C

1

1nF

C

2

150pF

R

3

549Ω

R

1

549Ω

R

2

1.24kΩ

V+

V

OUT

V

IN

V−

OPA365

V

OUT

V

OPA365

R

3

150kΩ

R

2

19.5kΩ

C

3

220pF

R

1

1.8kΩ

C

1

3.3nF

C

2

47pF

OPA365-EP

www.ti.com

SLOS735

– AUGUST 2011

Active Filtering

The OPA365 is well-suited for active filter applications requiring a wide bandwidth, fast slew rate, low-noise,

single-supply operational amplifier. Figure 9 shows a 500-kHz, 2nd-order, low-pass filter utilizing 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 ideal for applications

requiring predictable gain characteristics such as the anti-aliasing filter used ahead of an ADC.

Figure 9. Second-Order Butterworth 500kHz Low-Pass Filter

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 these options: 1)

adding an inverting amplifier; 2) adding an additional 2nd-order MFB stage; or 3) using a noninverting filter

topology such as the Sallen-Key (shown in Figure 10). MFB and Sallen-Key, low-pass and high-pass filter

synthesis is quickly accomplished using TI's FilterPro program. This software is available as a free download at

www.ti.com.

Figure 10. Configured as a 3-Pole, 20kHz, Sallen-Key Filter

Driving an Analog-to-Digital Converter

Very wide common-mode input range, rail-to-rail input and output voltage capability and high speed make the

OPA365 an ideal driver for modern ADCs. Also, because it is free of the input offset transition characteristics

inherent to some rail-to-rail CMOS op amps, the OPA365 provides low THD and excellent linearity throughout

the input voltage swing range. Figure 11 shows the OPA365 driving an ADS8326, 16-bit, 250kSPS converter.

The amplifier is connected as a unity-gain, noninverting buffer and has an output swing to 0 V, making it directly

compatible with the ADC minus full-scale input level. The 0-V level is achieved by powering the OPA365 V

−pin

with a small negative voltage established by the diode forward voltage drop. A small, signal-switching diode or

Schottky diode provides a suitable negative supply voltage of

−0.3 V to −0.7 V. The supply rail-to-rail is equal to

V+, plus the small negative voltage.

Copyright

© 2011, Texas Instruments Incorporated

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