ADA4511-2ARMZ-R7 datasheet(25/28 Pages) AD

25 / 28 page

Data Sheet

ADA4511-2

APPLICATIONS INFORMATION

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Rev. A | 25 of 28

Figure 84. Compensating the TIA, CF = 3.1 pF

Figure 85. Compensating the TIA, CF = 5.5 pF

Figure 86. Pulse Response for Various CF

ACTIVE FILTER

Figure 87. 4-Pole Low-Pass Filter with −3 dB Bandwidth of 10 kHz

Active filters are used to separate signals, passing signals of inter-

est and attenuating signals at unwanted frequencies. For example,

low-pass filters are often used as antialiasing filters in data acquisi-

tion systems or as noise filters to limit high-frequency noise.

The high-input impedance, high bandwidth, low-input bias current,

and DC precision make the ADA4511-2 a good fit for active filter

applications. Figure 87 shows the ADA4511-2 in a 4-pole Sallen-

Key Butterworth low-pass filter configuration. The 4-pole low-pass

filter has two complex conjugate pole pairs and is implemented

by cascading two 2-pole low-pass filters. Section A and B are

configured as 2-pole low-pass filters in unity gain. Table 7 shows

the quality factor (Q) requirement and pole position associated with

each stage of the Butterworth filter. For pole locations on the s

plane and Q requirements for filters of a different order, refer to the

Chapter 8, Analog Filters, in the Linear Circuit Design Handbook,

available at www.analog.com/AnalogDialogue.

Table 7. Q Requirements and Pole Positions

Section

Poles

−0.9239 ± j0.3827

0.5412

−0.3827 ± j0.9239

1.3065

The Sallen-Key topology is widely used due to its simple design

with few circuit elements. This topology provides the user the

flexibility of implementing either a low-pass or a high-pass filter by

simply interchanging the resistors and capacitors. The ADA4511-2

is configured in unity gain with a corner frequency at 10 kHz. An

active filter requires an op amp with a unity-gain bandwidth that is

at least 100 times greater than the product of the corner frequency

(fC) and the Q. The resistors and capacitors are also important in

determining the performance over manufacturing tolerances, time,

and temperature. At least 1% or better tolerance resistors and 5%

or better tolerance capacitors are recommended.

Figure 88 shows the frequency response of the low-pass Sallen-

Key filter, where:

VOUT1 is the output of the first stage.

VOUT2 is the output of the second stage.

VOUT1 shows a 40 dB/decade roll-off and VOUT2 shows an 80 dB/

decade roll-off. The transition band becomes sharper as the order

of the filter increases.

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