Datasheet

21
LTC1966
sn1966 1966fas
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 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 C
AVE
should be based on the lowest fundamental input frequency
divided as such:
f
f
CF
DESIGN
INPUT MIN
=
()
•–32
APPLICATIO S I FOR ATIO
WUUU
using the same design curves presented in Figures 6, 8,
17 and 18. 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:
f
f
CF
DESIGN
INPUT MIN
=
()
•–62
The effects of crest factor and DC offsets are cumulative.
So for example, a 10% duty cycle pulse train from 0V
PEAK
to 1V
PEAK
(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
==
30
6 3 16 2
378
•.–
.
for the purposes of Figures 6, 8, 17 and 18.
Figure 20. Settling Time with DC-Accurate Post Filter
Figure 19. Settling Time with Buffered Post Filter
SETTLING TIME (SEC)
0.01
0.1
SETTLING ACCURACY (%)
1
10
10.1 10 100
1066 F14
C = 100µFC = 47µFC = 22µFC = 10µFC = 4.7µFC = 2.2µFC = 1.0µFC = 0.47µFC = 0.22µFC = 0.1µF
SETTLING TIME (SEC)
0.01
0.1
SETTLING ACCURACY (%)
1
10
10.1 10 100
1066 F20
C = 100µFC = 47µFC = 22µFC = 10µFC = 4.7µFC = 2.2µFC = 1.0µFC = 0.47µFC = 0.22µFC = 0.1µF