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Introduction to Analog-to-Digital Converters
Shraga Kraus ADC

Contents Background Time-Interleaved Structure
Some Basic Analog Circuits ADC Architectures Flash ADC Folding ADC Algorithmic ADCs Pipeline ADC Time-Interleaved Structure Characterization in the Lab Discussion

Background

ADC Model (1/2) Analog signal: continuous both in time and value
Digital signal: discrete both in time and value Discrete time (sampling)  aliasing Discrete value (resolution)  quantization

ADC Model (2/2) Modeled as a linear system + quantization noise
For easy analog treatment, noise is input-referred noise

Sampling for Dummies (1/3)
Sampling = multiplication by an impulse train Y (t ) = X (t ) · S (t ) Ts = sampling interval fs = 1/Ts = sampling frequency Ts t

Sampling for Dummies (2/3)
In the frequency domain: Y (f ) = X (f ) * S (f ) “Aliasing” is evident

Sampling for Dummies (3/3)
Nyquist sampling: Over-sampling: Under-sampling:

Anti-Aliasing Filter Nyquist sampling: Over-sampling: Under-sampling:

Incoherence – by Comics
Consider the following sinusoidal inputs, sampled at fs: t

Quantization (1/4) Δ = LSB m = num of bits Full scale amplitude: 7 6 5
3 2 1 A Δ

Quantization (2/4) For incoherent sinusoidal input:
Assuming uniform distribution of quantization noise from –Δ/2 to +Δ/2 Fqn 1/Δ x –Δ/2 +Δ/2

Quantization (3/4) For incoherent sinusoidal input with full scale amplitude: Signal power: Noise power:

Quantization (4/4) SNR: Effective number of bits (ENOB):

Example Simulated ideal 7-bit ADC: SNR = 43.8 dB  ENOB = 7

Practical Over-Sampling
Out-of-band noise is filtered out digitally OSR = 2  SNR x2 (+3dB)  ENOB +½

What is ½ Bit?

Non-Linear Effects (1/2)
Integral Non-Linearity (INL) output code 7 6 5 4 3 2 1 Vin Vref

Non-Linear Effects (2/2)
Differential Non-Linearity (DNL) output code 7 6 5 4 3 2 1 Vin Vref

Some Basic Analog Circuits

Differential Pair The core of every op amp Finite gain (Av = gmRD)
Finite bandwidth Finite slew rate Input capacitance Non-linearity

Voltage Buffer (1/2) Theoretically Vout = Vin
Finite gain results in output offest Finite bandwidth (esp. with 2 stages) Finite settling time Input capacitance reduced by feedback, but still exists

Voltage Buffer (2/2) Settling time: Tsettling damping Output Voltage
slew rate Time

Switch (CMOS Only!) (1/2) Has finite resistance
Resistance depends on the input voltage (linearity issues) Parasitic capacitances result in charge sharing Complicated correction circuits

Switch (CMOS Only!) (2/2) Resistance depends on the input voltage (linearity issues)

Comparator Basically an open-loop op amp Must make a decision quickly
Memory effect Input capacitance not reduced by feedback Latched comparator – triggered by clock

Sample & Hold Triggered by clock Finite settling time
Must be very accurate when placed at the ADC’s input (noise/linearity) Speed and accuracy are achieved only by very complicated circuits

10-Minute Break

ADC Architectures

Implementation Methods
Discrete time Requires switches Takes advantage of switched capacitors Continuous Time 1 clock cycle / decision Frequencies set by absolute R-C values

Flash ADC Continuous Time No. of comparators = 2m – 1
Output in thermometer code Thermometer code is converted to binary by simple logic Fastest topology = ‘101 = 5

Flash ADC Limitations Many comparators – a lot of area & power
Resistors must be matched (area) Input drives comparators’ capacitances Number of bits is limited (~ 5 bits)

Non-Linearity of Flash ADC
Resistor ladder mismatch Input buffer CLK/vin skew or input S&H non-linearity Comparators’ “memory effect”

Folding ADC Continuous Time No. of comparators = 2m/ 2 (approx.)
Fast with quite a high resolution Common in instrumentation vout vin VREF

Folding ADC Limitations
Flash drawbacks are alleviated, but still there The folding amplifier must fold accurately and be linear The folding amplifier introduces a delay and result in skew between the two flash ADCs

Non-Linearity of Folding ADC
Inherited flash non-linearity Non-linearity of the folding amplifier CLK/vin skew between the two flashes or input S&H non-linearity

Algorithmic ADCs Discrete Time
Small No. of comparators (reduced area & power) High resolution (up to 16 bits) Digital circuitry, usually plenty of switches Output data rate = fs /m or fs /2m (= slow…) Types: single/dual slope, successive approximation register (SAR), integrating (Agilent’s patent) Common in slow instrumentation and consumer devices (e.g. digital cameras)

Example: Single-Slope ADC
S&H Stop! Start! VREF Comparator’s output flips Counter stops Counter reset to 0 and starts counting Slope triggered vin t

Single-Slope ADC Limitations
Calibrations are required: Absolute R-C or L-C values Non-linearity of the slope Maximum time per decision: m clock cycles (sloooooooooooow) S&H must be as accurate as the ADC However : one slope + one counter can be used for many ADCs

Non-Linearity of Single-Slope
Non-linearity of the slope Input S&H non-linearity Incomplete capacitor discharge (“memory effect” of the slope)

Pipeline ADC Discrete Time No. of comparators = m
Switched capacitor circuitry Common in CMOS

Pipeline ADC – Example VREF = 1 V vin = 0.65 V Dout = ‘101 C2 C1 C0
‘1’ C1 –VREF/2 then x2 ‘0’ C0 x2 ‘1’

Pipeline ADC Limitations
Speed limited by switches and op amp settling time The first comparator must be extremely accurate (1½ bit arch.) Switches and op amps are lousy in contemporary CMOS technologies

Non-Linearity of Pipeline ADC
Input S&H non-linearity (if exists) Amplifiers’ gain error (low gain) Amplifiers’ gain different than x2 (feedback capacitor mismatch) Amplifiers’ settling time Inaccuracy in VREF /2 subtraction

Time-Interleaved Structure

The Principle Using many slow ADCs
Each ADC samples the signal at a different phase t

The Structure vin ADC 1 CK τ ADC 2 τ ADC 3 τ ADC 4

Limitations Many ADCs – area, power
Signal and clock distribution networks are required Signal and clock distributed with different delays Advanced RF techniques Complicated calibration

Characterization in the Lab

Effective Number of Bits
Pure Sine Signal Generator ADC Signal Generator

Linearity Dual Tone Signal Generator ADC Signal Generator SFDR

Discussion

Periodic Non-Uniform Sampling
Recall the example from Moshiko’s presentation: L = 7 p = 3 C = {0, 2, 3} Are there two adjacent elements from L in C ?

Two Adjacent Samples C = {0, 2, 3}
Speed constraints on the ADC are not relaxed Time-interleaved structure can benefit from omitting some of the ADCs 1 2 3 4 5 6

No Adjacent Samples C = {0, 2, 5}
Speed constraints on the ADC are now relaxed Clock generator implemented by a simple logic circuit Time-interleaved structure can benefit from omitting some of the ADCs 1 2 3 4 5 6

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