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Sample & hold amplifiersADC parameters
Alessandro SpinelliTel. (02 2399) 4001
[email protected]/spinelli
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Elettronica 75513 Alessandro Spinelli
Slides are supplementary material and are NOT a
replacement for textbooks and/or lecture notes
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Sample & hold amplifiers (SHAs)
Elettronica 75513 Alessandro Spinelli
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Basic SHA scheme
β’ R is the resistance of the non-ideal switchβ’ In the ideal case, β π‘π‘ β ππ = πΏπΏ(π‘π‘ β ππ)
Elettronica 75513 Alessandro Spinelli
CR
Ο
β(π‘π‘ β ππ)
t
sample
S/H
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Output S/N
β’ The output noise is
πππ π π π 2 = 4πππ΅π΅ππππ1
4πππ π =πππ΅π΅πππ π
β’ A gate time πππΊπΊ βͺ πππ π could be used for fast sampling (gated integrator). The output noise(for unity signal gain) would be
πππΊπΊπΊπΊ2 = 4πππ΅π΅ππππ1
2πππΊπΊβ« πππ π π π 2
Elettronica 75513 Alessandro Spinelli
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Parameters
Elettronica 75513 Alessandro Spinelli
From [1]
Here SHA behaves as an amplifier/followerβ parameters are similar
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Pedestal or Hold step
The error is due to parasitic injection throughthe non-ideal switch
Ξππ β Ξππππ/π»π»π π ππ
π π ππ + π π Elettronica 75513 Alessandro Spinelli
S/H
C
π π ππFrom [1]
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Aperture time and jitter
Elettronica 75513 Alessandro Spinelli
From [2] From [1]
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Droop rate
Elettronica 75513 Alessandro Spinelli
From [1]
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Input feedthrough
β’ Similar to hold step, Ξππ β πππππ π ππ/(π π ππ + π π )β’ π π ππ can be due to the switch as well as to
parasitic layout coupling,β¦Elettronica 75513 Alessandro Spinelli
From [3]
C
π π ππ
S/H
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Acquisition time
Elettronica 75513 Alessandro Spinelli
From [2]
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Improved scheme
Elettronica 75513 Alessandro Spinelli
β
+Vi
Voβ
+
CS/H
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Pros and Cons
β’ Advantagesβ Impedance decouplingβ Insensitive to ππππππ2
β’ Limitationsβ Limited bandwidth owing to the stability
requirement when the loop is closedβ Saturation of the first OPAMP in Hold mode β
long acquisition time
Elettronica 75513 Alessandro Spinelli
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Prevention of saturation
Elettronica 75513 Alessandro Spinelli
β
+Vi Voβ
+
CS/H
S/H
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SHA with integrator
Elettronica 75513 Alessandro Spinelli
β
+Vi
Voβ
+ C
S/HS/H
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Pros and Cons
β’ Advantagesβ The S/H switch operates always near zero bias
β’ Pedestal error is not input-dependentβ’ Input feedthrough during Hold mode is reduced
β’ Limitationsβ Limited bandwidth owing to the stability
requirement when the loop is closed
β’ Differential architectures with complementaryswitches can be used to improve performance
Elettronica 75513 Alessandro Spinelli
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ADC DC Parameters
Elettronica 75513 Alessandro Spinelli
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Quantization error
Elettronica 75513 Alessandro Spinelli
From [4]
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Offset error
Offset or Zero-scale error is the differencebetween actual and ideal first transition voltage
Elettronica 75513 Alessandro Spinelli
Modified from [4]
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Offset and dynamic range
Offset leads to a loss in the input range, i.e., a smaller dynamic rangeElettronica 75513 Alessandro Spinelli
From [5]
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Gain and FS errors
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DNL
β’ In an ideal ADC, the transitions are 1 LSB apartβ’ The difference between the actual and the
ideal code width is called DNLβ’ DNL is measured for each input range or
output code; however, only the maximum absolute value is reported
Elettronica 75513 Alessandro Spinelli
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DNL and missing codes
Elettronica 75513 Alessandro Spinelli
0 LSB;DNL(100) = -1
From [4]
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Remarks
β’ If DNL is defined for each output code (see[6]), obviously DNL = -1 β missing code!
β’ If DNL is defined for each input range (see [4]), it is aways > -1. DNL β₯ 1 does not imply the existence of missing codes
β’ The manufacturer usually specifies if the ADC has missing codes
β’ If |DNL| < 1 β ADC is monotonic
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INL
β’ INL is the maximum difference between the real characteristic and an ideal linear behavior
β’ INL is the sum of DNL errorsElettronica 75513 Alessandro Spinelli
From [4]
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Which straight line?
β’ Offset and gain errors are first removedβ’ End-point INL is easier and most practicalβ’ Best-fit INL gives lower values but is impractical in
most applicationsElettronica 75513 Alessandro Spinelli
Analog INAnalog IN
Digital OUT Digital OUT
INL INL
End-points INL Best-fit INL
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Other parametersβ¦
β’ Code-edge noiseβ’ Output noiseβ’ Voltage reference specs
β T driftβ Voltage noiseβ Long-term stabilityβ Load regulationβ β¦
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ADC AC Parameters
Elettronica 75513 Alessandro Spinelli
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SNR
β’ Maximum amplitude of sinusoid is π΄π΄ = ππππππππ/2β’ Signal power
ππππ2 = π΄π΄2 sin2 πππ‘π‘ =π΄π΄2
2=ππππππππ2
8=
22ππΞ2
8β’ Noise power = quantization noise Ξ2/12
ππππππ2 =22ππΞ2
812Ξ2
=32
22ππ
πππππππππ΅π΅ = 10 log1032
22ππ = 6.02ππ + 1.76
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SINAD
β’ If additional noise is present, SNR decreaseswith respect to the ideal value
β’ Another component is(harmonic) distorsion
πππΌπΌπππ΄π΄πΌπΌ =rms signal
rms (noise + distorsion)< SNR
Elettronica 75513 Alessandro Spinelli
From [7]
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SINAD vs. frequency
Elettronica 75513 Alessandro Spinelli
From [7]
πππ π
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ENOB
β’ SINAD is expressed as an effective number of bits (ENOB) via the SNR formula
πΈπΈπππΈπΈπΈπΈ =πππΌπΌπππ΄π΄πΌπΌπππ΅π΅ β 1.76
6.02< ππ
β’ Donβt confuse ENOB with resolution bits
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SFDR
SFDR represents the smallest signal that can be distinguished from a large interference
Elettronica 75513 Alessandro Spinelli
From [7]
πππππΌπΌππ =rms signal
rms worst spur
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ADC technologies
Elettronica 75513 Alessandro Spinelli
FLASH (Parallel) PIPELINE SAR SIGMA DELTA DUAL SLOPE (Integrating)
Pick This Architecture if you want:
Ultra-high speed when power consumption not a primary concern
High speeds, few Msps to 100+ Msps, 8 bits to 16 bits, lower power consumption than flash
Medium to high resolution (8 to 16bit), 5Msps and under, low power, small size
High resolution, low to medium speed, no precision external components, simultaneous 50/60Hz rejection, digital filter reduces anti-aliasing requirements
Monitoring DC signals, high resolution, low power consumption, good noise performance
Conversion Method
N bits β 2N - 1 comparators caps increase by a factor of 2 for each bit
Small parallel structure, each stage works on one to a few bits
Binary search algorithm, internal circuitry runs higher speed
Oversampling ADC, 5-Hz β60 Hz rejection programmable data output
Unknown input voltage is integrated and value compared against known reference value
Disadvantages
Sparkle codes / metastability, high power consumption, large size, expensive
Parallelism increases throughput at the expense of power and latency
Speed limited to ~5Msps. May require anti-aliasing filter
Higher order (4th order or higher) - multibit ADC and multibit feedback DAC
Slow conversion rate. High precision external components required to achieve accuracy
Conversion Time Does not change with increased resolution
Increases linearly with increased resolution
Increases linearly with increased resolution
Tradeoff between data output rate and noise free resolution
Doubles with every bit increase in resolution
ComponentMatchingRequirements
Typically limits resolution to 8 bits
Double with every bit increase in resolution
Double with every bit increase in resolution
Double with every bit increase in resolution
Does not increase with increase in resolution
Size
2^N-1 comparators, Die size and power increases exponentially with resolution
Die increases linearly with increase in resolution
Die increases linearly with increase in resolution
Core die size will not materially change with increase in resolution
Core die size will not materially change with increase in resolution
From [8]
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Resolution and conversion rate
Final choice will depend on application(see discussion in [9])
Elettronica 75513 Alessandro Spinelli
Flash
Integrating
From [9]
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References1. http://www.analog.com/static/imported-files/tutorials/MT-
090.pdf2. http://www.ti.com/lit/an/snoa223/snoa223.pdf3. http://www.analog.com/static/imported-
files/application_notes/4886613927731859762198006746AN270.pdf
4. http://www.unirioja.es/cu/lzorzano/ABCs_of_ADCs.pdf5. http://www.maximintegrated.com/app-notes/index.mvp/id/7486. http://www.freescale.com/files/microcontrollers/doc/app_note/A
N2438.pdf7. http://www.analog.com/static/imported-files/tutorials/MT-
003.pdf8. http://www.maximintegrated.com/app-notes/index.mvp/id/20949. http://www.ti.com/europe/downloads/Choose%20the%20right%2
0data%20converter%20for%20your%20application.pdf
Elettronica 75513 Alessandro Spinelli