Datasheet

13
LTC1966
sn1966 1966fas
APPLICATIO S I FOR ATIO
WUUU
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 averaging
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
2
. It is appropriate for most applications, in which
the output is fed to a circuit with an inherently band-limited
frequency response, such as a dual slope/integrating A/D
converter, a ∆Σ A/D converter or even a mechanical analog
meter.
Figure 6. DC Error vs Input Frequency
C = 4.7µF
INPUT FREQUENCY (Hz)
1
–2.0
DC ERROR (%)
–1.6
–1.2
–0.8
–0.4
10 20 50 60 100
1966 F06
0
–1.8
–1.4
–1.0
–0.6
–0.2
C = 10µF
C = 2.2µF
C = 1.0µF
C = 0.47µF
C = 0.22µF
C = 0.1µF
Figure 7. Output Ripple Exceeds DC Error
TIME
OUTPUT
1966 F07
DC
ERROR
(0.05%)
IDEAL
OUTPUT
DC
AVERAGE
OF ACTUAL
OUTPUT
PEAK
RIPPLE
(5%)
ACTUAL OUTPUT
WITH RIPPLE
f = 2 × f
INPUT
PEAK
ERROR =
DC ERROR +
PEAK RIPPLE
(5.05%)
However, if the output is examined on an oscilloscope with
a very low frequency input, the incomplete averaging 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
AVE
= 1µF and f
INPUT
= 10Hz.
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.
2
This frequency-dependent error is in additon to the static errors that affect all readings and are
therefore easy to trim or calibrate out. The “Error Analyses” section to follow discusses the effect
of static error terms.