Datasheet

AD8033/AD8034
Rev. D | Page 20 of 24
1V/DIV 100ns/DIV
02924-055
OUTPUT
INPUT
2
Figure 56. Peak Detector Response 4 V, 300 ns Pulse
Figure 56 shows the peak detector in Figure 55 capturing a
300 ns, 4 V pulse with 10 mV of kickback and a droop rate of
5 V/s. For larger peak-to-peak pulses, increase the time constants
of R1, C1 and R3, C3 to reduce overshoot. The best droop rate
occurs by isolating parasitic resistances from Node 3, which can
be accomplished using a guard band connected to the output of the
second amplifier that surrounds its summing junction (Node 3).
Increasing both time constants by a factor of 3 permits a larger
peak pulse to be captured and increases the output accuracy.
1V/DIV 200ns/DIV
02924-057
OUTPUT
INPUT
2
Figure 57. Peak Detector Response 5 V, 1 μs Pulse
Figure 57 shows a 5 V peak pulse being captured in 1 µs 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, determines the 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 are.
The Sallen-Key topology is the least dependent on the active
device, requiring that the bandwidth be flat to beyond the stop-
band frequency because 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 the linear input range of the amplifier.
Using an AD8033/AD8034, a 4-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 58.
–V
S
+V
S
C2
10pF
–V
S
+V
S
V
IN
R1
4.22kΩ
AD8034
1/2
AD8034
1/2
R2
6.49kΩ
R5
49.9Ω
C1
27pF
C3
33pF
R4
4.99kΩ
R3
4.99kΩ
C4
82pF
V
OUT
02924-058
Figure 58. 4-Pole Cascade Sallen-Key Filter
Component values are selected using a normalized cascaded,
2-stage Butterworth filter table and Sallen-Key 2-pole active
filter equations. The overall frequency response is shown in
Figure 59.
10M1M10k
FREQUENCY (Hz)
–100
REF LEVEL (dB)
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
100k
0
2924-059
Figure 59. 4-Pole Cascade Sallen-Key Filter Response