LTC1968 Linear Integrated Systems, LTC1968 Datasheet - Page 18

• Manufacturer Part Number: LTC1968
• Description: RMS-to-DC Converter
• Manufacturer: Linear Integrated Systems

APPLICATIO S I FOR ATIO
Crest factor, which is the peak to RMS ratio of a dynamic
signal, also effects the required C
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
should be based on the lowest fundamental input frequency
divided as such:
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:
LTC1968
18
www.DataSheet4U.com
f
0.1
0.1
10
10
DESIGN
1
1
0.01
0.01
C = 0.1µF
=
C = 0.22µF
3
f
•
INPUT MIN
C = 0.22µF
CF
U
(
C = 0.47µF
–
)
U
2
C = 0.47µF
C = 1µF
0.1
0.1
AVE
W
Figure 19. Settling Time with DC-Accurate Post Filter
C = 1µF
value. With a higher
Figure 18. Settling Time with Buffered Post Filter
C = 2.2µF
C = 2.2µF
U
C = 4.7µF
AVE
SETTLING TIME (SEC)
SETTLING TIME (SEC)
C = 4.7µF
C = 10µF
1
1
The effects of crest factor and DC offsets are cumulative.
So for example, a 10% duty cycle pulse train from 0V
to 1V
input is effectively only 30Hz due to the DC asymmetry and
is effectively only:
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
C = 10µF
f
f
DESIGN
DESIGN
PEAK
C = 22µF
(CF = √10 = 3.16) repeating at 16.67ms (60Hz)
=
=
C = 22µF
6
6
C = 47µF
f
•
•
INPUT MIN
CF
3 16
.
30
(
C = 47µF
–
C = 100µF
–
)
2
10
10
2
=
C = 100µF
3 78
C = 220µF
.
Hz
C = 220µF
C = 470µF
C = 470µF
1968 F18
1968 F19
PEAK
100
100
1968f