• Clocks
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Modulator Implementation
Single clock for entire design (4 x OSR)
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Measured PSD (OSR = 32)
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Measured PSD (OSR = 64)
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Sinc2 Decimator Filter (OSR=64)
H(z) = [1 / OSR x (1 – z-OSR) / (1 – z-1) ]2
fb/2 + m fs/2
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Sinc2 Decimator Filter
• Integration: in hardware using Type 1 decimator block
• Differentiation: in software at downconversion rate (4 x
OSR)
• Interrupts at 4 x OSR / fs
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Timer ISR
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API Routines
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API Routines
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Example
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Modeling of jitter noise
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Switch Thermal Noise
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Switch Thermal Noise
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Switch Thermal Noise
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OpAmp Noise
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Modeling of Slew Rate & Saturation
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Second Order DS Modulator
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Second-order DS Modulator
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PSoC Implementation
©Alex Doboli 2006