= 1592pF.
= 1/(2 鈥?/div>
蟺
鈥?R
1
鈥?C
1
)
8
9
10
11
12
13
14
15
16
FIGURE 2. Diode-Clamped Nonlinear Filter (can improve
0.01% settling time for a conventional filter by
2/1 for a 20V step).
NONLINEAR FILTER
To understand how a nonlinear filter can improve settling
time, consider the simple diode clamped nonlinear filter,
shown in Figure 2. Settling time is improved because the
filter capacitor, C
1
, is charged faster through the low forward
biased diode impedance (R
ON
) during the initial portion of a
large input step change. When the difference between the
input and output voltage becomes less than the forward
biased diode drop (about 0.6V), the diode turns off and C
1
reacts with R
1
alone. At this point, the circuit behaves like a
normal single-pole RC filter.
Assuming diode R
ON
is negligible, the improvement in
settling time depends on the ratio of the input step voltage to
the forward biased diode voltage. For a step of 鈥?0V to
+10V (a 20V step), the improvement is ln(0.60/20) or 3.5
time constants. In other words, for a 20V step, the simple
AB-022
Printed in U.S.A. January, 1991
鈥?
IN
)
鈥?100 = %
therefore
t
S
= 鈥搇n(%/100) 鈥?R
1
鈥?C
1
Where:
t
S
= settling time (s)
% = percent accuracy at t
S
R
1
鈥?C
1
= RC time constant (鈩?鈥?F) or (s)
For example, if a settling to 0.01% is needed,
ln(0.01/100) = 鈥?.2
In other words, it takes 9.2 R
1
鈥?C
1
time constants for an input
step to settle to within 0.01% of its final value.
漏
1991 Burr-Brown Corporation
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