LINEAR AN106-1 Data Sheet

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 BEFORE PROCEEDING ANY FURTHER, THE READER of the Square) measurement of waveforms furnishes IS WARNED THAT CAUTION MUST BE USED IN THE the most accurate amplitude information.1 Rectify-and- CONSTRUCTION, TESTING AND USE OF THIS CIRCUIT. average schemes, usually calibrated to a sine wave, are HIGH VOLTAGE, LETHAL POTENTIALS ARE PRESENT IN only accurate for one waveshape. Departures from this THIS CIRCUIT. EXTREME CAUTION MUST BE USED IN waveshape result in pronounced errors. Although accurate, WORKING WITH, AND MAKING CONNECTIONS TO, THIS RMS conversion often entails limited bandwidth, restricted CIRCUIT. REPEAT: THIS CIRCUIT CONTAINS DANGER- range, complexity and diffi cult to characterize dynamic OUS, HIGH VOLTAGE POTENTIALS. USE CAUTION. and static errors. Recent developments address these

Figure 3’s AC power line monitor has 0.5% accuracy over

issues while simultaneously improving accuracy. Figure 1

a sensed 90VAC to 130VAC input and provides a safe, fully

shows the LTC®1966/LTC1967/LTC1968 device family. Low

isolated output. RMS conversion provides accurate report-

frequency accuracy, including linearity and gain error, is

ing of AC line voltage regardless of waveform distortion,

inside 0.5% with 1% error at bandwidths extending to

which is common.

500k Hz. These converters employ a sigma-delta based

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

computational scheme to achieve their performance.2

All other trademarks are the property of their respective owners. 1See Appendix A, “RMS-to-DC Conversion” for complete discussion of

Figure 2’s pinout descriptions and basic circuits reveal an

RMS measurement.

easily applied device. An output fi lter capacitor is all that is

2Appendix A details sigma-delta based RMS-to-DC converter operation.

required to form a functional RMS-to-DC converter. Split and single supply powered variants are shown. Such ease of implementation invites a broad range of application; examples begin with Figure 3.

LINEARITY CONVERSION 1% ERROR 3d B ERROR ERROR GAIN ERROR BANDWIDTH BANDWIDTH SUPPLY VOLTAGE ISUPPLY

PART NUMBER TYP/MAX (%) TYP/MAX (%) (k Hz) (k Hz) MAX(V) MAX (µA)MIN(V)

LTC1966 0.02/0.15 0.1/0.3 2.7 ±5 LTC1967 0.02/0.15 0.1/0.3 4MHz 4.5 5.5 LTC1968 0.02/0.15 0.1/0.3 15MHz 4.5 5.5 2.3m A

Figure 1. Primary Differences in RMS to DC Converter Family are Bandwidth and Supply Requirements. All Devices Have Rail-to-Rail Differential Inputs and Output

an106f

AN106-1