LTC1968 查看數據表（PDF） - Linear Technology

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LTC1968

Precision Wide Bandwidth, RMS-to-DC Converter

Linear Technology 

LTC1968

PI FU CTIO S

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:

(VOUT – OUT RTN) =

Average

⎡⎣⎢(IN2

–

IN1)2

⎤

⎦⎥

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”:

VRMS = V2

1V DC +– R

1V ACRMS

R

SAME

HEAT

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

WAVEFORM

Square Wave

VRMS

1.000

AVERAGE

RECTIFIED

(V)

1.000

ERROR*

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

Θ = 90°

0.637

– 29.3%

SCR at 1/4 Power, 1.000

Θ = 114°

0.536

– 40.4%

1V (AC + DC) RMS

R

1968 F01

Figure 1

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

1968f

7

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