AD8033 datasheet(17/20 Pages) AD

17 / 20 page

REV. B

AD8033/AD8034

–17–

INPUT

OUTPUT

1.00V/DIV

200nS/DIV

Figure 7. Peak Detector Response 5 V 1

µs 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 fC = 1 MHz and over 80 dB of

stop-band attenuation, as shown in Figure 8.

–VS

+VS

10pF

–VS

+VS

VIN

4.22k

AD8034

1/2

AD8034

1/2

6.49k

49.9

27pF

33pF

4.99k

82pF

VOUT

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

10M

FREQUENCY – Hz

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

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 fC = 4 MHz can be constructed that

has a relatively low Q of 0.707 while still maintaining 15 dB

of attenuation an octave above fC and 35 dB of stop-band

attenuation. The filter and response are shown in Figures 10

and 11, respectively.

–VS

+VS

VIN

2.49k

22pF

VOUT

AD8033

2.49k

49.9

10pF

Figure 10. Two-Pole Butterworth Active Filter

100k

100M

10M

FREQUENCY – Hz

–45

–40

–35

–30

–25

–20

–15

–10

–5

Figure 11. Two-Pole Butterworth Active Filter Response

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