LT1175-5 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

Список матч

LT1175-5

Precision Micropower ∆∑ RMS-to-DC Converter

Linear Technology 

LTC1966

Applications Information

More detail of the LTC1966 inner workings is shown in

the Simplified Schematic towards the end of this data

sheet. Note that the internal scalings are such that the ∆Σ

output duty cycle is limited to 0% or 100% only when VIN

exceeds ± 4 • VOUT.

Linearity of an RMS-to-DC Converter

Linearity may seem like an odd property for a device that

implements a function that includes two very nonlinear

processes: squaring and square rooting.

However, an RMS-to-DC converter has a transfer function,

RMS volts in to DC volts out, that should ideally have a

1:1 transfer function. To the extent that the input to output

transfer function does not lie on a straight line, the part

is nonlinear.

A more complete look at linearity uses the simple model

shown in Figure 5. Here an ideal RMS core is corrupted by

both input circuitry and output circuitry that have imperfect

transfer functions. As noted, input offset is introduced in

the input circuitry, while output offset is introduced in the

output circuitry.

Any nonlinearity that occurs in the output circuity will cor-

rupt the RMS in to DC out transfer function. A nonlinearity

in the input circuitry will typically corrupt that transfer

function far less, simply because with an AC input, the

RMS-to-DC conversion will average the nonlinearity from

a whole range of input values together.

But the input nonlinearity will still cause problems in an

RMS-to-DC converter because it will corrupt the accuracy

as the input signal shape changes. Although an RMS-to-DC

converter will convert any input waveform to a DC output,

the accuracy is not necessarily as good for all waveforms

as it is with sine waves. A common way to describe dy-

namic signal wave shapes is crest factor. The crest factor

is the ratio of the peak value relative to the RMS value of

a waveform. A signal with a crest factor of 4, for instance,

has a peak that is four times its RMS value. Because this

peak has energy (proportional to voltage squared) that is

16 times (42) the energy of the RMS value, the peak is

necessarily present for at most 6.25% (1/16) of the time.

The LTC1966 performs very well with crest factors of 4

or less and will respond with reduced accuracy to signals

with higher crest factors. The high performance with crest

factors less than 4 is directly attributable to the high linear-

ity throughout the LTC1966.

The LTC1966 does not require an input rectifier, as is com-

mon with traditional log/antilog RMS-to-DC converters.

Thus, the LTC1966 has none of the nonlinearities that are

introduced by rectification.

The excellent linearity of the LTC1966 allows calibration to

be highly effective at reducing system errors. See System

Calibration section following the Design Cookbook.

INPUT

INPUT CIRCUITRY

• VIOS

• INPUT NONLINEARITY

IDEAL

RMS-TO-DC

CONVERTER

OUTPUT CIRCUITRY

• VOOS

OUTPUT

• OUTPUT NONLINEARITY

1966 F05

Figure 5. Linearity Model of an RMS-to-DC Converter

1966fb

12

Share Link: