8
LTC1968
1968f
the lowpass filter. The input to the LPF is the calculation
from the multiplier/divider; (V
IN
)
2
/V
OUT
. The lowpass
filter will take the average of this to create the output,
mathematically:
APPLICATIO S I FOR ATIO
U
U
U
currents. The power delivered to the load depends on the
firing angle, as well as any parasitic losses such as switch
ON voltage drop. Real circuit waveforms will also typi-
cally have significant ringing at the switching transition,
dependent on exact circuit parasitics. For the purposes of
this data sheet, SCR Waveforms refers to the ideal
chopped sine wave, though the LTC1968 will do faithful
RMS-to-DC conversion with real SCR waveforms as well.
The case shown is for ?= 90? which corresponds to 50%
of available power being delivered to the load. As noted in
Table 1, when ?= 114? only 25% of the available power
is being delivered to the load and the power drops quickly
as ?approaches 180?
With an average rectification scheme and the typical
calibration to compensate for errors with sine waves, the
RMS level of an input sine wave is properly reported; it is
only with a non-sinusoidal waveform that errors occur.
Because of this calibration, and the output reading in
V
RMS
, the term True-RMS got coined to denote the use of
an actual RMS-to-DC converter as opposed to a calibrated
average rectifier.
CONTROL
V
LOAD
AC
MAINS
V
LINE
V
THY
1968 F02a
+
+
+
I
LOAD
V
LINE
?/DIV>
V
LOAD
V
THY
I
LOAD
1968 F02b
Figure 2a
Figure 2b
How an RMS-to-DC Converter Works
Monolithic RMS-to-DC converters use an implicit compu-
tation 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 topology is
easy and starts by identifying the inputs and the output of
V
V
V
V
V
V
so
V
V
V
and
V V or
V V RMSV
OUT
IN
OUT
OUT
IN
OUT
OUT
IN
OUT
OUT IN
OUT IN
IN
=
( )
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
( )
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
( )
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
( )
?/DIV>
?/DIV>
?/DIV>
?/DIV>
( )
=
( )
=
( )
=
( )
2
2
2
2
2 2
2
,
,
,
,
Because V is DC,
V
OUT
IN
Figure 3. RMS-to-DC Converter with Implicit Computation
V
IN
V
OUT
1968 F03
?/DIV>
?/DIV>
LPF
V
V
IN
OUT
( )
2
Unlike the prior generation RMS-to-DC converters, the
LTC1968 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 band-
width changes with the signal amplitude and the gain drifts
with temperature.
How the LTC1968 RMS-to-DC Converter Works
The LTC1968 uses a completely new topology for RMS-to-
DC conversion, in which a ?modulator acts as the
divider, and a simple polarity switch is used as the multi-
plier
1
as shown in Figure 4.
1
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