LTC1966IMS8 Linear Technology, LTC1966IMS8 Datasheet - Page 20

• Manufacturer Part Number: LTC1966IMS8
• Description: IC PREC RMS/DC CONV MCRPWR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1966IMS8

• 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: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC1966
Figures 19 and 20 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
These are comparable to the curves in Figure 12 (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.
The other difference is the settling behavior of the filters
below the 1% level. Unlike the case of a 1st order filter, any
3rd order filter can have overshoot and ringing. The filter
designs presented here have minimal overshoot and
ringing, but are somewhat sensitive to component mis-
matches. Even the 12% tolerance of the LTC1966 output
impedance can be enough to cause some ringing. The
dashed lines indicate what can happen when 5% capaci-
tors and 1% resistors are used.
20
–0.2
–0.4
–0.6
–2.0
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–0.2
–0.4
–0.6
–0.8
–1.6
–1.8
–2.0
–1.0
–1.2
–1.4
0
0
1
1
C = 10 F
C = 10 F
C = 4.7 F
C = 4.7 F
U
U
Figure 18. Peak Error vs Input Frequency with DC-Accurate Post Filter
Figure 17. Peak Error vs Input Frequency with Buffered Post Filter
C = 2.2 F
C = 2.2 F
W
AVE
U
C = 1.0 F
value.
C = 1.0 F
INPUT FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
10
10
Although the settling times for the post-filtered configura-
tions shown on Figures 19 and 20 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
C = 0.47 F
C = 0.47 F
C = 0.22 F
C = 0.22 F
C = 0.1 F
C = 0.1 F
sn1966 1966fas
1966 F17
1966 F18
100
100