LTC1968 datasheet(17/28 Pages) LINER

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LTC1968

1968f

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 CAVE 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 CAVE should be based on half of the lowest

input frequency, using the same design curves presented

in Figures 6, 8, 16 and 17.

Figure 17. Peak Error vs Input Frequency with DC-Accurate Post Filter

Figure 16. Peak Error vs Input Frequency with Buffered Post Filter

INPUT FREQUENCY (Hz)

–1.2

–1.0

–0.8

–0.6

–0.4

100

1000

1968 F08

–1.4

–1.6

–1.8

–2.0

–0.2

C = 22µF

C = 100µF

C = 47µF

C = 10µF

C = 4.7µF

C = 2.2µF

C = 0.47µF

C = 0.22µF

C = 0.1µF

C =1µF