OPA348, 2348, 4348
8
SBOS213C
www.ti.com
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 55% to 13% (see the typical charac-
teristic “Small-Signal Overshoot vs. Load Capacitance”).
However, when large valued resistors cannot be avoided, a
small (4pF to 6pF) capacitor, C
FB
, can be inserted in the
feedback, as shown in Figure 6. This significantly reduces
overshoot by compensating the effect of capacitance, C
IN
,
which includes the amplifier's input capacitance and PC
board parasitic capacitance.
FIGURE 6. Improving Capacitive Load Drive.
Normally, input currents are 0.5pA. However, large inputs
(greater than 500mV beyond the supply rails) can cause
excessive current to flow in or out of the input pins. There-
fore, as well as keeping the input voltage below the maxi-
mum rating, it is also important to limit the input current to
less than 10mA. This is easily accomplished with an input
voltage resistor, as shown in Figure 4.
R
I
OPA348
V
IN
V
OUT
R
F
C
FB
C
IN
C
L
FIGURE 4. Input Current Protection for Voltages Exceeding
the Supply Voltage.
5kΩ
OPA348
10mA max
+5V
V
IN
V
OUT
I
OVERLOAD
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 18mV from supply
rail. With moderate resistive loads (10kΩ to 50kΩ), the output
voltage can typically swing to within 100mV of the supply
rails while maintaining high open-loop gain (see the typical
characteristic “Output Voltage Swing vs Output Current”).
CAPACITIVE LOAD AND STABILITY
The OPA348 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 typical characteristic “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 5.
This significantly reduces ringing while maintaining DC per-
formance for purely capacitive loads. However, if there is a
resistive load in parallel with the capacitive load, a voltage
divider is created, introducing a Direct Current (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.
FIGURE 5. Series Resistor in Unity-Gain Buffer Configura-
tion Improves Capacitive Load Drive.
10Ω to
20Ω
OPA348
V+
V
IN
V
OUT
R
S
R
L
C
L
DRIVING A/D CONVERTERS
The OPA348 series op amps are optimized for driving
medium-speed sampling Analog-to-Digital Converters (ADCs).
The OPA348 op amps buffer the ADCs input capacitance
and resulting charge injection while providing signal gain.
The OPA348 in a basic noninverting configuration driving the
ADS7822, see Figure 7. The ADS7822 is a 12-bit,
micro
POWER sampling converter in the MSOP-8 package.
When used with the low-power, miniature packages of the
OPA348, the combination is ideal for space-limited, low-
power applications. In this configuration, an RC network at
the ADC’s input can be used to provide for anti-aliasing filter
and charge injection current.
The OPA348 in noninverting configuration driving ADS7822
limited, low-power applications. In this configuration, an RC
network at the ADC’s input can be used to provide for anti-
aliasing filter and charge injection current. See Figure 8 for
the OPA2348 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 microphone. This circuit
will operate with V
S
= 2.7V to 5V with less than 250µA typical
quiescent current.
