ADA4510-2ARMZ-R7 datasheet(29/32 Pages) AD

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Data Sheet

ADA4510-2

APPLICATIONS INFORMATION

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Rev. B | 29 of 32

Figure 87. Compensating the TIA, CF = 3.1 pF

Figure 88. Compensating the TIA, CF = 5.5 pF

Figure 89. Pulse Response for Various CF

ACTIVE FILTER

Figure 90. 4-Pole Low Pass Filter with −3dB Bandwidth of 10kHz

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 ADA4510-2 a good fit for active filter

applications. Figure 90 shows the ADA4510-2 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 Section B are

configured as 2-pole low-pass filters in unity gain. Table 8 shows

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

each stage of the Butterworth filter. Refer to Chapter 8, Analog

Filters, in Linear Circuit Design Handbook, available at www.ana-

log.com/AnalogDialogue, for pole locations on the s plane and Q

requirements for filters of a different order.

Table 8. 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 ADA4510-2

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 91 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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