ADA4941-1 Analog Devices, ADA4941-1 Datasheet - Page 21

• Manufacturer Part Number: ADA4941-1
• Description: Single Supply Differential 18-Bit ADC Driver
• Manufacturer: Analog Devices

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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.
562Ω
VIN
3.9nF
562Ω
Figure 54. Sallen-Key, Low-Pass Filter with 100 kHz Cutoff Frequency
10nF
Figure 55. Frequency Response of the Circuit Shown in Figure 54
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
562Ω
0
10
1.69kΩ
560pF
100
1
8
FB
IN
6
–5V
1k
Rev. A | Page 21 of 24
3
A1
V
V
+5V
S+
S–
FREQUENCY (Hz)
2
10k
REF
500Ω
1kΩ
100k
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
A2
1M
V
O, dm
0.1µF
0.1µF
10M
= 3V p-p
OUT+
OUT–
100M
4
5
33Ω
33Ω
2.7nF
2.7nF
V
O, dm
+
–
ADA4941-1