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Identifying Noise in DAQ

Measuring electrical signals with data acquisition systems is not always as simple as wiring the signal source leads to the measurement equipment. Data integrity is dependent on the transmission of clean electrical signals from the signal source and a reliable DAQ system.

In this 1-hour, complimentary webcast, you will learn the basic sources of electrical noise in data-acquisition systems. You will also learn how to easily identify and isolate this noise sources and the basic steps to remove these noise sources. These noise sources include:
• Quantization noise
• A/D internal noise
• Power line noise
• Aliasing noise
• Common mode noise
• Radiated noise (EMI)

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Identifying Noise in DAQ

  1. 1. Identifying and Overcoming Noise in Data Acquisition William Chen Product Manager
  2. 2. 2 Copyright © Yokogawa Meters & Instruments Corporation Kristina Neahr Marketing Specialist Yokogawa Corporation of America Newnan, GA kristina.neahr@us.yokogawa.com 1-800-888-6400 ext. 2611 tmi.yokogawa.com Host & Panelist
  3. 3. 3 Copyright © Yokogawa Meters & Instruments Corporation PC’s Speakers - Audio Broadcast To hear the audio through your PC, select the Communicate Tab and Join the Audio Broadcast. Recorded Presentation A recording of this presentation will be posted under our technical library of our web page. Chat: Select and send all questions to “Panelist” during the Webinar presentation. Yokogawa Webinar – Housekeeping Issues
  4. 4. 4 Copyright © Yokogawa Meters & Instruments Corporation William Chen Product Manager Yokogawa Corporation of America Newnan, GA William.Chen@us.yokogawa.com 1-800-888-6400 Ext 2537 tmi.yokogawa.com Presenter
  5. 5. 5 Copyright © Yokogawa Meters & Instruments Corporation If you have any questions for one of these Webinar Topics, please send them to the below e-mail. I will try to answer them during the Webinar or directly afterwards. Ø webinarwednesdays@us.yokogawa.com Questions
  6. 6. 6 Copyright © Yokogawa Meters & Instruments Corporation ■ 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
  7. 7. 7 Copyright © Yokogawa Meters & Instruments Corporation 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
  8. 8. 8 Copyright © Yokogawa Meters & Instruments Corporation 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
  9. 9. 9 Copyright © Yokogawa Meters & Instruments Corporation 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
  10. 10. 10 Copyright © Yokogawa Meters & Instruments Corporation 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
  11. 11. 11 Copyright © Yokogawa Meters & Instruments Corporation 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
  12. 12. 12 Copyright © Yokogawa Meters & Instruments Corporation 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
  13. 13. 13 Copyright © Yokogawa Meters & Instruments Corporation Source 1: Quantization Noise ■ Most commonly affects: •  Thermocouple measurements •  Low voltage measurements •  Ripple measurements •  High speed measurements on any voltage
  14. 14. 14 Copyright © Yokogawa Meters & Instruments Corporation 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
  15. 15. 15 Copyright © Yokogawa Meters & Instruments Corporation 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 = ​ 𝑅 𝑎𝑛𝑔𝑒/𝐶𝑜𝑢𝑛𝑡𝑠  𝑜𝑟 ​ 𝑅 𝑎𝑛𝑔𝑒/​2↑𝑏𝑖𝑡𝑠 𝑜𝑓 𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛   •  8 bit: ​10 𝑉 −(−10 𝑉)/​2↑8   = ​20 𝑉/256 = 78.125 mV •  12 bit: ​10 𝑉 −(−10 𝑉)/​2↑12   = ​20 𝑉/4096  = 4.883 mV +10V -10V b0 b1 b2 b3 b4 b5 b6 b7 A/DPGIA Full scale 1 LSB
  16. 16. 16 Copyright © Yokogawa Meters & Instruments Corporation Quantization Noise Solution ■ Typical result achieved with digital software filter (poor) ■ SW Filter used: Sharp, Lowpass, fcutoff=2%*fsample, 88th order
  17. 17. 17 Copyright © Yokogawa Meters & Instruments Corporation 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
  18. 18. 18 Copyright © Yokogawa Meters & Instruments Corporation 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)
  19. 19. 19 Copyright © Yokogawa Meters & Instruments Corporation 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
  20. 20. 20 Copyright © Yokogawa Meters & Instruments Corporation 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
  21. 21. 21 Copyright © Yokogawa Meters & Instruments Corporation 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
  22. 22. 22 Copyright © Yokogawa Meters & Instruments Corporation 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 Voltage Signal with power supply noise Voltage corresponding to the value after the conversion For Integration-type A/D Time Voltage Signal with power supply noise Voltage corresponding to the value after the conversion
  23. 23. 23 Copyright © Yokogawa Meters & Instruments Corporation 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
  24. 24. 24 Copyright © Yokogawa Meters & Instruments Corporation 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
  25. 25. 25 Copyright © Yokogawa Meters & Instruments Corporation Principle of Time Skew ■ Multiplexing Sampling channel clock [inter-channel skew] scan clock Ch1 Ch2 Ch3 Ch4 … ■ Simultaneous Sampling Ch1 Ch2 Ch3 Ch4 …
  26. 26. 26 Copyright © Yokogawa Meters & Instruments Corporation Source 5: Aliasing Noise ■ Aliasing or Fold-Over Distortion ■ Occurs when higher frequency content exists beyond the sampling bandwidth
  27. 27. 27 Copyright © Yokogawa Meters & Instruments Corporation Principle of Aliasing ■ Any Analog Signal or Non-sinusoidal waveform, actually consists of Sine Waves of various: ■  Amplitudes ■  Frequencies ■  Phase fmax
  28. 28. 28 Copyright © Yokogawa Meters & Instruments Corporation Principle of Aliasing ■ Red trace is actual signal, Green dots are measured values ■ Original waveform with 20 samples per period (fs = 20 f0) ■ Original waveform with 5 samples per period (fs = 5 f0) ■ Original waveform with 2 samples per period (fs = 2 f0)
  29. 29. 29 Copyright © Yokogawa Meters & Instruments Corporation Principle of Aliasing ■ 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.
  30. 30. 30 Copyright © Yokogawa Meters & Instruments Corporation 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
  31. 31. 31 Copyright © Yokogawa Meters & Instruments Corporation 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
  32. 32. 32 Copyright © Yokogawa Meters & Instruments Corporation 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
  33. 33. 33 Copyright © Yokogawa Meters & Instruments Corporation 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
  34. 34. 34 Copyright © Yokogawa Meters & Instruments Corporation 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
  35. 35. 35 Copyright © Yokogawa Meters & Instruments Corporation Principle of Common Mode ■ Voltage difference between sensor “ground” and instrument ground ■ Can cause permanent damage to measurement hardware 5V 0+Vcm Vcm+5
  36. 36. 36 Copyright © Yokogawa Meters & Instruments Corporation 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
  37. 37. 37 Copyright © Yokogawa Meters & Instruments Corporation Common Mode and Normal Mode Common Mode Noise   Noise Normal/Differential Mode Noise   Actual signal and noise
  38. 38. 38 Copyright © Yokogawa Meters & Instruments Corporation 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
  39. 39. 39 Copyright © Yokogawa Meters & Instruments Corporation 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.
  40. 40. 40 Copyright © Yokogawa Meters & Instruments Corporation 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
  41. 41. 41 Copyright © Yokogawa Meters & Instruments Corporation 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)
  42. 42. 42 Copyright © Yokogawa Meters & Instruments Corporation 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
  43. 43. 43 Copyright © Yokogawa Meters & Instruments Corporation Module to Module Isolation ■ [V2] Channel-to-Channel isolation ■ Each module is isolated from each other ■ Provides noise immunity and overvoltage protection
  44. 44. 44 Copyright © Yokogawa Meters & Instruments Corporation Channel to Channel Isolation   [V3] Channel-to-Channel isolation   Provides channel to channel protection from:   Crosstalk   High energy transients
  45. 45. 45 Copyright © Yokogawa Meters & Instruments Corporation 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
  46. 46. 46 Copyright © Yokogawa Meters & Instruments Corporation 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
  47. 47. 47 Copyright © Yokogawa Meters & Instruments Corporation 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)
  48. 48. 48 Copyright © Yokogawa Meters & Instruments Corporation 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
  49. 49. 49 Copyright © Yokogawa Meters & Instruments Corporation 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 OutputVoltagefromCell 1.03V (Theoretical) Anode side Cathode side Reaction Resistance Electric Bilayer Capacitance Solution Resistance Fuel Cell Equivalent Circuit
  50. 50. 50 Copyright © Yokogawa Meters & Instruments Corporation 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
  51. 51. 51 Copyright © Yokogawa Meters & Instruments Corporation 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
  52. 52. 52 Copyright © Yokogawa Meters & Instruments Corporation Isolated DAQ Instruments Copyright © Yokogawa Electric Corporation
  53. 53. 53 Copyright © Yokogawa Meters & Instruments Corporation 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 Isolated DAQ Instruments Copyright © Yokogawa Electric Corporation
  54. 54. 54 Copyright © Yokogawa Meters & Instruments Corporation 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
  55. 55. 55 Copyright © Yokogawa Meters & Instruments Corporation Questions? Copyright © Yokogawa Electric Corporation
  56. 56. 56 Copyright © Yokogawa Meters & Instruments Corporation Thank You for Attending Copyright © Yokogawa Electric Corporation

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