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.

1. Instrumentation: Test and
Measurement Methods and Solutions
Reference Designs and System Applications
Walt Kester, Applications Engineer, Greensboro, NC, US

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. AD5933 Used with AFE for Measuring Ground-
Referenced Impedance in Blood-Coagulation
Measurement System
16
Ground-referenced
Unknown Z

15. Blood Clotting Factor Measurements
17

16. Liquid Quality Impedance Measurement
18
CONDUCTANCE
LIQUID
MEASUREMENT
SWITCHES
AFE
AD5933/
AD5934
CONTROLLER
CALIBRATION
IMPEDANCE
UNKNOWN
IMPEDANCE

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. 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. 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. 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. 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. 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. 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. 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. ADXL203 Dual Axis Accelerometer
27
1 mg resolution for BW = 60 Hz
700 µA current @ 5 V

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 4-LEAD
BRIDGE
RLEAD
+
–RLEAD
RSENSE
VREF
VO
I
I
I
I =
VREF
RSENSE
Constant Current Excitation also
Minimizes Wiring Resistance Errors
39

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. 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. 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. 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. 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. 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. Precision Weigh Scales-Industrial and
High Precision Commercial
46
Laboratory scales
Process control
 Hopper scales
 Conveyor scales
Stock control
 Counting scales
Retail scales

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. CN0216 Evaluation Board and Software
58
Complete design files
 Schematic
 Bill of material
 PADs layout
 Gerber files
 Assembly drawing

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. 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. 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. CN0102 Precision Weigh Scale System
62

60. AD7190 Sinc4 Filter Response, 50 Hz Output
Data Rate
63

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. 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. 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. 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. 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. 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. 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