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Instrumentation: Test and
Measurement Methods and Solutions
Reference Designs and System Applications
Walt Kester, Applica...
Today’s Agenda
Understand challenges of precision data acquisition in sensing
applications
 Complex impedance measuremen...
Circuits from the Lab
 Circuits from the Lab reference circuits are engineered and
tested for quick and easy system integ...
System Demonstration Platform (SDP-B, SDP-S)
 The SDP (System Demonstration Platform) boards provide intelligent USB
comm...
Impedance Measurement Applications
Consumer and biomedical markets
 High end biomedical equipment
 Resistivity/conducti...
Impedance Measurement Devices
Impedance measurement is a
difficult signal processing task
Need to measure complex
impeda...
Impedance Measurement Challenge
Problem:
 How to analyze a complex
impedance
 How to control ADC sampling
frequency wit...
VDD/2
DAC
Z(ω)
SCL
SDA
DVDDAVDDMCLK
AGND DGND
ROUT VOUT
AD5933
RFB
VIN
05324-001
1024-POINT DFT
I2C
INTERFACE
IMAGINARY
RE...
CN0217: High Accuracy Impedance
Measurements Using 12-Bit Impedance Converters
Circuit features
 Wide impedance range
 ...
50kΩ
50kΩ
50kΩ
50kΩ
RFB
20kΩ
20kΩ
47nF
ZUNKNOWN
VDD
VDD
VDD
+
+
−
−
A1
A2
A1, A2 ARE
½ AD8606
1.48V
1.98V p-p
VDD/2
1.98V ...
High Accuracy Performance from the
AD5933/AD5934 with External AFE
12
30 35 40
FREQUENCY (kHz)
45 50
8160
8180
8200
8220
8...
Low RON SPDT CMOS Switch Used to Switch
Between RCAL and Unknown Z
13
50kΩ
ZUNKNOWN RCAL
S1
D
S2
RFB
VDD
IN
ADG849
50kΩ
A1...
CN0217 Evaluation Board, EVAL-CN0217-EB1Z
14
 Complete design files
 Schematic
 Bill of material
 PADs layout
 Gerber...
AD5933 Used with AFE for Measuring Ground-
Referenced Impedance in Blood-Coagulation
Measurement System
16
Ground-referenc...
Blood Clotting Factor Measurements
17
Liquid Quality Impedance Measurement
18
CONDUCTANCE
LIQUID
MEASUREMENT
SWITCHES
AFE
AD5933/
AD5934
CONTROLLER
CALIBRATION
...
Precision Tilt Measurements
 Fundamentals of iMEMS (micro electro mechanical systems)
accelerometers
 Single axis tilt m...
Why Use Accelerometers to Measure Tilt?
 Pendulums/potentiometers wear out
 Accuracy and bandwidth is limited
 Reliabil...
Applications of iMEMS Accelerometers
Tilt or inclination
 Car alarms
 Patient monitors
Inertial forces
 Laptop comput...
Tilt Measurements Using Low g Accelerometers
Need accuracy over full 360° arc
Output error less than 0.5°
Single-supply...
ADXL-Family Micromachined iMEMS
Accelerometers (Top View of IC)
23
FIXED
OUTER
PLATES
CS1 CS1 < CS2= CS2
DENOTES ANCHOR
BE...
ADXL-Family iMEMS Accelerometers
Internal Signal Conditioning
24
OSCILLATOR A1
SYNCHRONOUS
DEMODULATOR
BEAM
PLATE
PLATE
CS...
Using a Single Axis Accelerometer to
Measure Tilt
25
X
0°
+90°
θ
1g
Acceleration
X
–90°
–1g
0°
+1g
+90°
Acceleration = 1g ...
Single Axis vs. Dual Axis Acceleration
Measurements
26
Output Acceleration vs. Angle of Inclination Output Acceleration vs...
ADXL203 Dual Axis Accelerometer
27
1 mg resolution for BW = 60 Hz
700 µA current @ 5 V
CN0189: Tilt Measurement Using a Dual Axis
Accelerometer
28
Circuit features
 Dual axis tilt measurement
 0.5° accuracy...
CN0189 Dual Axis Tilt Measurement Circuit
29
AD7887 ADC
■ 12-bit, 125 kSPS SAR
■ 850 µA current @ 5 V
AD8608 Quad Op Amp...
Output Error for arcsin(X), arccos(Y), and
arctan(X/Y) Calculations
30
OUTPUT = arcsin(X)
OUTPUT = arccos(Y)
OUTPUT = arct...
CN0189 Dual Axis Tilt Measurement Hardware
and Demonstration Software
32
SDP-S BOARD
POWER CONNECTOR
SOFTWARE OUTPUT DISPL...
Precision Load Cell (Weigh Scales)
Wheatstone bridge solutions
Low level signal conditioning issues
High common-mode vo...
Resistance-Based Sensor Examples
34
Strain gages 120 Ω, 350 Ω, 3500 Ω
Weigh scale load cells 350 Ω to 3500 Ω
Pressure s...
VO
R4
R1
R3
R2
VB
VO
R
R R
VB
R
R R
VB=
+
−
+
1
1 4
2
2 3
=
−
+





 +






R
R
R
R
R
R
R
R
VB
1
4
2
3
1
1
4...
Output Voltage and Linearity Error for Constant
Voltage Drive Bridges
36
R R
R R+∆R
R+∆R
R+∆R R+∆R R+∆R
R−∆R R+∆R R−∆RR R
...
R R
R R+∆R
R+∆R
R+∆R R+∆R R+∆R
R−∆R R+∆R R−∆RR R
R R−∆R
VO
VO VO
VO
IB IB IB IB
VO:
Linearity
Error:
0.25%/% 0 0 0
IBR
4
∆...
Kelvin (4-Wire) Sensing Minimizes Errors
Due to Lead Resistance for Voltage Excitation
38
6-LEAD
BRIDGE
RLEAD
RLEAD
+SENSE...
4-LEAD
BRIDGE
RLEAD
+
–RLEAD
RSENSE
VREF
VO
I
I
I
I =
VREF
RSENSE
Constant Current Excitation also
Minimizes Wiring Resist...
ADC Architectures, Applications, Resolution,
Sampling Rates
40
10 100 1k 10k 100k 1M 10M 100M 1G
8
10
12
14
16
18
20
22
24...
SAR vs. Sigma-Delta Comparison
41
Successive approximation
(SAR)
 Fast settling, ideal for multiplexing
 Data available...
Sigma-Delta Concepts: Oversampling, Digital
Filtering, Noise Shaping, and Decimation
42
fs
2
fs
Kfs
2
Kfs
Kfs
Kfs
2
fs
2
f...
First-Order Sigma-Delta ADC
43
∑ ∫ +
_
+VREF
–VREF
DIGITAL
FILTER
AND
DECIMATOR
+
_
CLOCK
Kfs
VIN
N-BITS
fs
fs
A
B
1-BIT D...
Sigma-Delta ADC Architecture Benefits
High resolution
 24 bits, no missing codes
 22 bits, effective resolution (RMS)
...
Typical Applications of High Resolution
Sigma-Delta ADCs
Process control
 4 mA to 20 mA
Sensors
 Weigh scale
 Pressur...
Precision Weigh Scales-Industrial and
High Precision Commercial
46
Laboratory scales
Process control
 Hopper scales
 C...
Weigh Scale Product Definition
47
Capacity 2 kg
Sensitivity 0.1 g
Other features
 Accuracy 0.1 %
 Linearity ±0.1 g
 ...
Characteristics of Tedea Huntleigh
505H-0002-F070 Load Cell
48
Full load 2 kg
Sensitivity 2 mV/V
Excitation 5 V
Other ...
Characteristics of Tedea Huntleigh
505H-0002-F070 Load Cell
49
 Full load 2 kg
 Sensitivity 2 mV/V
 Excitation 5 V
 VF...
Input-Referred Noise of ADC Determines the
"Noise-Free Code Resolution"
50
n n+1 n+2 n+3 n+4n–1n–2n–3n–4
NUMBER OF
OCCURAN...
Performance Requirement – Resolution
51
Required: 0.1 g in 2 kg
 # Noise free counts = full-scale/p-p noise in g
 # Noi...
Definition of "Noise-Free" Code Resolution and
"Effective" Resolution
52
Effective
Resolution
= log2
Full-Scale Range
RMS ...
Terminology for Resolution Based on Peak-to-
Peak and RMS Noise
Peak-to-peak noise:
 Noise-free code resolution
 Noise-...
Options for Conditioning Load Cell Outputs
54
+
−
+
−
+
−
+
−
+
−
A:
EXTERNAL IN-AMP
B:
DIFFERENTIAL INPUT ADC
EXTERNAL IN...
CN0216: Load Cell Signal Conditioning with
Differential Input ADC and External In-Amp
Circuit features
 Gain of 375 low ...
CN0216: Load Cell Conditioning with
Differential Input ADC and External In-Amp
56
G = 375
FS = 10mV
FS = 3.75V
INPUT RANGE...
CN0216 Noise Performance
57
Data rate = 9.5 Hz
VP-P NOISE = 159 counts × 0.596 µV = 94.8 µV
VFS = 3.75 V
Noise-free co...
CN0216 Evaluation Board and Software
58
Complete design files
 Schematic
 Bill of material
 PADs layout
 Gerber files...
AD7190, 24-Bit Sigma-Delta ADC: Weigh Scale
with Ratiometric Processing
59
IN+
IN-
OUT- OUT+
+5V
2mV/V
SENSITIVITY
Load c...
AD7190 Sigma-Delta System On-Chip Features
Analog input buffer options
 Drives Σ-Δ modulator, reduces dynamic input curr...
CN0102: Precision Weigh Scale System
Circuit features
 Integrated solution with PGA
 16.8 noise-free bits
Circuit bene...
CN0102 Precision Weigh Scale System
62
AD7190 Sinc4 Filter Response, 50 Hz Output
Data Rate
63
AD7190 Noise and Resolution, Sinc4 Filter,
Chop Disabled
64
For G = 128
VREF = 5 V,
FS = 80 mV p-p
17.5
for
10 mV p-p
Only...
CN0102 Load Cell Test Results, 500 Samples
65
System resolution with load cell connected
 Load cell: full-scale output =...
CN0102 Evaluation Board and Load Cell
66
EVAL-AD7190EBZ
Software Display
Complete design files
 Schematic
 Bill of mate...
Tweet it out! @ADI_News #ADIDC13
What We Covered
Fundamentals of making complex impedance measurements using
integrated s...
Tweet it out! @ADI_News #ADIDC13
Visit the Impedance Measurement Demo in the
Exhibition Room
Measuring complex impedances...
Tweet it out! @ADI_News #ADIDC13
Visit the Tilt Measurement Demo in the
Exhibition Room
69
Measure tilt using the ADXL203...
Tweet it out! @ADI_News #ADIDC13
Visit the Weigh Scale Demo in the Exhibition
Room
70
Measure weights from
0.1 g to 2000 ...
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Instrumentation: Test and Measurement Methods and Solutions - VE2013

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Tilt Measurement: Tilt measurement is fast becoming a fundamental analysis tool in many fields including automotive, industrial, and healthcare. Navigation, vehicle dynamic control, building sway indication, and motion detection systems all rely on this simple, cheap, and precise way of angle monitoring. MEMS accelerometers are better suited to inclination measurement than other methodologies. This session will address the challenges encountered when designing a dual-axis tilt sensor using a MEMS accelerometer including measurement resolution, signal conditioning, single- vs. dual-axis, angle computation, and calibration.

Impedance Measurement: The measurement of complex impedance is widely used across industrial, commercial, automotive, healthcare, and consumer markets, and can include applications such as proximity sensing, inductive transducers, metallurgy and corrosion detection, loudspeaker impedance, biomedical, virus detection, blood coagulation factor, and network impedance analysis. This session will cover the concepts, approaches, and challenges of performing complex impedance measurements and will present a system-level solution for impedance conversion.

Weigh Scale Measurement: Most common industrial weigh scale applications use a bridge-type load-cell sensor, with a voltage output that is directly proportional to the load weight placed on it. This session examines the basic parameters of a bridge-type load-cell sensor, such as the number of varying elements, impedance, excitation, sensitivity (mV/V), errors, and drift. It will also discuss the various components of the signal conditioning chain and present solutions with high dynamic range.

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Transcript of "Instrumentation: Test and Measurement Methods and Solutions - VE2013"

  1. 1. Instrumentation: Test and Measurement Methods and Solutions Reference Designs and System Applications Walt Kester, Applications Engineer, Greensboro, NC, US
  2. 2. Today’s Agenda Understand challenges of precision data acquisition in sensing applications  Complex impedance measurements over a wide range (CN0217)  Tilt measurements over full 360° range using dual axis low-g iMEMS® accelerometers (CN0189)  Weigh scale signal conditioning and digitization of low level signals with high noise-free code resolution (CN0216, CN0102) Applications selected to illustrate important design principles applicable to a variety of precision sensor conditioning circuits including MEMS See tested and verified Circuits from the Lab® signal chain solutions chosen to illustrate design principles  Low cost evaluation hardware and software available  Complete documentation packages:  Schematics, BOM, layout, Gerber files, assemblies 3
  3. 3. Circuits from the Lab  Circuits from the Lab reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. 4  Evaluation board hardware Design files and software  Windows evaluation software  Schematic  Bill of material  PADs layout  Gerber files  Assembly drawing  Product device drivers
  4. 4. System Demonstration Platform (SDP-B, SDP-S)  The SDP (System Demonstration Platform) boards provide intelligent USB communications between many Analog Devices evaluation boards and Circuits from the Lab boards and PCs running the evaluation software 5 USB USB EVALUATION BOARD SDP-B SDP-S EVALUATION BOARD POWER POWER  SDP-S (USB to serial engine based)  One 120-pin small footprint connector  Supported peripherals:  I2C  SPI  GPIO  SDP-B (ADSP-BF527 Blackfin® based)  Two 120-pin small footprint connectors  Supported peripherals:  I2C  SPI  SPORT  Asynchronous parallel port  PPI (parallel pixel interface)  Timers
  5. 5. Impedance Measurement Applications Consumer and biomedical markets  High end biomedical equipment  Resistivity/conductivity of biomedical tissues  Medical sample analysis  Consumer  Medical sample analysis (e.g., glucose) Industrial and instrumentation markets  Electro impedance spectrometry  Corrosion analysis  Liquid condition analysis  Sensor interface (sensor impedance changes with some external event) 6
  6. 6. Impedance Measurement Devices Impedance measurement is a difficult signal processing task Need to measure complex impedances, not just R, L, or C Impedance conversion  …is becoming more important in many sensor/diagnostic related applications  …is traditionally accomplished using discrete solutions  …usually requires a high level of analog design skill to extract frequency responses of the unknown impedance 7
  7. 7. Impedance Measurement Challenge Problem:  How to analyze a complex impedance  How to control ADC sampling frequency with respect to DDS output frequency (windowing vs. coherent sampling)?  How to manage component selection?  Must develop software to control DDS  Software required for FFT  How to calculate error budget?  What about temperature effects?  Usually ends up consuming greater board area and cost? 8 Excitation/Stimulus Frequency Response Analysis Integrated Single-Chip SolutionAD5933 DDS Filter Buffer ADC
  8. 8. VDD/2 DAC Z(ω) SCL SDA DVDDAVDDMCLK AGND DGND ROUT VOUT AD5933 RFB VIN 05324-001 1024-POINT DFT I2C INTERFACE IMAGINARY REGISTER REAL REGISTER OSCILLATOR DDS CORE (27 BITS) TEMPERATURE SENSOR ADC (12 BITS) LPF GAIN AD5933/AD5934 Impedance Converter  1 kΩ to 10 MΩ impedance range  12-bit impedance resolution  100 kHz maximum excitation frequency  Adjustable voltage excitation  User programmable frequency sweep  Single frequency capability  1 MSPS SAR ADC (AD5933)  DFT carried out at each frequency point  Manual calibration routine  Single-chip solution with internal DSP  Output at each frequency is real and imaginary data word  Simple off-chip processing required to calculate magnitude and phase 9 I2C INTERFACE TO µC OR PC UNKNOWN IMPEDANCE EXCITATION FREQUENCY REAL AND IMAGINARY COMPONENT REGISTERS DDS ADJUSTABLE VOLTAGE EXITATION CURRENT TO VOLTAGE CONVERTER
  9. 9. CN0217: High Accuracy Impedance Measurements Using 12-Bit Impedance Converters Circuit features  Wide impedance range  12-bit accuracy  Analog front end (AFE) for impedance measurements less than 1 kΩ Circuit benefits  Self contained DDS excitation  DSP for calculating DFT  Complex impedance measurements 10 Target Applications Key Parts Used Interface/Connectivity Medical Consumer Industrial AD5933 AD8606 I2C (AD5933) USB (EVAL-AD5933EBZ)
  10. 10. 50kΩ 50kΩ 50kΩ 50kΩ RFB 20kΩ 20kΩ 47nF ZUNKNOWN VDD VDD VDD + + − − A1 A2 A1, A2 ARE ½ AD8606 1.48V 1.98V p-p VDD/2 1.98V p-p VDD/2 DAC SCL SDA DVDDAVDDMCLK AGND DGND ROUT VOUT AD5933/AD5934 RFB VIN 1024-POINT DFT I2C INTERFACE IMAGINARY REGISTER REAL REGISTER OSCILLATOR DDS CORE (27 BITS) TEMPERATURE SENSOR TRANSMIT SIDE OUTPUT AMPLIFIER ADC (12 BITS) LPF GAIN VDD VDD 09915-001 I-V CN0217 External AFE Signal Conditioning  External analog front end (AFE) allows impedance measurements below 1 kΩ  The solution is based on the AD8605/AD8606 op amp  Excitation stage: low Output Z (<1 Ω) up to 100 kHz  Receive stage: low bias current (<1 pA) 11 VDD = 3.3V
  11. 11. High Accuracy Performance from the AD5933/AD5934 with External AFE 12 30 35 40 FREQUENCY (kHz) 45 50 8160 8180 8200 8220 8240 8260 8280 IMPEDANCEMAGNITUDE(Ω) R3 IDEAL 09915-008 35 30 25 20 15 10 5 0 29.95 30.00 30.05 30.10 30.15 30.20 10.3Ω 30Ω 1µF 30.25 FREQUENCY (kHz) MAGNITUDE(Ω) 09915-003 Magnitude Results For ZC = 10 kΩ||10 nF, RCAL = 1 kΩ Magnitude Results For Low Impedance ZC = 8.21 kΩ, RCAL = 99.85 kΩ ZC = 217.25 kΩ, RCAL = 99.85 kΩ One calibration using 99.85 kΩ resistor covers wide range Allows low value impedance measurements Tracks R||C across frequency 30 35 40 FREQUENCY (kHz) 45 50 IMPEDANCEMAGNITUDE(kΩ) R4 09915-009 213.5 214.0 214.5 21.50 215.5 216.0 216.5 217.0 217.5 218.0 218.5 IDEAL 500 0 1000 1500 2000 2500 3000 3500 4000 4 24 44 64 84 104 IMPEDANCEMAGNITUDE(Ω) FREQUENCY (kHz) IDEAL MEASURED 09915-011
  12. 12. Low RON SPDT CMOS Switch Used to Switch Between RCAL and Unknown Z 13 50kΩ ZUNKNOWN RCAL S1 D S2 RFB VDD IN ADG849 50kΩ A1 A2 09915–013 Use low RON CMOS switch for switching from unknown impedance to calibration resistor RON = 0.5Ω
  13. 13. CN0217 Evaluation Board, EVAL-CN0217-EB1Z 14  Complete design files  Schematic  Bill of material  PADs layout  Gerber files  Assembly drawing PC Unknown Z USB
  14. 14. AD5933 Used with AFE for Measuring Ground- Referenced Impedance in Blood-Coagulation Measurement System 16 Ground-referenced Unknown Z
  15. 15. Blood Clotting Factor Measurements 17
  16. 16. Liquid Quality Impedance Measurement 18 CONDUCTANCE LIQUID MEASUREMENT SWITCHES AFE AD5933/ AD5934 CONTROLLER CALIBRATION IMPEDANCE UNKNOWN IMPEDANCE
  17. 17. Precision Tilt Measurements  Fundamentals of iMEMS (micro electro mechanical systems) accelerometers  Single axis tilt measurements  Dual axis tilt measurements for better accuracy (CN0189)  Signal conditioning 19
  18. 18. Why Use Accelerometers to Measure Tilt?  Pendulums/potentiometers wear out  Accuracy and bandwidth is limited  Reliability is lower  Takes up a large area  Out of plane sensitivity/mechanical interference  MEMS accelerometers are the latest proven technology for electronically measuring tilt  Good accuracy and bandwidth  Small board area  Low power  High reliability  Minimal out of plane sensitivity 20
  19. 19. Applications of iMEMS Accelerometers Tilt or inclination  Car alarms  Patient monitors Inertial forces  Laptop computer disc drive protection  Airbag crash sensors  Car navigation systems  Elevator controls Shock or vibration  Machine monitoring  Control of shaker tables  Data loggers to determine events/damage ADI accelerometer full-scale g-range: ±2g to ±100g ADI accelerometer frequency range: DC to 1 kHz 21
  20. 20. Tilt Measurements Using Low g Accelerometers Need accuracy over full 360° arc Output error less than 0.5° Single-supply operation Low power CN0189 illustrates the signal chain solution  Accelerometer signal conditioning  Easy to use SAR ADC  Low power, single supply  Hardware, software, and design files available 22
  21. 21. ADXL-Family Micromachined iMEMS Accelerometers (Top View of IC) 23 FIXED OUTER PLATES CS1 CS1 < CS2= CS2 DENOTES ANCHOR BEAM TETHER CS1 CS2 CENTER PLATE AT REST APPLIED ACCELERATION
  22. 22. ADXL-Family iMEMS Accelerometers Internal Signal Conditioning 24 OSCILLATOR A1 SYNCHRONOUS DEMODULATOR BEAM PLATE PLATE CS1 CS2 SYNC 0° 180° A2 VOUT CS2 > CS1 APPLIEDACCELERATION
  23. 23. Using a Single Axis Accelerometer to Measure Tilt 25 X 0° +90° θ 1g Acceleration X –90° –1g 0° +1g +90° Acceleration = 1g × sin θ θ0g –90°  Highest sensitivity between −45° and +45°  Ambiguous beyond ±90°
  24. 24. Single Axis vs. Dual Axis Acceleration Measurements 26 Output Acceleration vs. Angle of Inclination Output Acceleration vs. Angle of Inclination Single Axis Dual Axis Sensitivity equal over entire 360° range Removes ambiguity beyond ±90° X-Axis Y-Axis
  25. 25. ADXL203 Dual Axis Accelerometer 27 1 mg resolution for BW = 60 Hz 700 µA current @ 5 V
  26. 26. CN0189: Tilt Measurement Using a Dual Axis Accelerometer 28 Circuit features  Dual axis tilt measurement  0.5° accuracy over 360° arc Circuit benefits  Single supply  Low power  Conditioning circuits for ADXL203 outputs Target Applications Key Parts Used Interface/Connectivity Medical Consumer Industrial ADXL203 AD8608 AD7887 SPI (AD7887) SDP-S (EVAL-CN0189-SDPZ) USB (EVAL-SDP-CS1Z)
  27. 27. CN0189 Dual Axis Tilt Measurement Circuit 29 AD7887 ADC ■ 12-bit, 125 kSPS SAR ■ 850 µA current @ 5 V AD8608 Quad Op Amp ■ 65 µV input offset voltage ■ 1 pA input bias current ■ 4 mA quiescent current 0.5 Hz BW
  28. 28. Output Error for arcsin(X), arccos(Y), and arctan(X/Y) Calculations 30 OUTPUT = arcsin(X) OUTPUT = arccos(Y) OUTPUT = arctan(X/Y) Error increases at ±90° Error increases at 0° Uniform error distribution
  29. 29. CN0189 Dual Axis Tilt Measurement Hardware and Demonstration Software 32 SDP-S BOARD POWER CONNECTOR SOFTWARE OUTPUT DISPLAYEVAL-CN0189-SDPZ  Complete design files ■ Schematic ■ Bill of Material ■ PADs layout ■ Gerber files ■ Assembly drawing
  30. 30. Precision Load Cell (Weigh Scales) Wheatstone bridge solutions Low level signal conditioning issues High common-mode voltage with respect to signal voltage Weigh scale system requirements Understanding noise-free code resolution ΣΔ ADC vs. SAR ADC High performance instrumentation amp solution (CN0216) High resolution ΣΔ integrated solution (CN0102) 33
  31. 31. Resistance-Based Sensor Examples 34 Strain gages 120 Ω, 350 Ω, 3500 Ω Weigh scale load cells 350 Ω to 3500 Ω Pressure sensors 350 Ω to 3500 Ω Relative humidity 100 kΩ to 10 MΩ Resistance temperature devices (RTDs) 100 Ω, 1000 Ω Thermistors 100 Ω to 10 MΩ
  32. 32. VO R4 R1 R3 R2 VB VO R R R VB R R R VB= + − + 1 1 4 2 2 3 = − +       +       R R R R R R R R VB 1 4 2 3 1 1 4 1 2 3 AT BALANCE, VO IF R R R R = =0 1 4 2 3 + - Wheatstone Bridge for Precision Resistance Measurements 35
  33. 33. Output Voltage and Linearity Error for Constant Voltage Drive Bridges 36 R R R R+∆R R+∆R R+∆R R+∆R R+∆R R−∆R R+∆R R−∆RR R R R−∆R VB VB VB VB VO VO VO VO (A) Single-Element Varying (B) Two-Element Varying (1) (C) Two-Element Varying (2) (D) All-Element Varying Linearity Error: VO: 0.5%/% 0.5%/% 0 0 VB 4 ∆R ∆R 2 R + VB 2 ∆R ∆R 2 R + VB 2 ∆R R VB ∆R R R
  34. 34. R R R R+∆R R+∆R R+∆R R+∆R R+∆R R−∆R R+∆R R−∆RR R R R−∆R VO VO VO VO IB IB IB IB VO: Linearity Error: 0.25%/% 0 0 0 IBR 4 ∆R ∆R 4 R + IB 2 ∆R IB ∆RIB 2 ∆R (A) Single-Element Varying (B) Two-Element Varying (1) (C) Two-Element Varying (2) (D) All-Element Varying R Output Voltage and Linearity Error for Constant Current Drive Bridges 37
  35. 35. Kelvin (4-Wire) Sensing Minimizes Errors Due to Lead Resistance for Voltage Excitation 38 6-LEAD BRIDGE RLEAD RLEAD +SENSE – SENSE +FORCE – FORCE + + +VB – – VO
  36. 36. 4-LEAD BRIDGE RLEAD + –RLEAD RSENSE VREF VO I I I I = VREF RSENSE Constant Current Excitation also Minimizes Wiring Resistance Errors 39
  37. 37. ADC Architectures, Applications, Resolution, Sampling Rates 40 10 100 1k 10k 100k 1M 10M 100M 1G 8 10 12 14 16 18 20 22 24 Σ-∆ SAR PIPELINE INDUSTRIAL MEASUREMENT DATA ACQUISITION HIGH SPEED INSTRUMENTATION, VIDEO, IF SAMPLING, SOFTWARE RADIO SAMPLING RATE (Hz) APPROXIMATE STATE-OF-THE-ART (2013) RESOLUTION
  38. 38. SAR vs. Sigma-Delta Comparison 41 Successive approximation (SAR)  Fast settling, ideal for multiplexing  Data available immediately after conversion (no "pipeline" delay)  Easy to use (minimal programming)  Requires external in-amp  Has 1/f noise (need lots of external filtering)  Analog filter can be difficult Sigma-Delta  Digital filter limits settling  More difficult to use (some programming required)  Some have internal PGA  Some have chopping (removes 1/f noise)  Internal digital filter (removes power line noise)  Oversampling relaxes requirement on analog filter
  39. 39. Sigma-Delta Concepts: Oversampling, Digital Filtering, Noise Shaping, and Decimation 42 fs 2 fs Kfs 2 Kfs Kfs Kfs 2 fs 2 fs 2 DIGITAL FILTER REMOVED NOISE REMOVED NOISE QUANTIZATION NOISE = q / 12 q = 1 LSBADC ADC DIGITAL FILTER Σ∆ MOD DIGITAL FILTER fs Kfs Kfs DEC fs NYQUIST OPERATION OVERSAMPLING + DIGITAL FILTER + DECIMATION OVERSAMPLING + NOISE SHAPING + DIGITAL FILTER + DECIMATION A B C DEC fs
  40. 40. First-Order Sigma-Delta ADC 43 ∑ ∫ + _ +VREF –VREF DIGITAL FILTER AND DECIMATOR + _ CLOCK Kfs VIN N-BITS fs fs A B 1-BIT DATA STREAM1-BIT DAC LATCHED COMPARATOR (1-BIT ADC) 1-BIT, Kfs Ʃ-∆ MODULATOR INTEGRATOR
  41. 41. Sigma-Delta ADC Architecture Benefits High resolution  24 bits, no missing codes  22 bits, effective resolution (RMS)  19 bits, noise-free code resolution (peak-to-peak)  On-chip PGAs High accuracy  INL 2 ppm of full-scale ~ 1 LSB in 19 bits  Gain drift 0.5ppm/°C More digital, less analog  Programmable balance between speed × resolution Oversampling and digital filtering  50 Hz/60 Hz rejection  High oversampling rate simplifies antialiasing filter Wide dynamic range Low cost 44
  42. 42. Typical Applications of High Resolution Sigma-Delta ADCs Process control  4 mA to 20 mA Sensors  Weigh scale  Pressure  Temperature Instrumentation  Gas monitoring  Portable instrumentation  Medical instrumentation 45 WEIGH SCALE
  43. 43. Precision Weigh Scales-Industrial and High Precision Commercial 46 Laboratory scales Process control  Hopper scales  Conveyor scales Stock control  Counting scales Retail scales
  44. 44. Weigh Scale Product Definition 47 Capacity 2 kg Sensitivity 0.1 g Other features  Accuracy 0.1 %  Linearity ±0.1 g  Temperature drift (±20 ppm at 10°C ~ 30°C)  Data rate 5 Hz to 10 Hz  Power (120 V AC)  Dimensions (7.5” × 8.6” × 2.6”)  Qualification (“legal for trade”)
  45. 45. Characteristics of Tedea Huntleigh 505H-0002-F070 Load Cell 48 Full load 2 kg Sensitivity 2 mV/V Excitation 5 V Other features  Impedance 350 Ω  Total error 0.025%  Hysteresis 0.025%  Repeatability 0.01  Temperature drifts 10 ppm/°C  Overload 150% Four strain gages
  46. 46. Characteristics of Tedea Huntleigh 505H-0002-F070 Load Cell 49  Full load 2 kg  Sensitivity 2 mV/V  Excitation 5 V  VFS = VEXC × Sensitivity  VFS = 5 V × 2 mV/V = 10 mV  VCM = 2.5 V  Full-scale voltage 10 mV  Proportional to excitation  “Ratiometric”
  47. 47. Input-Referred Noise of ADC Determines the "Noise-Free Code Resolution" 50 n n+1 n+2 n+3 n+4n–1n–2n–3n–4 NUMBER OF OCCURANCES RMS NOISE P-P INPUT NOISE ≈ 6.6 × RMS NOISE OUTPUT CODE “GROUNDED INPUT HISTOGRAM"
  48. 48. Performance Requirement – Resolution 51 Required: 0.1 g in 2 kg  # Noise free counts = full-scale/p-p noise in g  # Noise free counts = 2000 g/0.1 g = 20,000  20,000 counts  VFS = 10 mV at 5 V excitation  V P-P NOISE < VFS/# counts  VP-P NOISE < 10 mV/20,000 = 0.0005 mV  0.5 µV p-p noise  VRMS NOISE ≈ VP-P NOISE/6.6  VRMS NOISE ≈ 0.5 µV/6.6 = 0.075 µV  75 nV RMS noise  Noise-free bits = log2( VFS/VP-P NOISE)  Noise-free bits = log10(VFS/VP-P NOISE) / log10(2)  Noise-free bits = log10(10 mV/0.0005 mV)/0.3  Noise-free bits = 14.3 (minimum)  14.3 bits p-p in 10 mV range:  Bits RMS = log10( VFS/VRMS NOISE)/log10(2)  Bits RMS = log10( 10 mV/0.000075)/0.3  17.0 bits RMS in 10 mV range
  49. 49. Definition of "Noise-Free" Code Resolution and "Effective" Resolution 52 Effective Resolution = log2 Full-Scale Range RMS Noise Bits Noise-Free Code Resolution = log2 Full-Scale Range P-P Noise Bits P-P Noise = 6.6 × RMS Noise Noise-Free Code Resolution = log2 Full-Scale Range 6.6 × RMS Noise Bits = Effective Resolution – 2.72 Bits log2 (x) = log10 (x) log10 (2) = log10 (x) 0.301
  50. 50. Terminology for Resolution Based on Peak-to- Peak and RMS Noise Peak-to-peak noise:  Noise-free code resolution  Noise-free bits  Flicker-free bits  Peak-to-peak resolution RMS noise:  Effective resolution  RMS resolution  The term "Effective Number of Bits" (ENOB) applies to high speed ADCs with sine wave inputs: ENOB = log2 (RMS value of FS sine wave/RMS noise) This should not be confused with "Effective Resolution" 53
  51. 51. Options for Conditioning Load Cell Outputs 54 + − + − + − + − + − A: EXTERNAL IN-AMP B: DIFFERENTIAL INPUT ADC EXTERNAL IN-AMP (SEE CN0216) C: DIFFERENTIAL INPUT ADC INTERNAL IN-AMP OR PGA (SEE CN0102) ADC SAR or Σ-Δ RG RG VCM LOAD CELL LOAD CELL LOAD CELL IN-AMP FUNNEL AMP (AD8475) 10mV FS 10mV FS 10mV FS ADC SAR or Σ-Δ ADC SAR or Σ-Δ ADC Σ-Δ PGA ~12 NOISE-FREE BITS FOR 10mV FS ~12 NOISE-FREE BITS FOR 10mV FS 15 NOISE-FREE BITS FOR 10mV FS 16 NOISE-FREE BITS FOR 10mV FS SEE CN0251) LOW NOISE OP AMPS
  52. 52. CN0216: Load Cell Signal Conditioning with Differential Input ADC and External In-Amp Circuit features  Gain of 375 low noise in-amp  15.3 noise-free bits of resolution Circuit benefits  Precision load cell conditioning  Zero-drift in-amp  Single +5 V operation Inputs  10 mV full-scale 55 Target Applications Key Parts Used Interface/Connectivity Load cell Weigh scales AD7791 ADA4528-1 ADP3301 SPI (AD7791) SDP (EVAL-CN0216-SDPZ) USB (EVAL-SDP-CB1Z)
  53. 53. CN0216: Load Cell Conditioning with Differential Input ADC and External In-Amp 56 G = 375 FS = 10mV FS = 3.75V INPUT RANGE = 10V p-p 1 LSB = 10V/224 = 0.596µV 24-BIT Σ-Δ ADC BW = 4.3Hz DIFF BW = 8Hz CM BW = 160Hz
  54. 54. CN0216 Noise Performance 57 Data rate = 9.5 Hz VP-P NOISE = 159 counts × 0.596 µV = 94.8 µV VFS = 3.75 V Noise-free counts = VFS / VP-P NOISE = 3.75 V/94.8 µV = 39,557 Noise-free bits = log2(39,557) = 15.3 bits
  55. 55. CN0216 Evaluation Board and Software 58 Complete design files  Schematic  Bill of material  PADs layout  Gerber files  Assembly drawing
  56. 56. AD7190, 24-Bit Sigma-Delta ADC: Weigh Scale with Ratiometric Processing 59 IN+ IN- OUT- OUT+ +5V 2mV/V SENSITIVITY Load cell: ■ 2 mV/V typically => with +5 V excitation, full-scale signal from load cell = 10 mV. AD7190 ■ With VREF = 5 V, gain = 128, full-scale signal = ±40 mV (80 mV p-p). ■ 12.5% of range used by load cell signal (10 mV ÷ 80 mV = 0.125). ■ The load cell has an offset (~50%) and full-scale error (~±20%). The wider range available from the AD7190 prevents the offset and full-scale error from overloading the AD7190. ■ Ratiometric operation eliminates need for external voltage reference.
  57. 57. AD7190 Sigma-Delta System On-Chip Features Analog input buffer options  Drives Σ-Δ modulator, reduces dynamic input current Differential AIN, REFIN  Ratiometric configuration eliminates need for accurate reference Multiplexer PGA Calibrations  Self calibration, system calibration, auto calibration Chopping options  No offset and offset drifts  Minimizes effects of parasitic thermocouples 60
  58. 58. CN0102: Precision Weigh Scale System Circuit features  Integrated solution with PGA  16.8 noise-free bits Circuit benefits  Single supply  Optimized for weigh scales Inputs  10 mV full-scale 61 Target Applications Key Parts Used Interface/Connectivity Weigh scales Load cells AD7190 ADP3303 SPI (AD7190) USB (EVAL-AD7190EBZ) EVAL-AD7190EBZ
  59. 59. CN0102 Precision Weigh Scale System 62
  60. 60. AD7190 Sinc4 Filter Response, 50 Hz Output Data Rate 63
  61. 61. AD7190 Noise and Resolution, Sinc4 Filter, Chop Disabled 64 For G = 128 VREF = 5 V, FS = 80 mV p-p 17.5 for 10 mV p-p Only using 10 mV out of 80 mV range
  62. 62. CN0102 Load Cell Test Results, 500 Samples 65 System resolution with load cell connected  Load cell: full-scale output = 10 mV (2 mV/V sensitivity, VEXC = 5 V)  Measured RMS noise = 12 nV at 4.7 Hz data rate (G = 128)  Measured peak-to-peak noise = 88 nV  Noise-free counts = {full-scale output/peak-to-peak noise} = 10 mV/88 nV = 113,600  Noise-free resolution: log2 (113,600) = 16.8 bits Compared to 17.5 bits for AD7190 alone  If a 2 kg load cell is used, resolution is 2000 g/113,600 = 0.02 g
  63. 63. CN0102 Evaluation Board and Load Cell 66 EVAL-AD7190EBZ Software Display Complete design files  Schematic  Bill of material  PADs layout  Gerber files  Assembly drawing
  64. 64. Tweet it out! @ADI_News #ADIDC13 What We Covered Fundamentals of making complex impedance measurements using integrated solutions (CN0217)  Applications  Extending the range of measurement using analog front end circuit  Measurement results and applications Tilt measurements using dual axis accelerometers (CN0189)  Applications  Advantages of dual axis vs. single axis  Accelerometer conditioning circuits Precision load cells (weigh scales) (CN0216, CN0102)  Applications and requirements  Bridge fundamentals  Sigma-delta ADC fundamentals  Noise considerations and definition of noise-free code resolution  Solution using external in-amp  Solution using integrated PGA 67
  65. 65. Tweet it out! @ADI_News #ADIDC13 Visit the Impedance Measurement Demo in the Exhibition Room Measuring complex impedances with the AD5933 68 This demo board is available for purchase: www.analog.com/DC13-hardware SOFTWARE OUTPUT DISPLAY
  66. 66. Tweet it out! @ADI_News #ADIDC13 Visit the Tilt Measurement Demo in the Exhibition Room 69 Measure tilt using the ADXL203 dual axis accelerometer This demo board is available for purchase: www.analog.com/DC13-hardware SDP-S BOARDSOFTWARE OUTPUT DISPLAY EVAL-CN0189-SDPZ
  67. 67. Tweet it out! @ADI_News #ADIDC13 Visit the Weigh Scale Demo in the Exhibition Room 70 Measure weights from 0.1 g to 2000 g This demo board is available for purchase: www.analog.com/DC13-hardware SOFTWARE OUTPUT DISPLAY EVAL-CN0216-SDPZ SDP BOARD
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