OPA340-EP Datasheet, PDF (11/17 Pages) Texas Instruments – SINGLE-SUPPLY RAIL-TO-RAIL OPERATIONAL AMPLIFIERS

OPA340-EP
OPA2340-EP
OPA4340-EP
www.ti.com ................................................................................................................................................................................................ SBOS433 – AUGUST 2008
CAPACITIVE LOAD AND STABILITY
OPA340 series operational amplifiers can drive a
wide range of capacitive loads. However, all
operational amplifiers under certain conditions may
become unstable. Op amp configuration, gain, and
load value are just a few of the factors to consider
when determining stability. An operational amplifier in
unity gain configuration is most susceptible to the
effects of capacitive load. The capacitive load reacts
with the operational amplifier’s output resistance,
along with any additional load resistance, to create a
pole in the small-signal response which degrades the
phase margin. In unity gain, OPA340 series
operational amplifiers perform well, with a pure
capacitive load up to approximately 1000 pF.
Increasing gain enhances the amplifier’s ability to
drive more capacitance. See the typical performance
curve Small-Signal Overshoot vs Capacitive Load.
One method of improving capacitive load drive in the
unity gain configuration is to insert a 10-Ω to 20-Ω
resistor in series with the output, as shown in
Figure 26. This significantly reduces ringing with large
capacitive loads. However, if there is a resistive load
in parallel with the capacitive load, it creates a
voltage divider introducing a dc error at the output
and slightly reduces output swing. This error may be
insignificant. For example, with R_L = 10 kΩ and
R_S = 20 Ω, there is only approximately 0.2% error at
the output.
DRIVING A/D CONVERTERS
OPA340 series operational amplifiers are optimized
for driving medium speed (up to 100 kHz) sampling
A/D converters. However, they also offer excellent
performance for higher speed converters. The
OPA340 series provides an effective means of
buffering the A/D converter’s input capacitance and
resulting charge injection while providing signal gain.
Figure 27 and Figure 28 show the OPA340 driving an
ADS7816. The ADS7816 is a 12-bit, micro-power
sampling converter in the tiny MSOP-8 package.
When used with the miniature package options of the
OPA340 series, the combination is ideal for
space-limited and low-power applications. For further
information consult the ADS7816 data sheet. With the
OPA340 in a noninverting configuration, an RC
network at the amplifier’s output can be used to filter
high-frequency noise in the signal (see Figure 27). In
the inverting configuration, filtering may be
accomplished with a capacitor across the feedback
resistor (see Figure 28).
V+
R_S
OPAx340
V_OUT
V_IN
10W to
20W
R_L
C_L
Figure 26. Series Resistor in Unity-Gain
Configuration Improves Capacitive Load Drive
Copyright © 2008, Texas Instruments Incorporated
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