OPA365 Datasheet, PDF (10/16 Pages) Burr-Brown (TI) – 2.2V, 50MHz, Low-Noise, Single-Supply Rail-to-Rail OPERATIONAL AMPLIFIERS

OPA365
OPA2365
SBOS365A − JUNE 2006 − REVISED JULY 2006
ACHIEVING AN OUTPUT LEVEL OF
ZERO VOLTS (0V)
Certain single-supply applications require the op amp
output to swing from 0V to a positive full-scale voltage
and have high accuracy. An example is an op amp
employed to drive a single-supply ADC having an input
range from 0V to +5V. Rail-to-rail output amplifiers with
very light output loading may achieve an output level
within millivolts of 0V (or +V_S at the high end), but not
0V. Furthermore, the deviation from 0V only becomes
greater as the load current required increases. This in-
creased deviation is a result of limitations of the CMOS
output stage.
When a pull-down resistor is connected from the ampli-
fier output to a negative voltage source, the OPA365
can achieve an output level of 0V, and even a few milli-
volts below 0V. Below this limit, nonlinearity and limiting
conditions become evident. Figure 7 illustrates a circuit
using this technique.
A pull-down current of approximately 500µA is required
when OPA365 is connected as a unity-gain buffer.
A practical termination voltage (V_NEG) is −5V, but
other convenient negative voltages also may be
used. The pull-down resistor R_L is calculated from
R_L = [(V_O −V_NEG)/(500µA)]. Using a minimum output
voltage (V_O) of 0V, R_L = [0V−(−5V)]/(500µA)] = 10kΩ.
Keep in mind that lower termination voltages result in
smaller pull-down resistors that load the output during
positive output voltage excursions.
Note that this technique does not work with all op amps
and should only be applied to op amps such as the
OPA365 that have been specifically designed to oper-
ate in this manner. Also, operating the OPA365 output
at 0V changes the output stage operating conditions,
resulting in somewhat lower open-loop gain and band-
width. Keep these precautions in mind when driving a
capacitive load because these conditions can affect cir-
cuit transient response and stability.
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 8 shows a
500kHz, 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
−40dB/dec. The Butterworth response is ideal for ap-
plications requiring predictable gain characteristics
such as the anti-aliasing filter used ahead of an ADC.
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V+ = +5V
OPA365
V_OUT
V_IN
500µA
R_P = 10kΩ
Op Amps
Negative
Supply
Grounded
−V = −5V
(Additional
Negative Supply)
Figure 7. Swing-to-Ground
R₃
549Ω
C₂
150pF
R₁
549Ω
V_IN
R₂
1.24kΩ
C₁
1nF
V+
OPA365
V−
V_OUT
Figure 8. Second-Order Butterworth 500kHz
Low-Pass Filter
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