OPA348_15 Datasheet, PDF (10/32 Pages) Texas Instruments – 1MHz, 45uA, CMOS, Rail-to-Rail OPERATIONAL AMPLIFIERS Value Line

OPA348
OPA2348
OPA4348
SBOS213G – NOVEMBER 2001 – REVISED MARCH 2013
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
decreases overshoot from 55% to 13% (see the
typical characteristic Small-Signal Overshoot vs Load
Capacitance, Figure 13). 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 24. This significantly reduces
overshoot by compensating the effect of capacitance,
C_IN, which includes the amplifier's input capacitance
and PC board parasitic capacitance.
C_FB
R_I
V_IN
R_F
OPA348
C_IN
V_OUT
C_L
Figure 24. Improving Capacitive Load Drive
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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 25. The ADS7822 is
a 12-bit, microPOWER 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 antialiasing filter and charge
injection current. See Figure 26 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.
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