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LT1175-5

Precision Micropower ∆∑ RMS-to-DC Converter

Linear Technology 

LTC1966

Applications Information

Design Cookbook

The LTC1966 RMS-to-DC converter makes it easy to

implement a rather quirky function. For many applications

all that will be needed is a single capacitor for averaging,

appropriate selection of the I/O connections and power

supply bypassing. Of course, the LTC1966 also requires

power. A wide variety of power supply configurations are

shown in the Typical Applications section towards the end

of this data sheet.

Capacitor Value Selection

The RMS or root-mean-squared value of a signal, the root

of the mean of the square, cannot be computed without

some averaging to obtain the mean function. The LTC1966

true RMS-to-DC converter utilizes a single capacitor on

the output to do the low frequency averaging required for

RMS-to-DC conversion. To give an accurate measure of a

dynamic waveform, the averaging must take place over a

sufficiently long interval to average, rather than track, the

lowest frequency signals of interest. For a single averag-

ing capacitor, the accuracy at low frequencies is depicted

in Figure 6.

Figure 6 depicts the so-called DC error that results at a

given combination of input frequency and filter capacitor

values1. It is appropriate for most applications, in which

the output is fed to a circuit with an inherently band lim-

ited frequency response, such as a dual slope/integrating

A/D converter, a ∆Σ A/D converter or even a mechanical

analog meter.

However, if the output is examined on an oscilloscope

with a very low frequency input, the incomplete averag-

ing will be seen, and this ripple will be larger than the

error depicted in Figure 6. Such an output is depicted in

Figure 7. The ripple is at twice the frequency of the input

because of the computation of the square of the input.

The typical values shown, 5% peak ripple with 0.05% DC

error, occur with CAVE = 1µF and fINPUT = 10Hz.

If the application calls for the output of the LTC1966 to feed

a sampling or Nyquist A/D converter (or other circuitry that

will not average out this double frequency ripple) a larger

averaging capacitor can be used. This trade-off is depicted

in Figure 8. The peak ripple error can also be reduced by

additional lowpass filtering after the LTC1966, but the

simplest solution is to use a larger averaging capacitor.

1This frequency dependent error is in addition to the static errors that affect all readings and are

therefore easy to trim or calibrate out. The Error Analyses section to follow discusses the effect

of static error terms.

ACTUAL OUTPUT

WITH RIPPLE

f = 2 × fINPUT

PEAK

RIPPLE

(5%)

IDEAL

OUTPUT

DC

ERROR

(0.05%)

PEAK

ERROR =

DC ERROR +

PEAK RIPPLE

(5.05%)

DC

AVERAGE

OF ACTUAL

OUTPUT

TIME

1966 F07

Figure 7. Output Ripple Exceeds DC Error

0

–0.2 C = 4.7µF C = 10µF

–0.4

–0.6

C = 2.2µF

–0.8

–1.0

–1.2

–1.4

–1.6

–1.8

–2.0

1

C = 1.0µF

C = 0.47µF

C = 0.22µF

C = 0.1µF

10

20

INPUT FREQUENCY (Hz)

Figure 6. DC Error vs Input Frequency

50 60

100

1966 F06

1966fb

13

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