LTC1968 Datasheet, PDF (11/28 Pages) Linear Technology – Precision Wide Bandwidth, RMS-to-DC Converter

LTC1968
APPLICATIO S I FOR ATIO
0
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
1
C = 220µF
C = 100µF
C = 47µF
C = 22µF
C = 10µF
C = 4.7µF
C = 2.2µF
10
100
INPUT FREQUENCY (Hz)
Figure 8. Peak Error vs Input Frequency with One Cap Averaging
C =1µF
1000
1968 F08
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 = 10µF and f_INPUT = 6Hz.
If the application calls for the output of the LTC1968 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 LTC1968,
but the simplest solution is to use a larger averaging
capacitor.
A 10µF capacitor is a good choice for many applications.
The peak error at 50Hz/60Hz will be <1% and the DC error
will be <0.1% with frequencies of 10Hz or more.
Note that both Figure 6 and Figure 8 assume AC-coupled
waveforms with a crest factor less than 2, such as sine
waves or triangle waves. For higher crest factors and/or
AC + DC waveforms, a larger C_AVE will generally be
required. See “Crest Factor and AC + DC Waveforms.”
Capacitor Type Selection
The LTC1968 can operate with many types of capacitors.
The various types offer a wide array of sizes, tolerances,
parasitics, package styles and costs.
Ceramic chip capacitors offer low cost and small size, but
are not recommended for critical applications. The value
stability over voltage and temperature is poor with many
types of ceramic dielectrics. This will not cause an RMS-
to-DC accuracy problem except at low frequencies, where
it can aggravate the effects discussed in the previous
section. If a ceramic capacitor is used, it may be neces-
sary to use a much higher nominal value in order to
assure the low frequency accuracy desired.
Another parasitic of ceramic capacitors is leakage, which
is again dependent on voltage and particularly tempera-
ture. If the leakage is a constant current leak, the I • R drop
of the leak multiplied by the output impedance of the
LTC1968 will create a constant offset of the output voltage.
If the leak is Ohmic, the resistor divider formed with the
LTC1968 output impedance will cause a gain error. For
< 0.1% gain accuracy degradation, the parallel impedance
of the capacitor leakage will need to be >1000 times the
LTC1968 output impedance. Accuracy at this level can be
hard to achieve with a ceramic capacitor, particularly with
a large value of capacitance and at high temperature.
For critical applications, a film capacitor, such as metal-
ized polyester, will be a much better choice. Although
more expensive, and larger for a given value, the value
stability and low leakage make metal-film capacitors a
trouble-free choice.
With any type of capacitor, the self-resonance of the
capacitor can be an issue with the switched capacitor
LTC1968. If the self-resonant frequency of the averaging
capacitor is 1MHz or less, a second smaller capacitor
should be added in parallel to reduce the impedance seen
by the LTC1968 output stage at high frequencies. A
capacitor 100 times smaller than the averaging capacitor
will typically be small enough to be a low cost ceramic with
a high quality dielectric such as X7R or NPO/COG.
1968f
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