OPA347NA Datasheet, PDF (9/33 Pages) Texas Instruments – microPower, Rail-to-Rail Operational Amplifiers

RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is
used to achieve rail-to-rail output. This output stage is ca-
pable of driving 5kΩ loads connected to any potential be-
tween V+ and ground. For light resistive loads (> 100kΩ), the
output voltage can typically swing to within 5mV from supply
rail. With moderate resistive loads (10kΩ to 50kΩ), the output
can swing to within a few tens of millivolts from the supply
rails while maintaining high open-loop gain (see the typical
characteristic Output Voltage Swing vs Output Current).
CAPACITIVE LOAD AND STABILITY
The OPA347 in a unity-gain configuration can directly drive
up to 250pF pure capacitive load. Increasing the gain en-
hances the amplifier’s ability to drive greater capacitive loads
(see the characteristic curve Small-Signal Overshoot vs
Capacitive Load). In unity-gain configurations, capacitive
load drive can be improved by inserting a small (10Ω to 20Ω)
resistor, R_S, in series with the output, as shown in Figure 6.
This significantly reduces ringing while maintaining Direct
Current (DC) performance for purely capacitive loads. How-
ever, if there is a resistive load in parallel with the capacitive
load, a voltage divider is created, introducing a DC error at
the output and slightly reducing the output swing. The error
introduced is proportional to the ratio R_S/R_L, and is generally
negligible.
V+
R_S
OPA347
V_OUT
V_IN
10Ω to
20Ω
R_L
C_L
FIGURE 6. Series Resistor in Unity-Gain Buffer Configura-
tion Improves Capacitive Load Drive.
In unity-gain inverter configuration, phase margin can be
reduced by the reaction between the capacitance at the op
amp input, and the gain setting resistors, thus degrading
capacitive load drive. Best performance is achieved by using
small valued resistors. For example, when driving a 500pF
load, reducing the resistor values from 100kΩ to 5kΩ de-
creases overshoot from 40% to 8% (see the characteristic
curve Small-Signal Overshoot vs Load Capacitance). How-
ever, when large-valued resistors can not be avoided, a
small (4pF to 6pF) capacitor, C_FB, can be inserted in the
feedback, as shown in Figure 7. This significantly reduces
overshoot by compensating the effect of capacitance, C_IN,
which includes the amplifier input capacitance and PC board
parasitic capacitance.
R_I
V_IN
C_FB
R_F
OPA347
C_IN
V_OUT
C_L
FIGURE 7. Adding a Feedback Capacitor In the Unity-Gain
Inverter Configuration Improves Capacitative
Load.
DRIVING ADCs
The OPA347 series op amps are optimized for driving
medium-speed sampling Analog-to-Digital Converters (ADCs).
The OPA347 op amps buffer the ADC’s input capacitance
and resulting charge injection while providing signal gain.
See Figure 8 for the OPA347 in a basic noninverting configu-
ration driving the ADS7822. The ADS7822 is a 12-bit,
microPower sampling converter in the MSOP-8 package.
When used with the low-power, miniature packages of the
OPA347, the combination is ideal for space-limited, low-
power applications. In this configuration, an RC network at
the ADC input can be used to provide for anti-aliasing filter
and charge injection current.
See Figure 9 for the OPA2347 driving an ADS7822 in a
speech bandpass filtered data acquisition system. This small,
low-cost solution provides the necessary amplification and
signal conditioning to interface directly with an electret micro-
phone. This circuit will operate with V_S = 2.7V to 5V with less
than 250µA typical quiescent current.
OPA347, 2347, 4347
9
SBOS167D
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