11001101011010

10101010101010

10101010111010

01010011101101

010101

SAR ADC’s and Delta Sigma ADC’s: Different Architectures for Different Applications

1

Selecting ADC Topology

ADC Topology Data Rate Resolution Comments

SAR

ADS7xxx

ADS8xxx

≤ 4 Msps

≤ 1.25 Msps

≤ 16-bit

≤ 18-bit

• Easy to Use

• Zero Latency

• Low Power

Delta-Sigma

ADS10xx/11xx

ADS12xxx

ADS13xxx

ADS16xx

≤ 4 Ksps

≤ 4 Msps

≤ 10 Msps

> 24-bit

≤ 24-bit

≤ 16-bit

• High Resolution

• High Integration

Pipeline

≤ 200 Msps

≤ 250 Msps

≤ 1000 Msps

≤ 16-bit

≤ 14-bit

≤ 12-bit

• Higher Speed

• Higher Power

2

Selecting ADC Topology

ADC Topology Data Rate Resolution Comments

SAR

ADS7xxx

ADS8xxx

≤ 4 Msps

≤ 1.25 Msps

≤ 16-bit

≤ 18-bit

• Easy to Use

• Zero Latency

• Low Power

Delta-Sigma

ADS10xx/11xx

ADS12xxx

ADS13xxx

ADS16xx

≤ 4 Ksps

≤ 4 Msps

≤ 10 Msps

> 24-bit

≤ 24-bit

≤ 16-bit

• High Resolution

• High Integration

Pipeline

≤ 200 Msps

≤ 250 Msps

≤ 1000 Msps

≤ 16-bit

≤ 14-bit

≤ 12-bit

• Higher Speed

• Higher Power

3

Selecting ADC Topology

ADC Topology Data Rate Resolution Comments

SAR

ADS7xxx

ADS8xxx

≤ 4 Msps

≤ 1.25 Msps

≤ 16-bit

≤ 18-bit

• Easy to Use

• Zero Latency

• Low Power

Delta-Sigma

ADS10xx/11xx

ADS12xxx

ADS13xxx

ADS16xx

≤ 4 Ksps

≤ 4 Msps

≤ 10 Msps

> 24-bit

≤ 24-bit

≤ 16-bit

• High Resolution

• High Integration

Pipeline

≤ 200 Msps

≤ 250 Msps

≤ 1000 Msps

≤ 16-bit

≤ 14-bit

≤ 12-bit

• Higher Speed

• Higher Power

4

Selecting ADC Topology

ADC Topology Data Rate Resolution Comments

SAR

ADS7xxx

ADS8xxx

≤ 4 Msps

≤ 1.25 Msps

≤ 16-bit

≤ 18-bit

• Easy to Use

• Zero Latency

• Low Power

Delta-Sigma

ADS10xx/11xx

ADS12xxx

ADS13xxx

ADS16xx

≤ 4 Ksps

≤ 4 Msps

≤ 10 Msps

> 24-bit

≤ 24-bit

≤ 16-bit

• High Resolution

• High Integration

Pipeline

≤ 200 Msps

≤ 250 Msps

≤ 1000 Msps

≤ 16-bit

≤ 14-bit

≤ 12-bit

• Higher Speed

• Higher Power

5

SAR ADC takes “snapshots” Each conversion command captures the

signal level, at that point in time, onto the

sample/hold

ADC calculates an average The signal is sampled continuously

What is the ADC actually converting?

SAR vs. Delta-Sigma

SAR

6

• SAR conversions have Start Conversion Signal

• Delta-Sigma is always sampling/converting

SAR Converter

Start Conversion

Conversion Done

Delta-Sigma Converter

Input Sampling

Conversion Done

How does the ADC control happen?

SAR vs. Delta-Sigma

7

?

How Does a SAR ADC Work?

• Similar to a balance scale

½ ¼ 1

8

How Does a SAR ADC Work?

• Similar to a balance scale

½ ¼ 1

?

9

How Does a SAR ADC Work?

• Similar to a balance scale

½ ¼ 1

?

10

How Does a SAR ADC Work?

• Similar to a balance scale

½ ¼

? MSB

1

1

The MSB is determined first

11

How Does a SAR ADC Work?

• Similar to a balance scale

½

¼

?

MSB

1

1

The test is repeated for each

Binary weighted bit

Mid

0

12

How Does a SAR ADC Work?

• Similar to a balance scale

½

?

MSB

1

1

Mid

0

¼

LSB

1

The LSB is determined last

13

Typical Topology of a SAR ADC

14

SAR ADC Acquisition Phase

COMPARATOR

SAMPLE & HOLD

VIN DAC

C

S1

VIN

S2

N-bit Search

DAC

Data Register

SAR

15

SAR ADC Acquisition Phase

VSH0

VIN

tAQ Time

1/2 LSB

VCSH(t)

t0

COMPARATOR

SAMPLE & HOLD

VIN DAC

C

S1

VIN

S2

N-bit Search

DAC

Data Register

SAR

16

SAR ADC Acquisition Phase

)1()]([)()( 00

t

CSHINCSHCSH etVVtVtV

VSH0

VIN

tAQ Time

1/2 LSB

VCSH(t)

t0

SHS CR 1

COMPARATOR

SAMPLE & HOLD

VIN DAC

17

SAR ADC Conversion Phase

Analog

Input

VD

AC

FS

0

1/4FS

1/2FS

3/4FS

TEST

MSB

TEST

MSB -1

TEST

LSB

TEST

MSB -2

Bit = 1

Bit = 0

Bit = 1 Bit = 0

Digital Output Code = 10100

Time

Bit = 0

TEST

MSB -3

DAC Output 18

SAR ADCs

• Very Popular Topology

• Attractive in “Point in Time” or Multiplexed Measurements

• Advantages

– “no latency”

• input is sampled once

• “balancing” done internally

– good tradeoff between speed, resolution and power

• Speed: DC to 4MSPS

• Resolution: 8 to 18 bits; and moving towards higher resolutions

• TI Part Numbers:

– ADS7xxx

– ADS8xxx

SAR ADC

19

Delta Sigma Topology

Delta-Sigma Modulator

Analog Input

Digital Filter

Decimator

Digital Output

Digital Decimating Filter

20

Delta Sigma

Modulator

Digital

Filter Decimator

Delta Sigma Topology (2)

Delta-Sigma Modulator

Analog Input

Digital Filter

Decimator

Digital Output

Digital Decimating Filter

SAMPLING RATE (Fs)

21

Delta Sigma

Modulator

Digital

Filter Decimator

High frequency, 1 bit PCM data stream

(Samples at High Frequency)

Delta Sigma Topology (3)

Delta-Sigma Modulator

Analog Input

Digital Filter

Decimator

Digital Output

Digital Decimating Filter

SAMPLING RATE (Fs)

DATA RATE (Fd)

22

Delta Sigma

Modulator

Digital

Filter Decimator

High frequency, 1 bit PCM data stream

Lower data rate, very high

resolution digital output

Input Oversampling

23

Oversampling (1)

fs/2 fs

Power

Average Noise energy distributed from DC to

fs/2

Ideal N-Bit ADC SNR= 6.02 N + 1.76 dB

DC

Input Signal

Average Noise Floor

24

Oversampling (2)

– On a Delta-Sigma Converter, the analog input is sampled at a Frequency much higher than the Nyquist rate

fs/2 fs

Power

Average Noise energy distributed from DC to

fs/2

Ideal N-Bit ADC SNR= 6.02 N + 1.76 dB

DC

Input Signal

Average Noise Floor

Average Noise Floor

K fs/2 K fs

Power

Average Noise energy distributed over a wider range from DC to K fs/2

SNR= 6.02 N + 1.76 dB +10 log (OSR)

DC

Digital Low Pass filter

Oversampling

Delta Sigma Modulator

25

Delta Sigma

Modulator

Digital

Filter Decimato

r

First Order Delta-Sigma Modulator (1)

Noise Shaping

Integrator

(Low-Pass)

A(f)=1/f

Input

Signal

Xi

∑ +

∑

-

ei

Quantization

Noise

1-Bit

ADC

1-Bit

DAC

Yi

26

First Order Delta-Sigma Modulator (2)

Noise Shaping

Noise

Transfer

Function

Signal

Transfer

Function

Integrator

(Low-Pass)

A(f)=1/f

Input

Signal

Xi

∑ +

∑

-

ei

Quantization

Noise

1-Bit

ADC

1-Bit

DAC

Yi

𝑌 = 𝑋 − 𝑌 𝐴 𝑓 + 𝑒 𝑛 (1)

𝑌 = 𝑒 𝑛𝑓

1 + 𝑓+ 𝑋

1

1 + 𝑓 (2)

27

First Order Delta-Sigma Modulator (3)

Noise Shaping

Signal

Ma

gn

itu

de

Quantization

Noise

Noise

Transfer

Function

Signal

Transfer

Function

Integrator

(Low-Pass)

A(f)=1/f

Input

Signal

Xi

∑ +

∑

-

ei

Quantization

Noise

1-Bit

ADC

1-Bit

DAC

Yi

𝑌 = 𝑋 − 𝑌 𝐴 𝑓 + 𝑒 𝑛 (1)

𝑌 = 𝑒 𝑛𝑓

1 + 𝑓+ 𝑋

1

1 + 𝑓 (2)

28

First Order Delta-Sigma Modulator (4)

Noise Shaping

Modulator Output: TIME DOMAIN

0

1

Believe it or not, the sine wave is in there!

(drawing is approximate)

Sig

nal

29

First Order Delta-Sigma Modulator (5)

Noise Shaping

Modulator Output: TIME DOMAIN

0

1

Believe it or not, the sine wave is in there!

(drawing is approximate)

Sig

nal

30

First Order Delta-Sigma Modulator (6)

Noise Shaping

Modulator Output: TIME DOMAIN

Modulator Output: FREQUENCY DOMAIN

0

1

Believe it or not, the sine wave is in there!

(drawing is approximate)

QUANTIZATION

NOISE

Fs

SIGNAL

Sig

nal

31

Frequency FS

Third Order

Modulator

Second Order

Modulator

First Order

Modulator

Higher Order Delta-Sigma Modulators

32

Delta-Sigma A/D Signal Path

33

Delta Sigma

Modulator

Digital

Filter Decimator

Frequency FS

Modulator Noise Shaping and Digital Filter(1)

34

Modulator

Noise Shaping

Frequency FS

Modulator Noise Shaping and Digital Filter(2)

35

Modulator

Noise Shaping

Frequency FS

Modulator Noise Shaping and Digital Filter(3)

36

Filter set by

Oversampling

Ratio

Modulator

Noise Shaping

37

Digital Filter

• Digital filter architecture determines overall ADC response.

• Common filters: “Sinc” and “Flat Passband”

Sinc Filter Flat Passband Filter

38

Sinc Digital Filter

Sinc filter response

-100

-80

-60

-40

-20

0

0 1 2 3 4 5 6

Frequency (x Fdata)

Att

en

tua

tio

n, d

B

Sinc 1

Sinc 3

Sinc 5

• Typically used for DC measurements, or slow moving signals

Advantages

• Economical silicon area, easy to

implement

– Low cost

– Low power

• Low latency

• Filter notches can target specific

frequencies (ex. 50/60 Hz)

Disadvantages

• Pass band signal droop

• Weak Stop band attenuation for

low-order Sinc filters

Fdata

Sinc Digital Filter Settling

0 1 2 3 0 1 2 3

3 full cycles 3 full cycles

Valid data Valid data

Fdata periods

Analog Inputs

Settling time for an input step change, Sinc3 filter

Need n cycles to settle for a Sincn filter

4 cycles

Uncertainty of Analog Edge

4 Data Cycles

39

• Zero cycle Latency =

– Zero latency

– Single cycle conversion

– Single cycle settling

– No Latency

Analog IN

Data OUT

Single Cycle

Conversion

Data

Invalid

N+0 N+1 N+2 N+3 N-1

N+1 N+2

N+3 N+0

Delta Sigma: Zero Cycle Latency (1)

40

• Zero cycle Latency =

– Zero cycle latency

– Single cycle conversion

– Single cycle settling

– No Latency

Analog IN

Data OUT

Single Cycle

Conversion

Data

Invalid

N+0 N+1 N+2 N+3 N-1

N+1 N+2

N+3 N+0

Delta-Sigma: Zero Cycle Latency (2)

“Hidden Conversions”

41

42

Flat Pass Band Filter

Advantages

• Frequency Response

• Very low ripple pass band

• Sharp Nyquist transition band

• Large stopband attenuation: lower than

-100dB (simplify aliasing requirement)

• Frequency response scalable with master

clock

Disadvantages

• Large area – Costly

• Higher-order / high-tap filter – large latency

100dB stop band

Low Ripple Passband

43

Delta-Sigma: Flat Passband Digital Filter Settling

• The latency of the filter depends on the number of delay blocks used

• Flat Passband filters require a lot delay blocks to maintain desired AC response

• Many Delta-Sigma Converters incorporate filters with programmable settings: – Optimize for lower latency, power consumption or for AC performance/higher resolution

delay delay delay delay delay

Σ

Modulator Data Input

Digital Filter

Output

FIR filter block

topology

ΔΣ ADCs: Simplifying the Signal Chain (1)

CT

Passive Network + Protection

Passive Network + Protection +

R1

R2

In

OutADC Drive Circuitry

Iso

ADCProcessor

MUX

Sensor Signal

Conditioning

and

Protection

Gain Stage Mux ADC Drive ADC

MCU

44

ΔΣ ADCs: Simplifying the Signal Chain (2)

CT

Passive Network + Protection

Passive Network + Protection +

R1

R2

In

OutADC Drive Circuitry

Iso

ADCProcessor

MUX

Sensor Signal

Conditioning

and

Protection

Gain Stage Mux ADC Drive ADC

MCU

Delta-Sigma ADCs integrate many signal chain elements into one device 45

Delta-Sigma ADC’s

• Highest Resolution and High Stability with moderate power consumption

• Incorporate a Digital Filter

• Frequency Response, and Latency dependent on Digital Filter

• Typically Highly Integrated devices:

– Digital Filter, Buffer, PGA, MUX, Vref, Calibration/diagnostics

• Typically Requires Configuration of Registers

46

ΔΣ ADC

SAR ADCs

• Very Popular Topology

• Attractive in “Point in Time” or Multiplexed Measurements

• Advantages

– “no latency”

• input is sampled once

• “balancing” done internally

– good tradeoff between speed, resolution and power

• Speed: DC to 4MSPS

• Resolution: 8 to 18 bits; and moving towards higher resolutions

• TI Part Numbers:

– ADS7xxx

– ADS8xxx

SAR ADC

47

48

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