Datasheet

ADA4941-1
Rev. C | Page 21 of 24
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 54 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 55.
1kΩ
1kΩ
500Ω
A2
A1
REF
IN
2
8
4
2.7nF
5
FB
+5V
–5V560pF
V
S+
V
S–
OUT+
+
–
V
O, dm
1
3
OUT–
VIN
562Ω
562Ω
562Ω
1.69kΩ
33Ω
33Ω
6
05704-058
0.1µF
0.1µF
2.7nF
3.9nF
10nF
Figure 54. Sallen-Key, Low-Pass Filter with 100 kHz Cutoff Frequency
0
–100
10 100M
FREQUENCY (Hz)
V
O, dm
/VIN (dB)
05704-059
–10
–20
–30
–40
–50
–60
–70
–80
–90
100 1k 10k 100k 1M 10M
V
O, dm
= 3V p-p
Figure 55. Frequency Response of the Circuit Shown in Figure 54