OPA365-EP_15 Datasheet, PDF (13/22 Pages) Texas Instruments – 50-MHz Low-Distortion High-CMRR Rail-to-Rail I/O, Single-Supply Operational Amplifier

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.
R₃
549Ω
C₂
150pF
R₁
549Ω
R₂
1.24kΩ
V_IN
C₁
1nF
V+
OPA365
V−
V_OUT
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.
C₃
220pF
V_IN = 1V_RMS
R₁
1.8kΩ
R₂
19.5kΩ
R₃
150kΩ
C₁
3.3nF
C₂
47pF
OPA365
V_OUT
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
Product Folder Link(s): OPA365-EP
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