LTC1966
12
1966fb
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 ∆S
output duty cycle is limited to 0% or 100% only when V
IN
exceeds ± 4 • V
OUT
.
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 (4
2
) 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.
Figure 5. Linearity Model of an RMS-to-DC Converter
INPUT CIRCUITRY
• V
IOS
• INPUT NONLINEARITY
IDEAL
RMS-TO-DC
CONVERTER
OUTPUT CIRCUITRY
• V
OOS
• OUTPUT NONLINEARITY
INPUT OUTPUT
1966 F05
