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LTC1968

1968f

GND (Pin 1): Ground. The power return pin.

IN1 (Pin 2): Differential Input. DC coupled (polarity is

irrelevant).

IN2 (Pin 3): Differential Input. DC coupled (polarity is

irrelevant).

VOUT (Pin 5): Output Voltage. Pin 5 is high impedance. The

RMS averaging is accomplished with a single shunt ca-

pacitor from Pin 5 to OUT RTN. The transfer function is

given by:

OUT RTN

Average IN

OUT ––

() =

()

⎡

⎣⎢

⎤

⎦⎥

PI FU CTIO S

OUT RTN (Pin 6): Output Return. The output voltage is

created relative to this pin. The VOUT and OUT RTN pins

are not balanced and this pin should be tied to a low

impedance, both AC and DC. Although Pin 6 is often tied

to GND, it can also be tied to any arbitrary voltage:

GND < OUT RTN < (V+ – Max Output)

V+ (Pin 7): Positive Voltage Supply. 4.5V to 5.5V.

ENABLE (Pin 8): An Active-Low Enable Input. LTC1968 is

debiased if open circuited or driven to V+. For normal

operation, pull to GND.

APPLICATIO S I FOR ATIO

RMS-TO-DC CONVERSION

Definition of RMS

RMS amplitude is the consistent, fair and standard way to

measure and compare dynamic signals of all shapes and

sizes. Simply stated, the RMS amplitude is the heating

potential of a dynamic waveform. A 1VRMS AC waveform

will generate the same heat in a resistive load as will 1V DC.

Mathematically, RMS is the “Root of the Mean of the

Square”:

RMS =

–

1V DC

1968 F01

SAME

HEAT

1V ACRMS

1V (AC + DC) RMS

Figure 1

Alternatives to RMS

Other ways to quantify dynamic waveforms include peak

detection and average rectification. In both cases, an

average (DC) value results, but the value is only accurate

at the one chosen waveform type for which it is calibrated,

typically sine waves. The errors with average rectification

are shown in Table 1. Peak detection is worse in all cases

and is rarely used.

Table 1. Errors with Average Rectification vs True RMS

AVERAGE

RECTIFIED

WAVEFORM

VRMS

(V)

ERROR*

Square Wave

1.000

11%

Sine Wave

1.000

0.900

*Calibrate for 0% Error

Triangle Wave

1.000

0.866

–3.8%

SCR at 1/2 Power,

1.000

0.637

–29.3%

Θ = 90°

SCR at 1/4 Power,

1.000

0.536

–40.4%

Θ = 114°

The last two entries of Table 1 are chopped sine waves as

is commonly created with thyristors such as SCRs and

Triacs. Figure 2a shows a typical circuit and Figure 2b

shows the resulting load voltage, switch voltage and load

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