Datasheet

10 toW
20W
OPAx340
V+
V
IN
V
OUT
R
S
R
L
C
L
ADS7816
12-BitA/D
DCLOCK
D
OUT
CS/SHDN
OPA340
+5V
V
IN
V+
2
+In
3
-In
V
REF
8
4GND
Serial
Interface
1
0.1mF 0.1mF
7
6
5
NOTE:A/DInput=0toV
REF
V =0Vto5Vfor
IN
0Vto5Voutput.
RCnetworkfiltershighfrequencynoise.
500W
3300pF
OPA340-EP
www.ti.com SBOS433A – AUGUST 2008–REVISED APRIL 2011
CAPACITIVE LOAD AND STABILITY DRIVING A/D CONVERTERS
The OPA340 can drive a wide range of capacitive The OPA340 is optimized for driving medium speed
loads. However, all operational amplifiers under (up to 100 kHz) sampling A/D converters. However, it
certain conditions may become unstable. Op amp also offers excellent performance for higher speed
configuration, gain, and load value are just a few of converters. The OPA340 provides an effective means
the factors to consider when determining stability. An of buffering the A/D converter’s input capacitance and
operational amplifier in unity gain configuration is resulting charge injection while providing signal gain.
most susceptible to the effects of capacitive load. The Figure 27 and Figure 28 show the OPA340 driving an
capacitive load reacts with the operational amplifier’s ADS7816. The ADS7816 is a 12-bit, micro-power
output resistance, along with any additional load sampling converter in the tiny MSOP-8 package.
resistance, to create a pole in the small-signal When used with the miniature package options of the
response which degrades the phase margin. In unity OPA340 series, the combination is ideal for
gain, OPA340 series operational amplifiers perform space-limited and low-power applications. For further
well, with a pure capacitive load up to approximately information consult the ADS7816 data sheet. With the
1000 pF. Increasing gain enhances the amplifier’s OPA340 in a noninverting configuration, an RC
ability to drive more capacitance. See the typical network at the amplifier’s output can be used to filter
performance curve Small-Signal Overshoot vs high-frequency noise in the signal (see Figure 27). In
Capacitive Load. the inverting configuration, filtering may be
accomplished with a capacitor across the feedback
One method of improving capacitive load drive in the
resistor (see Figure 28).
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.
Figure 26. Series Resistor in Unity-Gain
Configuration Improves Capacitive Load Drive
Copyright © 2008–2011, Texas Instruments Incorporated 11