V

O

V = 1 V

I

RMS

OPA320-Q1

150 kΩ

19.5 kΩ

220 pF

1.8 kΩ

3.3 nF

47 pF

V

(V+)

V

(V–)

C1

1 nF

C2

150 pF

R3

549 Ω

R1

549 Ω

R2

1.24 kΩ

V

(V+)

V

O

V

I

V

(V–)

OPA320-Q1

21

OPA320-Q1, OPA2320-Q1

www.ti.com

SLOS884B – SEPTEMBER 2014 – REVISED DECEMBER 2018

Product Folder Links: OPA320-Q1 OPA2320-Q1

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Copyright © 2014–2018, Texas Instruments Incorporated

8.2.4 Active Filter

The OPAx320-Q1 is an excellent choice for active filter applications that require a wide bandwidth, fast slew rate,

low-noise, single-supply operational amplifier. Figure 43 shows a 500 kHz, second-order, low-pass filter using the

multiple-feedback (MFB) topology. The components have been selected to provide a maximally-flat Butterworth

response. Beyond the cutoff frequency, roll-off is –40 dB/dec. The Butterworth response is excellent for

applications requiring predictable gain characteristics, such as the antialiasing filter used in front of an ADC.

One point to observe when considering the MFB filter is that the output is inverted, relative to the input. If this

inversion is not required, or not desired, a noninverting output can be achieved through one of the following

options:

1. adding an inverting amplifier

2. adding an additional second-order MFB stage

3. using a noninverting filter topology, such as the Sallen-Key

Figure 43. Second-Order Butterworth 500-kHz Low-Pass Filter

Figure 44. OPAx320-Q1 Configured as a Three-Pole, 20-kHz, Sallen-Key Filter