LTC1967_15 Datasheet, PDF (17/28 Pages) Linear Technology – Precision Extended Bandwidth, RMS-to-DC Converter

LTC1967
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 = 22µF
C = 10µF
C = 4.7µF
C = 2.2µF
C = 1µF
C = 0.47µF
10
INPUT FREQUENCY (Hz)
Figure 16. Peak Error vs Input Frequency with Buffered Post Filter
C = 0.22µF
100
1967 F16
0
–0.2
C = 10µF
–0.4
–0.6
–0.8
–1.0
C = 4.7µF
–1.2
–1.4
–1.6
–1.8
–2.0
1
C = 2.2µF
C = 1µF
C = 0.47µF
C = 0.22µF
10
INPUT FREQUENCY (Hz)
Figure 17. Peak Error vs Input Frequency with DC-Accurate Post Filter
C = 0.1µF
100
1967 F17
Figures 18 and 19 show the settling time versus settling
accuracy for the Buffered and DC accurate post filters,
respectively. The different curves represent different
scalings of the filters, as indicated by the C_AVE value. These
are comparable to the curves in Figure 11 (single capacitor
case), with somewhat less settling time for the buffered
post filter, and somewhat more settling time for the
DC-accurate post filter. These differences are due to the
change in overall bandwidth as mentioned earlier.
Although the settling times for the post-filtered configura-
tions shown on Figures 18 and 19 are not that much
different from those with a single capacitor, the point of
using a post filter is that the settling times are far better for
a given level peak error. The filters dramatically reduce the
low frequency averaging ripple with far less impact on
settling time.
Crest Factor and AC + DC Waveforms
In the preceding discussion, the waveform was assumed
to be AC coupled, with a modest crest factor. Both
assumptions ease the requirements for the averaging
capacitor. With an AC-coupled sine wave, the calculation
engine squares the input, so the averaging filter that
follows is required to filter twice the input frequency,
making its job easier. But with a sinewave that includes
DC offset, the square of the input has frequency content
at the input frequency and the filter must average out that
lower frequency. So with AC + DC waveforms, the re-
quired value for C_AVE should be based on half of the lowest
input frequency, using the same design curves presented
in Figures 6, 8, 16 and 17.
1967f
17