identifying and overcoming noise in data acquisition

Identifying and Overcoming Noise in Data Acquisition William Chen Product Manager

2 Copyright © Yokogawa Meters & Instruments Corporation

Kristina Neahr Marketing Specialist Yokogawa Corporation of America Newnan, GA [email protected] 1-800-888-6400 ext. 2611 tmi.yokogawa.com

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William Chen Product Manager Yokogawa Corporation of America Newnan, GA [email protected] 1-800-888-6400 Ext 2537 tmi.yokogawa.com

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■ Data Acquisition Overview • Applications By Speed and Signal Types

-  Low speed monitoring and recording, High speed single shot, Repetitive waveform monitoring, Memory blocks (Sequential store), High Speed continuous monitoring.

■ Quantization noise • Vertical resolution, LSB, Gain

■ Internal A/D noise • What do accuracy specifications mean and how do they reflect the noise

characteristics of the DAQ hardware? ■ Power line noise

• Filtering, Integrating A/D

Overview


Overview

■ Time skew •  Inter channel skew, and simultaneous sampling

■ Aliasing noise • Nyquist theory, Sampling rate/interval and frequency spectrum, AAF

■ Common mode noise • Ground loops, common mode, isolation

■ Radiated noise (EMI) • Crosstalk, DAQ product shielding, cable shielding

■ Application Example • Fuel Cell Impedance measurements


DAQ Applications

DC-1kHz Temperature, pressure, Static load, displacement Production monitoring Power line monitoring

1kHz-100kHz Mechanical Electronics Sound and Vibration Automotive, Aerospace

100kHz-20MHz Electrical performance Digital, timing, pattern I/O Consumer electronics

20MHz-1GHz + Component design RF, Microwave

PC-based Internal (most PC-centric) Pro: low cost, multi-function, bus speed Con: little/no signal conditioning, almost always multiplexed, poor noise immunity Benchtop (least PC-centric, DL850E)

Pro: separate/isolated power, portable, Better quality measuring hardware Con: less channel density, higher cost, large footprint, slower for automation

PC-based External (SL1000, MX100, GM10) Pro: most scalable, better quality measurement HW, fast performance for PC automation, lower cost, high bus speed, high channel density


DAQ Signal Types

■ Analog Input • DC/AC Voltages •  “Special” sensors

-  Accelerometer, ICP Microphone -  Strain Gage, Load Cell -  RTD, Thermistor, Resistance ■ Analog Output

• DC, Function Generation, Arbitrary, Sweeping

■ Digital Input / Output • TTL/CMOS, Static & Buffered

■ Timing Measurements • Event Counting, Delay, Period,

Frequency, Tachometer, Encoder, Time Stamps


Analog Input Applications ■ Low Speed Monitoring and Recording

•  Machine monitoring •  Process monitoring •  Certification testing •  Reliability testing

■ High Speed Single Shot •  Startup/shutdown monitoring •  Electrical response •  Device characterization •  Sweep testing •  Destructive/explosion testing


Analog Input Applications

■ Repetitive Waveform Monitoring •  Vibration •  Test stands/cells •  Engine or combustion monitoring •  Glitch measurements

■ Memory Blocks (Sequential store) •  Low re-arm time •  Continuity/glitch testing •  Engine R&D


Analog Input Applications

■ High Speed Continuous Monitoring •  Also called: Free-Run, Streaming to PC, Circular buffered acquisition, FIFO buffer •  In-vehicle/flight DAQ, high energy physics, real-time monitoring of multi-hour tests


Source 1: Quantization Noise

■ Most commonly affects: •  Thermocouple measurements •  Low voltage measurements •  Ripple measurements •  High speed measurements on

any voltage


Principle of Quantization Noise

■ Decimal System (Base 10) •  Used by humans •  Number of Digits •  1 digit = 0-9 [10 counts] •  2 digits = 00-99 [100 counts] •  3 digits = 000-999 [1000 counts] •  counts = 10(Number of Digits)

■ Binary System (Base 2) •  Used by Analog-Digital converters •  Number of Bits •  1 bit = 02-12 [2 counts] •  2 bits = 002-112 [4 counts] •  3 bits = 0002-1112 [8 counts] •  counts = 2(Number of Bits)

8 bit A/D = 28 counts = 256 10 bit A/D = 210 counts = 1024 12 bit A/D = 212 counts = 4096 14 bit A/D = 214 counts = 16384 16 bit A/D = 216 counts = 65536

Binary (8 bits)

Decimal (raw)

Voltage (scaled)

00000000 0 -10V

00000001 1 -9.921875

00000010 2 -9.84375

00000011 3 -9.765625

00000100 4 -9.6875

00000101 5 -9.609375

… … …

11111111 255 +10V


Principle of Quantization Noise

■ Bit Resolution determines how many counts exist across the full scale ■ A Least Significant Bit (LSB) is the smallest voltage change that can be measured

•  1 LSB = 𝑅𝑎𝑛𝑔𝑒/𝐶𝑜𝑢𝑛𝑡𝑠  𝑜𝑟 𝑅𝑎𝑛𝑔𝑒/2↑𝑏𝑖𝑡𝑠 𝑜𝑓 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛  

•  8 bit: 10𝑉 −(−10𝑉)/2↑8   = 20𝑉/256 = 78.125 mV

•  12 bit: 10𝑉 −(−10𝑉)/2↑12   = 20𝑉/4096  = 4.883 mV

+10V

-10V b0

b1

b2

b3

b4

b5

b6

b7

A/D PGIA

Full scale

1 LSB


Quantization Noise Solution

■ Typical result achieved with digital software filter (poor) ■ SW Filter used: Sharp, Lowpass, fcutoff=2%*fsample, 88th order


Quantization Noise Solution

■ Hardware solution: Decrease (match) the range, or increase the bit resolution (1 LSB = [range / counts]) ■ Quantization noise exists even if it is not visually present ■ Improving this will improve analysis accuracy


AC Coupling

■ AC coupling will dramatically reduce quantization noise when: •  You are interested in analyzing the AC content of a waveform •  A DC offset is present

Must Use ±2V Range Due to DC Offset

Enable AC/DC Coupling Circuit, allowing use of a lower Range (±50mV)


Source 2: A/D Internal Noise

■ Noise described by printed specifications (accuracy) ■ Main factors contributing to internal noise

•  Nonlinearity of the A/D converter itself (differential & integral nonlinearity)

•  PGIA & A/D block – gain and offset •  Thermal effects and thermal stability of the

entire digitizer ■ Accuracy specifications are highly

inconsistent in literature across vendors


Source 2: A/D Internal Noise

■ Various methods of reporting A/D internal noise •  Accuracy (gain and offset) •  Absolute Accuracy •  Signal to Noise Ratio (SNR) •  Effective Number of Bits (ENOB) •  Noise Floor (dB) •  Spurious Free Dynamic Range (SFDR)

■ Improving A/D internal noise: •  Choose a higher precision digitizer • Maintain a stable environmental temperature •  Reduce the Bandwidth used


Source 3: Power Line Noise

■ Power Line Noise also called “Pick Up” or “Hum” ■ Can be conducted (through DAQ device or power supply) or radiated

(through EMI), internal or external source ■ Noise frequency always occurs at the power line frequency (50/60/400Hz)

Clean Signal

Signal with power line noise


Integrating A/D Converter

■ Integrating A/D converters quantize by time rather than by voltage ■ Better linearity and accuracy, no missing codes ■ Eliminate noise occuring at the integration frequencies (and multiples) ■ i.e. 16.67msec integration period will filter out (60Hz, 120Hz, 180Hz, …)

noise ■ Best low speed solution

Normal Sampling Time

Volta

ge

Signal with power supply noise

Voltage corresponding to the value after the conversion

For Integration-type A/D Time

Volta

ge

Signal with power supply noise

Voltage corresponding to the value after the conversion


Other Power Line Noise

■ For Slow speed signals measurements: A “Moving Average” software filter is nearly as effective as an integrating A/D converter ■ Power Line Filters to remove high frequency RF noise from power ■ Isolated Input to Data Acquisition hardware


Source 4: Time Skew

■ Many Data Acquisition systems do not measure time aligned data ■ Differential measurements can have time skew between + and – terminals

(pseudo-differential)

Actual Signal Measured Signal


Principle of Time Skew

■ Multiplexing Sampling channel clock [inter-channel skew]

scan clock

Ch1 Ch2 Ch3 Ch4 …

■ Simultaneous Sampling

Ch1 Ch2 Ch3 Ch4 …


Source 5: Aliasing Noise

■ Aliasing or Fold-Over Distortion ■ Occurs when higher frequency content exists beyond the sampling

bandwidth


Principle of Aliasing

■  Any Analog Signal or Non-sinusoidal waveform, actually consists of Sine Waves of various: ■  Amplitudes ■  Frequencies ■  Phase

fmax



■ Red trace is actual signal, Green dots are measured values

■ Original waveform with 20 samples per period (fs = 20 f0)





■ Simple Example •  fs = 16 samples/sec

■ Adequately Sampled •  f0 = 1 Hz

■ Near “Nyquist frequency” or sampling bandwidth •  fN = ½ fs = 8 Hz •  f0 = 7 Hz

■ Near “Nyquist frequency” or sampling bandwidth

•  fN = ½ fs = 8 Hz •  f0 = 7 HzUnder sampled •  f0 = 11Hz •  In this case we should see a flat line (since

the signal is above our measurement bandwidth)

•  The incorrect signal introduces a type of distortion into the measurement.


Principle of Aliasing ■ More realistic example ■ Original Signal

•  3 Hz content, 1Vpp •  50 Hz content, 0.2Vpp

■ Aliased noise at 5 Hz

■ Sampled at 55 Hz ■ (fN = ½ fs = 27.5 Hz) ■ 27.5 – [50 - 27.5] = 5


Effect of Aliasing

■ Some Signals (i.e. a sawtooth wave) have infinite harmonics

■ If the fundamental frequency of the sawtooth wave is f0, and we sample at 20*f0, what happens?

Result = increased noise floor


Aliasing Solution ■ A Hardware filter (low pass) eliminates fold-over distortion ■ Set the filter frequency close to Nyquist frequency (½ fs) ■ The filter must reside in Hardware – Software filters are ineffective ■ Especially important for sound and vibration testing ■ External HW filter is an option, newest technology uses SW-selectable, built into

instrument


Reconstructing the Signal ■ In theory, the reconstructed waveform must not possess any frequency

content >= sampling frequency ■ To perfectly reconstruct, bandwidth limit the re-creation/output

•  (apply an ideal low pass filter with (cutoff frequency = Nyquist frequency)) ■ You can also use curve fitting to approximately reconstruct the signal

•  Linear interpolation, Sinusoidal interpolation, Spline

■  In practice, most engineers do not reconstruct waveforms prior to analysis! ■  Therefore, apply a Practical over-sampling criteria fsample > 4x fmax


Source 6: Common Mode Noise

■ Original signal with no common mode

■ Signal with Common mode input to a pseudo-differential device

■ Signal with Common mode measured by isolated digitizer

* Noise depends on CMRR

■ Signal with Common mode noise greater than digitizer specification


Principle of Common Mode

■ Voltage difference between sensor “ground” and instrument ground ■ Can cause permanent damage to measurement hardware

5V

0+Vcm

Vcm+5


Common Mode Noise ■ Typical causes of common mode voltages include:

•  Thermocouple measurements of powered devices •  Battery or fuel cell testing •  External sensor power supply •  “Floating” sensors in noisy EMI environment


Common Mode and Normal Mode

Common Mode Noise   Noise

Normal/Differential Mode Noise   Actual signal and noise


Ground Loops

■ Return paths for current referred to as “ground” ■ Occurs when more than one ground connection path exists between

devices

DAQ

DUT or sensor

Ground path through shield or negative terminal of SE measurement

Ground path through building or earth ground


Ground Loops

■ Three ways ground loops cause equipment problems •  Low currents circulating in the grounds generate voltages that can cause data

errors such as 60 Hz humming or high-frequency oscillations •  High-energy transients will clear through circuit ground instead of earth ground

causing inrush or switching currents to damage equipment • Ground loops can cause common-mode noise between phase, neutral and

ground in a power distribution system. Noise injected into the power supplies will pass on to the electronic components.


Common Mode Solution

■ Isolation Barriers •  Prevents ground loops and negates common mode voltage

■ Safety: keeps high voltage/current away from people and equipment ■ Integrity: rejects unwanted voltages from affecting measurement accuracy


Common mode solution

■ Built-in Isolation Systems ■ Four isolation specifications to consider

•  [V1] Channel-to-ground isolation •  [V2] Module-to-module isolation

(in a modular system) •  [V3] Channel-to-channel isolation •  [V4] Transient overvoltage protection

(or maximum withstand voltage)


Channel to Earth Isolation

  [V1] Channel-to-ground isolation ■ Accessible parts of instrument is safe ■ Prevents ground loops and common mode noise

Isolation Barrier

Analog Module A

Analog Module B

Analog Module C

Backplane


Module to Module Isolation

■ [V2] Channel-to-Channel isolation ■ Each module is isolated from each other ■ Provides noise immunity and overvoltage

protection


Channel to Channel Isolation

  [V3] Channel-to-Channel isolation   Provides channel to channel protection from:   Crosstalk   High energy transients


Source 7: Radiated noise (EMI) ■ Certain environments are especially prone to noise:

•  Industrial or manufacturing facilities •  Power engineering labs (supplies, UPS, etc.) • Motor or drive companies •  High Energy Physics Laboratories

■ Emission sources •  Fluorescent lighting (120 Hz sinusoidal) •  Bipolar Power supplies •  Internal components of desktop PC •  Crosstalk (other sensors, particularly

active sensors) ■ Manifest as Common mode and

Normal mode Voltage


Reducing EMI noise effects ■ Faraday Cage principle – Electric field within a closed surface

is zero ■ Shield Cabling

•  Use standard shielded cable types (coaxial/BNC, twisted pair) •  Use an external cable shield around each sensor-to-digitizer cable •  Tie the cable shield to ground on only one side •  Consider optical connections when feasible

■ Shield the Digitizer Module (vendor) ■ Shield the Station or Chassis (vendor) ■ Shield the Rack


Reducing EMI noise effects ■ More about Cabling ■ Avoid ribbon cables and unshielded terminal blocks at all

costs ■ Plug-in boards use high density connector, require terminal

block or external screw terminal ■ Many digitizer instruments have direct connections (clamp

terminals, NDIS, BNC) ■ Use true differential hardware with isolation to reject EMI

radiated as common mode voltage ■ For low speed/industrial applications, use current to transmit

signals (4-20mA)


Elements of a Noise Minimizing Digitizer ■ High Resolution A/D Converter (quantization noise) ■  Isolation barrier (common mode noise) ■ Low pass or AA filter (power line, aliasing noise) ■ Programmable gain (quantization noise) ■ BNC input (radiated noise) ■ Simultaneous sampling, independent channel hardware (time skew) ■ Attenuation ■ Mechanically shielded and enclosed hardware (EMI) ■ Acquisition Memory


Application Example: Fuel Cell Impedance Testing

When a cell is used for a long time, its impedance will increase. This causes a degradation or inefficiency.

By measuring the fuel cell impedance, we can verify the electrical nature of the internal configuration of the fuel cell.

Current Density (Output current from Cell)

Conductor Resistance of electrode

Reaction Resistance at Anode side

Reaction Resistance at Cathode side

Electrolyte Resistance

Out

put V

olta

ge fr

om C

ell

1.03V (Theoretical)

Anode side Cathode side Reaction Resistance

Electric Bilayer Capacitance

Solution Resistance

Fuel Cell Equivalent Circuit


Application Example: Fuel Cell Impedance Testing

■ Key requirements: •  Isolated output •  Isolated input •  High resolution, 16 bit •  AC/DC coupling •  Programmable gain •  Hardware Filtering •  Simultaneous Sampling

Hydrogen flow

Hydrogen outlet

Air(Oxygen) flow

Water and air outlet

Electric Load DC component

Electric Load AC component

DC Voltage(cell voltage) AC Voltage(ripple)

(optional) switch box

Measured load current


Isolated DAQ Instruments

Copyright © Yokogawa Electric Corporation

ScopeCorder DL850E Series   1000 Vrms isolation   100 MS/s high-speed sampling   12-bit A/D resolution   Complete built-in isolation system


High-Speed PC Based SL1000 Series   12 types of input modules for

measuring:   Voltage   Strain   Temperature   Acceleration   Frequency

  1000 Vrms isolation   100 MS/s high-speed sampling




Summary – What We Hope We Did ■ Data Acquisition Overview

•  Applications By Speed and Signal Types -  Low speed monitoring and recording, High speed single shot, Repetitive waveform monitoring,

Memory blocks (Sequential store), High Speed continuous monitoring. ■ Quantization noise •  Vertical resolution, LSB, Gain ■ Internal A/D noise •  What do accuracy specifications mean and how do they reflect the noise

characteristics of the DAQ hardware? ■ Power line noise •  Filtering, Integrating A/D ■ Time skew •  Inter channel skew, and simultaneous sampling ■ Aliasing noise •  Nyquist theory, Sampling rate/interval and frequency spectrum, AAF ■ Common mode noise •  Ground loops, common mode, isolation ■ Radiated noise (EMI) •  Crosstalk, DAQ product shielding, cable shielding ■ Application Example •  Fuel Cell Impedance measurements


Questions?



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identifying and overcoming noise in data acquisition

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