
REV. B
AD8033/AD8034
–17–
INPUT
OUTPUT
1.00V/DIV
200nS/DIV
Figure 7. Peak Detector Response 5 V 1
μ
s Pulse
Figure 7 shows a 5 V peak pulse being captured in 1 s with less
than 1 mV of kickback. With this selection of time constants, up
to a 20 V peak pulse can be captured with no overshoot.
Active Filters
The response of an active filter varies greatly depending on the
performance of the active device. Open-loop bandwidth and gain,
along with the order of the filter, will determine stop-band
attenuation as well as the maximum cutoff frequency, while input
capacitance can set a limit on which passive components are used.
Topologies for active filters are varied, and some are more
dependent on the performance of the active device than others.
The Sallen-Key topology is the least dependent on the active
device, requiring that the bandwidth be flat to beyond the stop-
band frequency since it is used simply as a gain block. In the
case of high Q filter stages, the peaking must not exceed the
open-loop bandwidth and linear input range of the amplifier.
Using an AD8033/AD8034, a four-pole cascaded Sallen-Key
filter can be constructed with f
C
= 1 MHz and over 80 dB of
stop-band attenuation, as shown in Figure 8.
–V
S
+V
S
C2
10pF
–V
S
+V
S
V
IN
R1
4.22k
AD8034
AD8034
R2
6.49k
R5
49.9
C1
27pF
C3
33pF
R4
4.99k
R3
4.99k
C4
82pF
V
OUT
Figure 8. Four-Pole Cascade Sallen-Key Filter
Component values are selected using a normalized cascaded
two-stage Butterworth filter table and Sallen-Key two-pole
active filter equations. The overall frequency response is shown
in Figure 9.
10k
1M
10M
FREQUENCY – Hz
–100
R
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
100k
Figure 9. Four-Pole Cascade Sallen-Key Filter Response
The common-mode input capacitance should be considered
in the component selection.
Filter cutoff frequencies can be increased beyond 1 MHz using
the AD8033/AD8034, but limited open-loop gain and input
impedance begin to interfere with the higher Q stages. This can
cause early roll-off of the overall response.
Additionally, the stop-band attenuation will decrease with
decreasing open-loop gain.
Keeping these limitations in mind, a two-pole Sallen-Key
Butterworth filter with f
C
= 4 MHz can be constructed that
has a relatively low Q of 0.707 while still maintaining 15 dB
of attenuation an octave above f
C
and 35 dB of stop-band
attenuation. The filter and response are shown in Figures 10
and 11, respectively.
–V
S
+V
S
V
IN
R1
2.49k
C3
22pF
V
OUT
AD8033
R2
2.49k
R5
49.9
C1
10pF
Figure 10. Two-Pole Butterworth Active Filter
100k
100M
1M
10M
FREQUENCY – Hz
–45
G
–40
–35
–30
–25
–20
–15
–10
–5
0
5
Figure 11. Two-Pole Butterworth Active Filter Response