LTC1966CMS8#TRPBF Linear Technology, LTC1966CMS8#TRPBF Datasheet - Page 13

• 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

DESIGN COOKBOOK
The LTC1966 RMS-to-DC converter makes it easy to
implement a rather quirky function. For many applications
all that will be needed is a single capacitor for averaging,
appropriate selection of the I/O connections and power
supply bypassing. Of course, the LTC1966 also requires
power. A wide variety of power supply configurations are
shown in the Typical Applications section towards the end
of this data sheet.
Capacitor Value Selection
The RMS or root-mean-squared value of a signal, the root
of the mean of the square, cannot be computed without
some averaging to obtain the mean function. The LTC1966
true RMS-to-DC converter utilizes a single capacitor on
the output to do the low frequency averaging required for
RMS-to-DC conversion. To give an accurate measure of a
dynamic waveform, the averaging must take place over a
sufficiently long interval to average, rather than track, the
lowest frequency signals of interest. For a single averag-
ing capacitor, the accuracy at low frequencies is depicted
in Figure 6.
Figure 6 depicts the so-called DC error that results at a
given combination of input frequency and filter capacitor
values
the output is fed to a circuit with an inherently band lim-
ited frequency response, such as a dual slope/integrating
A/D converter, a ∆S A/D converter or even a mechanical
analog meter.
applicaTions inForMaTion
–0.2
–0.4
–0.6
–2.0
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
0
1
1
. It is appropriate for most applications, in which
C = 4.7µF
C = 2.2µF
C = 10µF
C = 1.0µF
Figure 6. DC Error vs Input Frequency
C = 0.47µF
INPUT FREQUENCY (Hz)
10
1
therefore easy to trim or calibrate out. The Error Analyses section to follow discusses the effect
of static error terms.
However, if the output is examined on an oscilloscope
with a very low frequency input, the incomplete averag-
ing will be seen, and this ripple will be larger than the
error depicted in Figure 6. Such an output is depicted in
Figure 7. The ripple is at twice the frequency of the input
because of the computation of the square of the input.
The typical values shown, 5% peak ripple with 0.05% DC
error, occur with C
If the application calls for the output of the LTC1966 to feed
a sampling or Nyquist A/D converter (or other circuitry that
will not average out this double frequency ripple) a larger
averaging capacitor can be used. This trade-off is depicted
in Figure 8. The peak ripple error can also be reduced by
additional lowpass filtering after the LTC1966, but the
simplest solution is to use a larger averaging capacitor.
This frequency dependent error is in addition to the static errors that affect all readings and are
C = 0.22µF
Figure 7. Output Ripple Exceeds DC Error
ACTUAL OUTPUT
WITH RIPPLE
f = 2 × f
RIPPLE
20
PEAK
(5%)
AVE
INPUT
PEAK RIPPLE
DC ERROR +
ERROR =
= 1µF and f
(5.05%)
C = 0.1µF
PEAK
TIME
(0.05%)
ERROR
DC
OUTPUT
IDEAL
INPUT
OF ACTUAL
AVERAGE
OUTPUT
50
DC
LTC1966
= 10Hz.
1966 F07
60
13
1966 F06
1966fb
100