LT1175-5 データシートの表示（PDF） - Linear Technology

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LT1175-5

Precision Micropower ∆∑ RMS-to-DC Converter

Linear Technology 

LTC1966

Applications Information

How an RMS-to-DC Converter Works

Monolithic RMS-to-DC converters use an implicit com-

putation to calculate the RMS value of an input signal.

The fundamental building block is an analog multiply/

divide used as shown in Figure 3. Analysis of this topol-

ogy is easy and starts by identifying the inputs and the

output of the lowpass filter. The input to the LPF is the

calculation from the multiplier/divider; (VIN)2/VOUT. The

lowpass filter will take the average of this to create the

output, mathematically:

VOUT

=





(VIN)2

VOUT



,

Because VOUT is DC,





(VIN)2

VOUT





=





(

VIN)2





VOUT

,

so

VOUT

=





(VIN

)2

VOUT

,

and

(VOUT)2 = (VIN)2, or

VOUT = (VIN)2 = RMS(VIN)

( )VIN 2

VOUT

VIN

×÷

LPF

VOUT

1966 F03

Figure 3. RMS-to-DC Converter with Implicit Computation

Unlike the prior generation RMS-to-DC converters, the

LTC1966 computation does NOT use log/antilog circuits,

which have all the same problems, and more, of log/antilog

multipliers/dividers, i.e., linearity is poor, the bandwidth

changes with the signal amplitude and the gain drifts with

temperature.

How the LTC1966 RMS-to-DC Converter Works

The LTC1966 uses a completely new topology for RMS-

to-DC conversion, in which a ∆Σ modulator acts as the

divider, and a simple polarity switch is used as the multiplier

as shown in Figure 4.

∆–∑

REF

D α VIN

VOUT

VIN

±1

LPF

VOUT

Figure 4. Topology of LTC1966

The ∆Σ modulator has a single-bit output whose average

duty cycle (D) will be proportional to the ratio of the input

signal divided by the output. The ∆Σ is a 2nd order modula-

tor with excellent linearity. The single bit output is used to

selectively buffer or invert the input signal. Again, this is a

circuit with excellent linearity, because it operates at only

two points: ±1 gain; the average effective multiplication

over time will be on the straight line between these two

points. The combination of these two elements again creates

a lowpass filter input signal proportional to (VIN)2/VOUT,

which, as shown above, results in RMS-to-DC conversion.

The lowpass filter performs the averaging of the RMS

function and must be a lower corner frequency than the

lowest frequency of interest. For line frequency measure-

ments, this filter is simply too large to implement on-chip,

but the LTC1966 needs only one capacitor on the output

to implement the lowpass filter. The user can select this

capacitor depending on frequency range and settling time

requirements, as will be covered in the Design Cookbook

section to follow.

This topology is inherently more stable and linear than

log/antilog implementations primarily because all of the

signal processing occurs in circuits with high gain op amps

operating closed loop.

1966fb

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