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LTC1966

sn1966 1966fas

APPLICATIO S I FOR ATIO

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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

AVE

value.

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.

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

INPUT FREQUENCY (Hz)

–2.0

PEAK ERROR (%)

–1.6

–1.2

–0.8

–0.4

10 100

1966 F18

–1.8

–1.4

–1.0

–0.6

–0.2

C = 10µF

C = 4.7µF

C = 2.2µF C = 1.0µF C = 0.47µF C = 0.22µF C = 0.1µF

Figure 18. Peak Error vs Input Frequency with DC-Accurate Post Filter