LINEAR True RMS Converter – Autoranging Design Note 339 by Philip Karantzalis and Jim Mahoney

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An Autoranging True RMS Converter – Design Note 339 Philip Karantzalis and Jim Mahoney

Introduction True RMS voltage detection is most commonly required to measure complex amplitude and time varying signals,

The LTC®1966 is a true RMS-to-DC converter that uses a

such as machine or engine vibration monitoring and

∆Σ computational technique to make it dramatically sim-

complex AC power line load monitoring. Sometimes

pler to use, significantly more accurate, lower in power

consumption and more flexible than conventional log- these applications require accurate input signal mea- +– surement over an extremely wide dynamic range—even

antilog RMS-to-DC converters. The LTC1966 RMS-to-DC

more than the 50d B range of the LTC1966. One solution

converter has an input signal range from 5m VRMS to

is to add an autoranging function to the LTC1966, thus

1.5VRMS (a 50d B dynamic range with a single 5V supply

rail) and a 3d B bandwidth of 800k Hz with signal crest , LTC and LT are registered trademarks of Linear Technology Corporation.

factors up to four.

RMS-DC CONVERTER LT1783 BUFFER

0.1µF VDD LT1783 OUTPUT

IN1 OUT

0.01µF LTC1966 1µF

IN2 0_RTN –5V

10k10k VSSGNDEN

0.1µF WINDOW

OUT V+1 COMPARATOR

AGND G2 10k

PROGRAMMABLE

10µF LTC6910-2 LTC6700-1

GAIN AMPLIFIER

ININPUT G1 400m V REF

SET GAIN

LOGIC BLOCK (UP/DN COUNTER AND CONTROL LOGIC) 5V 5V

GAIN TOO HIGH DOWN RANGE GAIN TOO LOW UP RANGE

Q0 Q1 DIGITAL

LATCHED OUTPUTS

Q2 OUTPUTS CLOCK Q3

OUT V+5

1µF LTC6900 0.1µF

VIN SHDN 1M

5V DIV GND

LTC1983-5

3.9Ω SET

DN339 F01

–5V VOUT GND

LOOP TIMING CLOCK

NEGATIVE VOLTAGE GENERATOR

Figure 1. An Autoranging True RMS-to-DC Converter