13

LTC1966

sn1966 1966fas

APPLICATIO S I FOR ATIO

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 averaging

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

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

frequency response, such as a dual slope/integrating A/D

converter, a

∆ΣA/Dconverterorevenamechanicalanalog

meter.

Figure 6. DC Error vs Input Frequency

C = 4.7

µF

INPUT FREQUENCY (Hz)

1

–2.0

–1.6

–1.2

–0.8

–0.4

10

20

50

60

100

1966 F06

0

–1.8

–1.4

–1.0

–0.6

–0.2

C = 10

µF

C = 2.2

µF

C = 1.0

µF

C = 0.47

µF

C = 0.22

µF

C = 0.1

µF

Figure 7. Output Ripple Exceeds DC Error

TIME

1966 F07

DC

ERROR

(0.05%)

IDEAL

OUTPUT

DC

AVERAGE

OF ACTUAL

OUTPUT

PEAK

RIPPLE

(5%)

ACTUAL OUTPUT

WITH RIPPLE

f = 2

PEAK

ERROR =

DC ERROR +

PEAK RIPPLE

(5.05%)

However, if the output is examined on an oscilloscope with

a very low frequency input, the incomplete averaging 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,

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.

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

of static error terms.