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Application Notes

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  Instrumentation Circuitry Using RMS-to-DC Converters &mdash AN106
  PDF, 354 Kb, File published: Feb 21, 2007
  It is widely acknowledged that RMS measurement of waveforms furnishes the most accurate amplitude information. Rectify-andaverage schemes, usually calibrated to a sine wave, are only accurate for one waveshape. Departures from this waveshape result in pronounced errors. Although accurate, RMS conversion often entails limited bandwidth, restricted range, complexity and difficult to characterize dynamic and static errors. The LTC1966/67/68 RMS converter family addresses these issues, making instrument grade applications practical. A variety of instrumentation oriented applications are presented. Included are basic circuits, a fully isolated AC line monitor, a distortionless AC line voltage regulator, wideband X1000 pre-amplifiers, a quartz crystal RMS current meter, a crystal stabilized AC voltage reference, an RMS amplitude leveled random noise generator and an RMS amplitude level controller. Appended sections cover RMS theory and converter operation, AC measurement and signal handling practice, test equipment recommendations, noise theory and noise diodes.
  Extract from the document

  Application Note 106
  February 2007
  Instrumentation Circuitry Using RMS-to-DC Converters
  RMS Converters Rectify Average Results
  Jim Williams
  INTRODUCTION Isolated Power Line Monitor It is widely acknowledged that RMS (Root of the Mean
  of the Square) measurement of waveforms furnishes
  the most accurate amplitude information.1 Rectify-andaverage schemes, usually calibrated to a sine wave, are
  only accurate for one waveshape. Departures from this
  waveshape result in pronounced errors. Although accurate,
  RMS conversion often entails limited bandwidth, restricted
  range, complexity and difficult to characterize dynamic
  and static errors. Recent developments address these
  issues while simultaneously improving accuracy. Figure 1
  shows the LTCВ®1966/LTC1967/LTC1968 device family. Low
  frequency accuracy, including linearity and gain error, is
  inside 0.5% with 1% error at bandwidths extending to
  500kHz. These converters employ a sigma-delta based
  computational scheme to achieve their performance.2 BEFORE PROCEEDING ANY FURTHER, THE READER
  IS WARNED THAT CAUTION MUST BE USED IN THE
  CONSTRUCTION, TESTING AND USE OF THIS CIRCUIT.
  HIGH VOLTAGE, LETHAL POTENTIALS ARE PRESENT IN
  THIS CIRCUIT. EXTREME CAUTION MUST BE USED IN …

Design Notes

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  Precision LVDT Signal Conditioning Using Direct RMS to DC Conversion &mdash DN362
  PDF, 75 Kb, File published: Mar 23, 2005
  Extract from the document

  Precision LVDT Signal Conditioning Using Direct RMS to DC
  Conversion – Design Note 362
  Cheng-Wei Pei
  lowpass filtering of the rectified sine wave. However,
  the precision of the demodulation method depends on
  the accuracy of the phase adjustment. In addition, there
  are losses associated with demodulation, which usually
  involve switches and the associated charge injection
  and timing jitter. It is difficult to achieve 12-bit precision
  under these circumstances. Introduction
  Linear variable differential transformers (LVDTs) are
  theoretically infinite-resolution displacement measurement devices. An LVDT operates by comparing the
  magnetic flux coupled into two transformer secondary
  windings to determine the displacement of a moving
  transformer core. A low distortion sine wave acts as
  the input. The amplitude and phase of the output signal across the two secondary windings determine the
  distance and polarity of the LVDT core with respect to
  the center.
  Proper signal conditioning circuitry allows extremely
  precise measurements in demanding applications,
  such as in production manufacturing, fluid level measurements and structural/strain testing. One common
  application is to use an LVDT at the end of a Bourdon
  tube to measure minute changes in system or barometric pressure. A better approach to LVDT signal conditioning is shown …

Articles

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  Accurate and Simple AC Measurement to 500kHz &mdash LT Journal
  PDF, 166 Kb, File published: Mar 25, 2005
  Extract from the document

  DESIGN INFORMATION Accurate and Simple AC
  Measurement to 500kHz
  Introduction Figure 1. The LTC1967, LTC1968 are easy to
  hook-up. The only external components are an
  averaging capacitor and a bypass capacitor. pendent of the input amplitude being
  converted. This again is in contrast
  to older implementations which have
  their bias linked to the input amplitude, and therefore slow down with
  smaller inputs.
  Finally, the switched capacitor
  architecture makes it easy to get railto-rail operation at the input and the
  output. Level shifting the output is as
  easy as tying the OUT RTN pin to the
  desired output level. Advantages
  of the ∆Σ Topology Selecting the
  Averaging Capacitor 5V The LTC1967 and LTC1968 provide
  the easiest way to accurately measure
  the RMS value of any AC waveform with
  input signal frequencies as high as
  500kHz. They have 1% gain accuracy
  and noise out to 100kHz and 500kHz,
  respectively. Their phenomenal linearity of 0.02%, derived from the use of
  a Delta Sigma architecture, allows …

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  Low-Distortion Sine Wave Oscillator with Precise RMS Amplitude Stability &mdash LT Journal
  PDF, 111 Kb, File published: Mar 25, 2005
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  DESIGN IDEAS Low-Distortion Sine Wave Oscillator
  with Precise RMS Amplitude Stability
  by Cheng-Wei Pei
  Many applications require a frequency
  and/or amplitude-stable sine wave
  as a reference for calibration or measurement. Low harmonic distortion is
  also required for meaningful results
  in applications such as LVDT signal
  conditioning, ADC testing, and, of
  course, harmonic distortion testing.
  Many sine wave generation techniques simply cannot achieve the low
  harmonic distortion and amplitude
  stability required of a precision sine
  wave reference. The technique shown
  here generates a sine wave with less
  than 0.003% distortion and 0.1%
  amplitude stability.
  Figure 1 shows a simple oscillator
  circuit consisting of a Wien bridge
  oscillator core and an amplitude
  stabilization loop. The LT1632 highspeed low-distortion amplifier and its
  positive feedback RC network generate
  the oscillations. The amplitude, and …

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Manufacturer's Classification

• Signal Conditioning > RMS-DC Conversion

Other Names:

LTC1968IMS8PBF, LTC1968IMS8 PBF