LTC1966CMS8#TRPBF Linear Technology, LTC1966CMS8#TRPBF Datasheet - Page 12

• Manufacturer Part Number: LTC1966CMS8#TRPBF
• Description: IC PREC RMS/DC CONV MCRPWR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1966CMS8#TRPBF

• Current - Supply: 155µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LTC1966
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
duty cycle is limited to 0% or 100% only when V
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 func-
tion, 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 imper-
fect 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
corrupt the RMS in to DC out transfer function. A nonlin-
12
4 • V
OUT
.
U
INPUT
U
INPUT CIRCUITRY
• V
• INPUT NONLINEARITY
W
IOS
Figure 5. Linearity Model of an RMS-to-DC Converter
U
IN
exceeds
output
CONVERTER
RMS-TO-DC
IDEAL
earity 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 dynamic 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
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
linearity throughout the LTC1966.
OUTPUT CIRCUITRY
• V
• OUTPUT NONLINEARITY
OOS
1966 F05
2
OUTPUT
) the energy of the RMS value,
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