OPA353NA Datasheet, PDF (9/26 Pages) Texas Instruments – High-Speed, Single-Supply, Rail-to-Rail PERATIONAL AMPLIFIERS

A double-folded cascode adds the signal from the two input
pairs and presents a differential signal to the class AB output
stage. Normally, input bias current is approximately 500fA.
However, large inputs (greater than 300mV beyond the
supply rails) can turn on the OPA353’s input protection
diodes, causing excessive current to flow in or out of the
input pins. Momentary voltages greater than 300mV beyond
the power supply can be tolerated if the current on the input
pins is limited to 10mA. This is easily accomplished with an
input resistor as shown in Figure 3. Many input signals are
inherently current-limited to less than 10mA, therefore, a
limiting resistor is not required.
I_OVERLOAD
10mA max
V_IN
5kΩ
V+
OPAx353
V_OUT
FEEDBACK CAPACITOR IMPROVES RESPONSE
For optimum settling time and stability with high-imped-
ance feedback networks, it may be necessary to add a
feedback capacitor across the feedback resistor, R_F, as
shown in Figure 4. This capacitor compensates for the zero
created by the feedback network impedance and the
OPA353’s input capacitance (and any parasitic layout
capacitance). The effect becomes more significant with
higher impedance networks.
R_IN
V_IN
R_IN • C_IN = R_F • C_F
C_F
R_F
V+
C_IN
OPA353
C_IN
V_OUT
C_L
FIGURE 3. Input Current Protection for Voltages Exceeding
the Supply Voltage.
Where C_IN is equal to the OPA353’s input
capacitance (approximately 9pF) plus any
parastic layout capacitance.
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is
used to achieve rail-to-rail output. For light resistive loads
(>10kΩ), the output voltage swing is typically ten millivolts
from the supply rails. With heavier resistive loads (600Ω to
10kΩ), the output can swing to within a few tens of milli-
volts from the supply rails and maintain high open-loop
gain. See the typical performance curves “Output Voltage
Swing vs Output Current” and “Open-Loop Gain vs Output
Voltage.”
CAPACITIVE LOAD AND STABILITY
OPA353 series op amps can drive a wide range of capacitive
loads. However, all op amps under certain conditions may
become unstable. Op amp configuration, gain, and load
value are just a few of the factors to consider when determin-
ing stability. An op amp in unity gain configuration is the
most susceptible to the effects of capacitive load. The
capacitive load reacts with the op amp’s output impedance,
along with any additional load resistance, to create a pole in
the small-signal response which degrades the phase margin.
In unity gain, OPA353 series op amps perform well with
large capacitive loads. Increasing gain enhances the
amplifier’s ability to drive more capacitance. The typical
performance curve “Small-Signal Overshoot vs Capacitive
Load” shows performance with a 1kΩ resistive load. In-
creasing load resistance improves capacitive load drive ca-
pability.
FIGURE 4. Feedback Capacitor Improves Dynamic Perfor-
mance.
It is suggested that a variable capacitor be used for the
feedback capacitor since input capacitance may vary be-
tween op amps and layout capacitance is difficult to
determine. For the circuit shown in Figure 4, the value of
the variable feedback capacitor should be chosen so that
the input resistance times the input capacitance of the
OPA353 (typically 9pF) plus the estimated parasitic layout
capacitance equals the feedback capacitor times the feed-
back resistor:
R_IN • C_IN = R_F • C_F
where C_IN is equal to the OPA353’s input capacitance
(sum of differential and common-mode) plus the layout
capacitance. The capacitor can be varied until optimum
performance is obtained.
DRIVING A/D CONVERTERS
OPA353 series op amps are optimized for driving medium
speed (up to 500kHz) sampling A/D converters. However,
they also offer excellent performance for higher speed
converters. The OPA353 series provides an effective means
of buffering the A/D’s input capacitance and resulting
charge injection while providing signal gain. For applica-
tions requiring high accuracy, the OPA350 series is recom-
mended.
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OPA353, 2353, 4353