AD8033AKS-R2 datasheet(21/25 Pages) AD

21 / 25 page

AD8033/AD8034

Rev. D | Page 20 of 24

1V/DIV 100ns/DIV

OUTPUT

INPUT

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

OUTPUT

INPUT

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 fC = 1 MHz and over 80 dB of stop-band

attenuation, as shown in Figure 58.

–VS

+VS

10pF

–VS

+VS

VIN

4.22kΩ

AD8034

1/2

AD8034

1/2

6.49kΩ

49.9Ω

27pF

33pF

4.99kΩ

82pF

VOUT

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.

10M

10k

FREQUENCY (Hz)

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

100k

Figure 59. 4-Pole Cascade Sallen-Key Filter Response

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