LTC1966MPMS8#TRPBF【PDF 21/38 页】IC RMS/DC CONV MICROPWR 8-MSOP

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LTC1966

1966fb

applicaTions inForMaTion

Although the settling times for the post filtered configu-

rations 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 as-

sumptions 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

required value for C

AVE

should be based on half of the

lowest input frequency, using the same design curves

presented in Figures 6, 8, 17 and 18.

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

signal, also effects the required C

AVE

value. With a higher

crest factor, more of the energy in the signal is concen-

trated 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

AVE

should be based on the lowest fundamental input

frequency divided as such:

DESIGN

INPUTMIN

( )

Figure 20. Settling Time with DC Accurate Post Filter

Figure 19. Settling Time with Buffered Post Filter

SETTLING TIME (SEC)

0.01

0.1

100

1066 F14

C = 100礔

C = 47礔

C = 22礔

C = 10礔

C = 4.7礔

C = 2.2礔

C = 1.0礔

C = 0.47礔

C = 0.22礔

C = 0.1礔

SETTLING TIME (SEC)

0.01

0.1

100

1066 F20

C = 100礔

C = 47礔

C = 22礔

C = 10礔

C = 4.7礔

C = 2.2礔

C = 1.0礔

C = 0.47礔

C = 0.22礔

C = 0.1礔

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