Instrumentation: Test and Measurement Methods and Solutions Reference Designs and System Applications
Nov 17, 2014
Instrumentation: Test and Measurement Methods and SolutionsReference Designs and System Applications
2
Legal Disclaimer
Notice of proprietary information, Disclaimers and Exclusions Of Warranties
The ADI Presentation is the property of ADI. All copyright, trademark, and other intellectual property and
proprietary rights in the ADI Presentation and in the software, text, graphics, design elements, audio and all other
materials originated or used by ADI herein (the "ADI Information") are reserved to ADI and its licensors. The ADI
Information may not be reproduced, published, adapted, modified, displayed, distributed or sold in any manner, in
any form or media, without the prior written permission of ADI.
THE ADI INFORMATION AND THE ADI PRESENTATION ARE PROVIDED "AS IS". WHILE ADI INTENDS THE ADI
INFORMATION AND THE ADI PRESENTATION TO BE ACCURATE, NO WARRANTIES OF ANY KIND ARE MADE
WITH RESPECT TO THE ADI PRESENTATION AND THE ADI INFORMATION, INCLUDING WITHOUT LIMITATION
ANY WARRANTIES OF ACCURACY OR COMPLETENESS. TYPOGRAPHICAL ERRORS AND OTHER
INACCURACIES OR MISTAKES ARE POSSIBLE. ADI DOES NOT WARRANT THAT THE ADI INFORMATION AND
THE ADI PRESENTATION WILL MEET YOUR REQUIREMENTS, WILL BE ACCURATE, OR WILL BE
UNINTERRUPTED OR ERROR FREE. ADI EXPRESSLY EXCLUDES AND DISCLAIMS ALL EXPRESS AND IMPLIED
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT OF
ANY THIRD PARTY INTELLECTUAL PROPERTY RIGHTS. ADI SHALL NOT BE RESPONSIBLE FOR ANY DAMAGE
OR LOSS OF ANY KIND ARISING OUT OF OR RELATED TO YOUR USE OF THE ADI INFORMATION AND THE ADI
PRESENTATION, INCLUDING WITHOUT LIMITATION DATA LOSS OR CORRUPTION, COMPUTER VIRUSES,
ERRORS, OMISSIONS, INTERRUPTIONS, DEFECTS OR OTHER FAILURES, REGARDLESS OF WHETHER SUCH
LIABILITY IS BASED IN TORT, CONTRACT OR OTHERWISE. USE OF ANY THIRD-PARTY SOFTWARE
REFERENCED WILL BE GOVERNED BY THE APPLICABLE LICENSE AGREEMENT, IF ANY, WITH SUCH THIRD
PARTY.
©2013 Analog Devices, Inc. All rights reserved.
3
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
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
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
EVALUATIONBOARD
SDP-BSDP-S
EVALUATIONBOARD
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
6
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)
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
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 Solution
AD5933
DDS Filter Buffer
ADC
VDD/2
DAC
Z(ω)
SCL
SDA
DVDDAVDDMCLK
AGND DGND
ROUT VOUT
AD5933RFB
VIN
05
324
-001
1024-POINT DFT
I2CINTERFACE
IMAGINARYREGISTER
REALREGISTER
OSCILLATOR
DDSCORE
(27 BITS)
TEMPERATURESENSOR
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
I2CINTERFACETO µC OR PC UNKNOWN
IMPEDANCE
EXCITATION FREQUENCY
REAL AND IMAGINARYCOMPONENTREGISTERS
DDS
ADJUSTABLEVOLTAGEEXITATION
CURRENT TOVOLTAGECONVERTER
10
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
Target Applications Key Parts Used Interface/Connectivity
MedicalConsumerIndustrial
AD5933AD8606
I2C (AD5933)USB (EVAL-AD5933EBZ)
50kΩ
50kΩ
50kΩ
50kΩ
RFB
20kΩ
20kΩ
47nF
ZUNKNOWN
VDD
VDD
VDD
+
+
−
−
A1
A2
A1, A2 ARE½ AD8606
1.48V
1.98V p-p
VDD/2
1.98V p-p
VDD/2
DAC
SCL
SDA
DVDDAVDDMCLK
AGND DGND
ROUT
VOUT
AD5933/AD5934RFB
VIN
1024-POINT DFT
I2CINTERFACE
IMAGINARYREGISTER
REALREGISTER
OSCILLATOR
DDSCORE
(27 BITS)
TEMPERATURESENSOR
TRANSMIT SIDEOUTPUT AMPLIFIER
ADC(12 BITS) LPF
GAIN
VDD VDD
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
12
High Accuracy Performance from the AD5933/AD5934 with External AFE
30 35 40
FREQUENCY (kHz)
45 508160
8180
8200
8220
8240
8260
8280
IMP
ED
AN
CE
MA
GN
ITU
DE
(Ω
)
R3
IDEAL
09
915-
008
35
30
25
20
15
10
5
029.95 30.00 30.05 30.10 30.15 30.20
10.3Ω
30Ω
1µF
30.25
FREQUENCY (kHz)
MA
GN
ITU
DE
(Ω
)
09
915
-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 kΩOne calibration using 99.85 kΩ resistor covers wide range
Allows low value impedancemeasurements
Tracks R||Cacross frequency
30 35 40
FREQUENCY (kHz)
45 50
IMP
ED
AN
CE
MA
GN
ITU
DE
(kΩ
)
R4
09
91
5-0
09213.5
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
IMP
ED
AN
CE
MA
GN
ITU
DE
(Ω
)
FREQUENCY (kHz)
IDEALMEASURED
09
915
-011
13
Low RON SPDT CMOS Switch Used to Switch Between RCAL and Unknown Z
50kΩ
ZUNKNOWN RCAL
S1
D
S2
RFB
VDD
IN
ADG849
50kΩ
A1
A2
Use low RON CMOS switch for switching from unknown impedance to calibration resistor
RON = 0.5Ω
14
CN0217 Evaluation Board, EVAL-CN0217-EB1Z
Complete design files Schematic Bill of material PADs layout Gerber files Assembly drawing
PC
Unknown Z
USB
15
AD5933 Web Based Demonstration Tool
Web tool demo:
Enter different impedance types
Generate frequency sweep
Examine impedance plot
16
AD5933 Used with AFE for Measuring Ground-Referenced Impedance in Blood-Coagulation Measurement System
Ground-referencedUnknown Z
17
Blood Clotting Factor Measurements
Liquid Quality Impedance Measurement
18
CONDUCTANCE LIQUIDMEASUREMENT
SWITCHES
AFE
AD5933/AD5934
CONTROLLER
CALIBRATION IMPEDANCE
UNKNOWN IMPEDANCE
19
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
20
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
21
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
22
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
23
ADXL-Family Micromachined iMEMS Accelerometers (Top View of IC)
FIXEDOUTERPLATES
CS1 CS1 < CS2= CS2
DENOTES ANCHOR
BEAM
TETHER
CS1 CS2
CENTERPLATE
AT REST APPLIED ACCELERATION
24
ADXL-Family iMEMS AccelerometersInternal Signal Conditioning
OSCILLATOR A1SYNCHRONOUSDEMODULATOR
BEAM
PLATE
PLATE
CS1
CS2
SYNC
0°
180°A2
VOUT
CS2 > CS1
AP
PL
IED
AC
CE
LE
RA
TIO
N
25
Using a Single Axis Accelerometer to Measure Tilt
X
0°
+90°
q1g
Acceleration
X
–90°
–1g
0°
+1g
+90°
Acceleration = 1g × sin q
q0g
–90°
Highest sensitivity between −45° and +45°
Ambiguous beyond ±90°
26
Single Axis vs. Dual Axis Acceleration Measurements
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
27
ADXL203 Dual Axis Accelerometer
1 mg resolution for BW = 60 Hz
700 µA current @ 5 V
28
CN0189: Tilt Measurement Using a Dual Axis Accelerometer 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
MedicalConsumerIndustrial
ADXL203AD8608AD7887
SPI (AD7887) SDP-S (EVAL-CN0189-SDPZ)USB (EVAL-SDP-CS1Z)
29
CN0189 Dual Axis Tilt Measurement Circuit
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
30
Output Error for arcsin(X), arccos(Y), andarctan(X/Y) Calculations
OUTPUT = arcsin(X)
OUTPUT = arccos(Y)
OUTPUT = arctan(X/Y)
Error increases at ±90°
Error increases at 0°
Uniform error distribution
31
Tilt Measurement Using Dual Axis Accelerometer (CN0189 Block Diagram)
ADXL203DUAL AXIS
ACCELEROMETER
AD8608 QUAD OP AMP
SIGNAL CONDITIONING
AD78872-CHANNEL
12-BIT, 125kSPS SAR ADC
SYSTEMDEMONSTRATION
PLATFORM(SDP)
EVAL-SDP-CB1Z
PC
USB
X
Y
X
Y
CN0189 EVALUATION BOARD (EVAL-CN0189-SDPZ)
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
33
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)
34
Resistance-Based Sensor Examples
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
35
VO
R4
R1
R3
R2
VB
VOR
R RVB
RR R
VB
11 4
22 3
RR
RR
RR
RR
VB
14
23
114
123
AT BALANCE,
VO IFRR
RR
014
23
+ -
Wheatstone Bridge for Precision Resistance Measurements
36
Output Voltage and Linearity Error for Constant Voltage Drive Bridges
R R
R R+DR
R+DR
R+DR R+DR R+DR
R−DR R+DR R−DRR R
R R−DR
VB VB VB VB
VOVO VO
VO
(A) Single-ElementVarying
(B) Two-ElementVarying (1)
(C) Two-ElementVarying (2)
(D) All-ElementVarying
LinearityError:
VO:
0.5%/% 0.5%/% 0 0
VB
4
DRDR2R +
VB
2
DRDR2R +
VB
2
DR
R VB
DR
R
R
37
R R
R R+DR
R+DR
R+DR R+DR R+DR
R−DR R+DR R−DRR R
R R-DR
VOVO VO
VO
IB IB IB IB
VO:
LinearityError: 0.25%/% 0 0 0
IBR
4
DRDR4R +
IB2
DR IBDRIB
2DR
(A) Single-ElementVarying
(B) Two-ElementVarying (1)
(C) Two-ElementVarying (2)
(D) All-ElementVarying
R
Output Voltage and Linearity Error for Constant Current Drive Bridges
Kelvin (4-Wire) Sensing Minimizes Errors Due to Lead Resistance for Voltage Excitation
38
6-LEADBRIDGE
RLEAD
RLEAD
+SENSE
– SENSE
+FORCE
– FORCE
+
+
+VB
–
–
VO
39
4-LEADBRIDGE
RLEAD
+
–RLEAD
RSENSE
VREF
VO
I
I
II =
VREF
RSENSE
Constant Current Excitation alsoMinimizes Wiring Resistance Errors
40
ADC Architectures, Applications, Resolution, Sampling Rates
10 100 1k 10k 100k 1M 10M 100M 1G8
10
12
14
16
18
20
22
24
S-D
SAR
PIPELINE
INDUSTRIALMEASUREMENT
DATA ACQUISITION
HIGH SPEEDINSTRUMENTATION,VIDEO, IF SAMPLING, SOFTWARE RADIO
SAMPLING RATE (Hz)
APPROXIMATE STATE-OF-THE-ART
(2013)
RE
SO
LU
TIO
N
41
SAR vs. Sigma-Delta Comparison
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
42
Sigma-Delta Concepts: Oversampling, Digital Filtering, Noise Shaping, and Decimation
fs
2
fs
Kfs
2
Kfs
KfsKfs
2
fs
2
fs
2
DIGITAL FILTER
REMOVED NOISE
REMOVED NOISE
QUANTIZATIONNOISE = q / 12 q = 1 LSBADC
ADCDIGITALFILTER
SDMOD
DIGITALFILTER
fs
Kfs
Kfs
DEC
fs
NYQUISTOPERATION
OVERSAMPLING+ DIGITAL FILTER+ DECIMATION
OVERSAMPLING+ NOISE SHAPING+ DIGITAL FILTER+ DECIMATION
A
B
C
DEC
fs
43
First-Order Sigma-Delta ADC
å ò +
_
+VREF
–VREF
DIGITALFILTER
ANDDECIMATOR
+
_
CLOCKKfs
VINN-BITS
fs
fs
A
B
1-BIT DATASTREAM1-BIT
DAC
LATCHEDCOMPARATOR(1-BIT ADC)
1-BIT,
Kfs
Ʃ-∆ MODULATOR
INTEGRATOR
44
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
45
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
WEIGH SCALE
46
Precision Weigh Scales-Industrial and High Precision Commercial Laboratory scales
Process control Hopper scales Conveyor scales
Stock control Counting scales
Retail scales
47
Weigh Scale Product Definition
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”)
48
Characteristics of Tedea Huntleigh 505H-0002-F070 Load Cell
Full load 2 kg
Sensitivity 2 mV/V
Excitation 5 V
Other features Impedance 350 W Total error 0.025% Hysteresis 0.025% Repeatability 0.01 Temperature drifts 10 ppm/°C Overload 150%
Four straingages
49
Characteristics of Tedea Huntleigh 505H-0002-F070 Load Cell
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”
50
Input-Referred Noise of ADC Determines the "Noise-Free Code Resolution"
n n+1 n+2 n+3 n+4n–1n–2n–3n–4
NUMBER OFOCCURANCES
RMS NOISE
P-P INPUT NOISE
» 6.6 × RMS NOISE
OUTPUT CODE
“GROUNDED INPUT HISTOGRAM"
51
Performance Requirement – Resolution
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
52
Definition of "Noise-Free" Code Resolution and "Effective" Resolution
EffectiveResolution
= log2
Full-Scale RangeRMS Noise Bits
Noise-FreeCode Resolution = log2
Full-Scale RangeP-P Noise Bits
P-P Noise = 6.6 × RMS Noise
Noise-FreeCode Resolution
= log2Full-Scale Range6.6 × RMS Noise
Bits
= Effective Resolution – 2.72 Bits
log2 (x) = log10 (x)
log10 (2)=
log10 (x)
0.301
53
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"
Options for Conditioning Load Cell Outputs
54
+
−
+
−
+
−
+
−
+
−
A: EXTERNAL IN-AMP
B:DIFFERENTIAL INPUT ADCEXTERNAL IN-AMP (SEE CN0216)
C: DIFFERENTIAL INPUT ADCINTERNAL IN-AMP OR PGA(SEE CN0102)
ADCSAR or Σ-Δ
RG
RG
VCM
LOADCELL
LOADCELL
LOADCELL
IN-AMP
FUNNEL AMP (AD8475)
10mVFS
10mVFS
10mVFS
ADCSAR or Σ-Δ
ADCSAR or Σ-Δ
ADCΣ-ΔPGA
~12 NOISE-FREE BITS
FOR 10mV FS
~12NOISE-FREE BITS
FOR 10mV FS
15NOISE-FREE BITS
FOR 10mV FS
16NOISE-FREE BITS
FOR 10mV FS
SEE CN0251)
LOW NOISE OP AMPS
55
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
Target Applications Key Parts Used Interface/Connectivity
Load cellWeigh scales
AD7791ADA4528-1ADP3301
SPI (AD7791) SDP (EVAL-CN0216-SDPZ)USB (EVAL-SDP-CB1Z)
CN0216: Load Cell Conditioning with Differential Input ADC and External In-Amp
56
G = 375
FS = 10mV
FS = 3.75VINPUT RANGE = 10V p-p1 LSB = 10V/224 = 0.596µV
24-BITΣ-Δ ADC
BW = 4.3Hz DIFF BW = 8HzCM BW = 160Hz
57
CN0216 Noise Performance
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
58
CN0216 Evaluation Board and Software
Complete design files Schematic Bill of material PADs layout Gerber files Assembly drawing
59
AD7190, 24-Bit Sigma-Delta ADC: Weigh Scale with Ratiometric Processing
IN+
IN-
OUT- OUT+
+5V
2mV/VSENSITIVITY
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.
60
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
61
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
Target Applications Key Parts Used Interface/Connectivity
Weigh scalesLoad cells
AD7190ADP3303
SPI (AD7190) USB (EVAL-AD7190EBZ)
EVAL-AD7190EBZ
62
CN0102 Precision Weigh Scale System
63
AD7190 Sinc4 Filter Response, 50 Hz Output Data Rate
64
AD7190 Noise and Resolution, Sinc4 Filter, Chop Disabled
For G = 128VREF = 5 V, FS = 80 mV p-p
17.5for 10 mV p-p
Only using 10 mV out of 80 mV range
65
CN0102 Load Cell Test Results, 500 Samples
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
66
CN0102 Evaluation Board and Load Cell
EVAL-AD7190EBZ
Software Display
Complete design files Schematic Bill of material PADs layout Gerber files Assembly drawing
67 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
68 Tweet it out! @ADI_News #ADIDC13
Visit the Impedance Measurement Demo in the Exhibition Room
Measuring complex impedances with the AD5933
This demo board is available for purchase: www.analog.com/DC13-hardware
SOFTWARE OUTPUT DISPLAY
69 Tweet it out! @ADI_News #ADIDC13
Visit the Tilt Measurement Demo in the Exhibition Room
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
70 Tweet it out! @ADI_News #ADIDC13
Visit the Weigh Scale Demo in the Exhibition Room
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
71 Tweet it out! @ADI_News #ADIDC13
Design Resources Covered in this Session
Design tools and resources:Name Description URL
AD5933/AD5934 Demonstration and Design Tool
Demonstrates impedance measurement using the AD5933/AD5934
http://designtools.analog.com/dt/ad593x/ad593x.html
CN0189 FMC-SDP Interposer and Evaluation Board/ Xilinx KC705 Reference Design
Using the EVAL-CN0189-SDPZ evaluation board, together with the Xilinx® KC705 FPGA board, the Xilinx Embedded Development Kit (EDK), and the Micrium µC-Probe run-time monitoring tool.
http://wiki.analog.com/resources/fpga/xilinx/interposer/cn0189
ADXL203 Simulink® Model
Simulink model http://www.analog.com/en/mems-sensors/mems-inertial-sensors/adxl203/products/tools-software-simulation-models/index.html?location=tools-software
CN0216 BeMicro FPGA
BeMicro FPGA for CN0216 with Nios driver
http://wiki.analog.com/resources/fpga/altera/bemicro/cn0216
72 Tweet it out! @ADI_News #ADIDC13
Design Resources Covered in this Session-2
Name Description URL
Signal Chain Designer
Complete engineering design environment
http://www.analog.com/scd
AD7190 Tools Tools, software, and simulation models
http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7190/products/tools-software-simulation-models/index.html?location=tools-softwareAD7887 Tools Tools, software, and
simulation modelshttp://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7887/products/tools-software-simulation-models/index.html?location=tools-softwareAD7791 Tools Tools, software, and
simulation modelshttp://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7791/products/tools-software-simulation-models/index.html?location=tools-software
73 Tweet it out! @ADI_News #ADIDC13
Design Resources Covered in this Session-3
Ask technical questions and exchange ideas online in our EngineerZone® Support Community Choose a technology area from the homepage:
ez.analog.com Access the Design Conference community here:
www.analog.com/DC13community
74 Tweet it out! @ADI_News #ADIDC13
Selection Table of Products Covered Today
Part number Description
AD5933 1 MSPS, 12-bit impedance converter, network analyzer
AD8606 Precision, low noise, RRIO, CMOS op amp (dual)
ADG849 3 V/5 V CMOS 0.5 Ω SPDT switch in SC70
AD8221 Precision instrumentation amplifier
AD820 Single-supply, rail-to-rail, low power, FET input op amp
ADXL203 Precision ±1.7 g, ±5 g, ±18 g dual axis iMEMS accelerometer
AD8608 Precision, low noise, RRIO, CMOS op amp (quad)
AD7887 2.7 V to 5.25 V, micropower, 2-channel, 125 kSPS, 12-bit ADC in 8-lead MSOP
75 Tweet it out! @ADI_News #ADIDC13
Selection Table of Products Covered Today-2
Part number Description
AD7791 24-bit, single-channel, ultralow power, Ʃ-∆ ADC
ADA4528-1 5.0 V ultralow noise, zero-drift, RRIO, single op amp
ADP3301 High accuracy anyCAP® 100 mA low dropout linear regulator
ADP7190 4.8 kHz ultralow noise 24-bit Ʃ-∆ ADC with PGA
ADP3303 High accuracy anyCAP 200 mA low dropout linear regulator
76 Tweet it out! @ADI_News #ADIDC13
References-1
Circuit Notes CN0217, Impedance Measurements
www.analog.com/CN0217 CN0189, Tilt Measurements
www.analog.com/CN0189 CN0216, Precision Weigh Scale, External In-Amp
www.analog.com/CN0216 CN0102, Precision Weigh Scale, Internal PGA
www.analog.com/CN0102 CN0251, A Flexible 4-Channel Analog Front End for Wide Dynamic Range
Signal Conditioning www.analog.com/CN0251
CN0260, Oversampled SAR ADC with PGA www.analog.com/CN0260
CN0189, 4 mA to 20 mA Loop-Powered Pressure Sensor Transmitter www.analog.com/CN0189
77 Tweet it out! @ADI_News #ADIDC13
References-2
Mini Tutorials MT-004, ADC Input Noise
www.analog.com/MT-004 MT-021, Successive Approximation (SAR) ADCs
www.analog.com/MT-021 MT-022, Sigma-Delta ADC Basics
www.analog.com/MT-022 MT-023, Sigma-Delta ADC Advanced Concepts
www.analog.com/MT-023 MT-061, In-Amp Basics
www.analog.com/MT-061 MT-062, Two Op Amp In-Amp
www.analog.com/MT-062 MT-063, Three Op Amp In-Amp
www.analog.com/MT-063
78 Tweet it out! @ADI_News #ADIDC13
References-3
Mini Tutorials MT-064, In-Amp DC Errors
www.analog.com/MT-064 MT-065, In-Amp Noise
www.analog.com/MT-065 MT-066, In Amp Bridge Circuit Error Analysis
www.analog.com/MT-066 MT-069, In-Amp Overvoltage Protection
www.analog.com/MT-069 MT-070, In-Amp Input RFI Protection
www.analog.com/MT-070
79 Tweet it out! @ADI_News #ADIDC13
References-4
Reference Books Sensor Signal Conditioning
www.analog.com/sensor_signal_conditioning Analog-Digital Conversion
http://www.analog.com/library/analogDialogue/archives/39-06/data_conversion_handbook.html
Op Amp Applications http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applic
ations_handbook.html Linear Circuit Design
http://www.analog.com/library/analogDialogue/archives/43-09/linear_circuit_design_handbook.html
Instrumentation Amplifier Handbook http://www.analog.com/en/specialty-amplifiers/instrumentation-amplifiers/prod
ucts/design-handbooks/cu_dh_designers_guide_to_instrumentation_amps/resources/fca.html