LTC1967CMS8 Linear Technology, LTC1967CMS8 Datasheet - Page 17

• Manufacturer Part Number: LTC1967CMS8
• Description: IC CONVERTER RMS-DC PREC 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1967CMS8

• Current - Supply: 320µA
• Voltage - Supply: 5.0V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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
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.
–0.2
–0.4
–0.6
–2.0
–0.2
–0.4
–0.6
–0.8
–2.0
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–1.0
–1.2
–1.4
–1.6
–1.8
0
0
1
1
C = 10 F
C = 22 F
C = 10 F
C = 4.7 F
U
U
Figure 17. Peak Error vs Input Frequency with DC-Accurate Post Filter
C = 4.7 F
C = 2.2 F
Figure 16. Peak Error vs Input Frequency with Buffered Post Filter
W
AVE
value. These
C = 2.2 F
U
C = 1 F
INPUT FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
10
10
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
input frequency, using the same design curves presented
in Figures 6, 8, 16 and 17.
C = 1 F
C = 0.47 F
AVE
should be based on half of the lowest
C = 0.47 F
C = 0.22 F
C = 0.22 F
C = 0.1 F
LTC1967
1967 F16
1967 F17
17
100
100
1967f