OPA340-EP_15 Datasheet, PDF (11/19 Pages) Texas Instruments – SINGLE-SUPPLY RAIL-TO-RAIL OPERATIONAL AMPLIFIERS

OPA340-EP
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CAPACITIVE LOAD AND STABILITY
The OPA340 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.
SBOS433A – AUGUST 2008 – REVISED APRIL 2011
DRIVING A/D CONVERTERS
The OPA340 is optimized for driving medium speed
(up to 100 kHz) sampling A/D converters. However, it
also offers excellent performance for higher speed
converters. The OPA340 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
+5V
0.1mF
0.1mF
V_IN
V_IN = 0V to 5V for
0V to 5V output.
OPA340
500W
3300pF
8 V+
+In
2
-In
3
ADS7816
12-Bit A/D
GND 4
RC network filters high frequency noise.
1 V_REF
7
DCLOCK
6
D_OUT
5
CS/SHDN
Serial
Interface
NOTE: A/D Input = 0 to V_REF
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