LTC1968 datasheet(18/28 Pages) LINER

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LTC1968

1968f

APPLICATIO S I FOR ATIO

Crest factor, which is the peak to RMS ratio of a dynamic

signal, also effects the required CAVE value. With a higher

crest factor, more of the energy in the signal is concentrated

into a smaller portion of the waveform, and the averaging

has to ride out the long lull in signal activity. For busy

waveforms, such as a sum of sine waves, ECG traces or

SCR-chopped sine waves, the required value for CAVE

should be based on the lowest fundamental input frequency

divided as such:

DESIGN

INPUT MIN

()

•–

using the same design curves presented in Figures 6, 8,

16 and 17. For the worst case of square top pulse trains,

that are always either zero volts or the peak voltage, base

the selection on the lowest fundamental input frequency

divided by twice as much:

DESIGN

INPUT MIN

()

•–

The effects of crest factor and DC offsets are cumulative.

So for example, a 10% duty cycle pulse train from 0VPEAK

to 1VPEAK (CF = √10 = 3.16) repeating at 16.67ms (60Hz)

input is effectively only 30Hz due to the DC asymmetry and

is effectively only:

fHz

DESIGN ==

3 16

378

•.

–

for the purposes of Figures 6, 8, 16 and 17.

Obviously, the effect of crest factor is somewhat simplified

above given the factor of two difference based on a

subjective description of the waveform type. The results

will vary somewhat based on actual crest factor and

Figure 19. Settling Time with DC-Accurate Post Filter

Figure 18. Settling Time with Buffered Post Filter

SETTLING TIME (SEC)

0.01

0.1

100

1968 F18

C = 0.22µF

C = 0.47µF

C = 1µF

C = 2.2µF

C = 4.7µF

C = 10µF

C = 22µF

C = 47µF

C = 100µF

C = 220µF

C = 470µF

SETTLING TIME (SEC)

0.01

0.1

100

1968 F19

C = 0.1µF

C = 0.22µF

C = 0.47µF

C = 1µF

C = 2.2µF

C = 4.7µF

C = 10µF

C = 22µF

C = 47µF

C = 100µF

C = 220µF

C = 470µF

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