Datasheet

R
I
OPA348
V
IN
V
OUT
R
F
C
FB
C
IN
C
L
OPA348
OPA2348
OPA4348
SBOS213G –NOVEMBER 2001–REVISED MARCH 2013 www.ti.com
In unity-gain inverter configuration, phase margin can DRIVING A/D CONVERTERS
be reduced by the reaction between the capacitance
The OPA348 series op amps are optimized for driving
at the op amp input, and the gain setting resistors,
medium-speed sampling Analog-to-Digital Converters
thus degrading capacitive load drive. Best
(ADCs). The OPA348 op amps buffer the ADCs input
performance is achieved by using small valued
capacitance and resulting charge injection while
resistors. For example, when driving a 500pF load,
providing signal gain.
reducing the resistor values from 100kΩ to 5kΩ
decreases overshoot from 55% to 13% (see the
The OPA348 in a basic noninverting configuration
typical characteristic Small-Signal Overshoot vs Load
driving the ADS7822, see Figure 25. The ADS7822 is
Capacitance, Figure 13). However, when large valued
a 12-bit, microPOWER sampling converter in the
resistors cannot be avoided, a small (4pF to 6pF)
MSOP-8 package. When used with the low-power,
capacitor, C
FB
, can be inserted in the feedback, as
miniature packages of the OPA348, the combination
shown in Figure 24. This significantly reduces
is ideal for space-limited, low-power applications. In
overshoot by compensating the effect of capacitance,
this configuration, an RC network at the ADC’s input
C
IN
, which includes the amplifier's input capacitance
can be used to provide for anti-aliasing filter and
and PC board parasitic capacitance.
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.
Figure 24. Improving Capacitive Load Drive
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