Data Sheet

ADA4941-1

Rev. D | Page 21 of 23

ADDING A 3-POLE, SALLEN-KEY FILTER

The noninverting amplifier in the ADA4941-1 can be used as

the buffer amplifier of a Sallen-Key filter. A 3-pole, low-pass

filter can be designed to limit the signal bandwidth in front of

an ADC. The input signal first passes through the noninverting

stage where it is filtered. The filtered signal is then passed through

the inverting stage to obtain the complementary output.

Figure 55 illustrates a 3-pole, Sallen-Key, low-pass filter with a

−3 dB cutoff frequency of 100 kHz. The 1.69 kΩ resistor is

included to minimize dc errors due to the input offset current

in A1. The passive RC filters on the outputs are generally

required by the ADC converter that is being driven. The

frequency response of the filter is shown in Figure 56.

1kΩ

1kΩ

500Ω

A2

A1

REF

IN

2

8

4

2.7nF

5

FB

+5V

–5V

560pF

VS+

VS–

OUT+

+

–

VO, dm

1

3

OUT–

VIN

562Ω

562Ω

562Ω

1.69kΩ

33Ω

33Ω

6

0.1µF

0.1µF

2.7nF

3.9nF

10nF

Figure 55. Sallen-Key, Low-Pass Filter with 100 kHz Cutoff Frequency

0

–100

10

100M

FREQUENCY (Hz)

–10

–20

–30

–40

–50

–60

–70

–80

–90

100

1k

10k

100k

1M

10M

VO, dm = 3V p-p

Figure 56. Frequency Response of the Circuit Shown in Figure 55