OPA365-Q1_16 Datasheet, PDF (15/22 Pages) Texas Instruments – Single-Supply Operational Amplifiers

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Application Information (continued)
V+=+5V
OPA365-Q1, OPA2365-Q1
SBOS512D – MARCH 2010 – REVISED DECEMBER 2015
OPA365
V_IN
500µA
Op Amp
Negative
Supply
Grounded
V_OUT
R_P = 10 kΩ
−V = −5V
(Add itio na l
Negative Supply)
Figure 25. Swing-to-Ground
A pulldown current of approximately 500 µA is required when OPA365-Q1 is connected as a unity-gain buffer. A
practical termination voltage (V_NEG) is −5 V, but other convenient negative voltages also may be used. The
pulldown resistor R_L is calculated from R_L = [(V_O −V_NEG)/(500 µA)]. Using a minimum output voltage (V_O) of 0 V,
R_L = [0 V−(−5V)]/(500 µA)] = 10 kΩ. Keep in mind that lower termination voltages result in smaller pulldown
resistors that load the output during positive output voltage excursions.
This technique does not work with all operational amplifier, and should only be applied to operational amplifiers,
such as the OPA365-Q1, that have been specifically designed to operate in this manner. Also, operating the
OPA365-Q1 output at 0 V changes the output stage operating conditions, resulting in somewhat lower open-loop
gain and bandwidth. Keep these precautions in mind when driving a capacitive load because these conditions
can affect circuit transient response and stability.
9.1.3 Active Filtering
The OPA365-Q1 device is well-suited for active filter applications requiring a wide bandwidth, fast slew rate, low-
noise, and single-supply operational amplifier. Figure 26 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 26. Second-Order Butterworth 500-kHz 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;
3. using a noninverting filter topology such as the Sallen-Key (shown in Figure 27).
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