OPA2354-Q1 Datasheet, PDF (13/23 Pages) Texas Instruments – 250 MHz, RAIL-TO-RAIL I/O, CMOS OPERATIONAL AMPLIFIERS

OPA354-Q1
OPA2354-Q1
www.ti.com ....................................................................................................................................................... SBOS492A – JUNE 2009 – REVISED AUGUST 2009
Driving Analog-to-Digital Converters
The OPA354 series op amps offer 60-ns settling time to 0.01%, making them a good choice for driving high- and
medium-speed sampling A/D converters and reference circuits. The OPA354 series provide an effective means
of buffering the A/D converter's input capacitance and resulting charge injection while providing signal gain. For
applications requiring high DC accuracy, the OPA350 series is recommended.
Figure 6 shows the OPA354 driving an A/D converter. With the OPA354 in an inverting configuration, a capacitor
across the feedback resistor can be used to filter high-frequency noise in the signal.
+5V
330pF
5kΩ
V_IN
+2.5V
5kΩ
OPA354
+In
−In
V+
V_REF
ADS7816, ADS7861,
or ADS7864
12−Bit A/D Converter
GND
V_IN = 0V to −5V for 0V to 5V output.
NOTE: A/D Converter Input = 0V to V_REF
Figure 6. OPA354 Inverting Configuration Driving the ADS7816
Capacitive Load and Stability
The OPA354 series op amps can drive a wide range of capacitive loads. However, all op amps 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 op amp in unity-gain configuration is most susceptible to the effects of
capacitive loading. The capacitive load reacts with the op amp's output resistance, along with any additional load
resistance, to create a pole in the small-signal response that degrades the phase margin. See the typical
characteristic curve Frequency Response vs Capacitive Load for details.
The OPA354's topology enhances its ability to drive capacitive loads. In unity gain, these op amps perform well
with large capacitive loads. Refer to the typical characteristic curve Recommended R_S vs Capacitive Load and
Frequency Response vs Capacitive Load for details.
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 7. This significantly reduces ringing with large capacitive loads—see
the typical characteristic curve Frequency Response vs Capacitive Load. However, if there is a resistive load in
parallel with the capacitive load, R_S creates a voltage divider. This introduces a DC error at the output and
slightly reduces output swing. This error may be insignificant. For instance, with R_L = 10 kΩ and R_S = 20 Ω, there
is only about a 0.2% error at the output.
V+
OPA354
V_IN
R_S
V_OUT
R_L
C_L
Figure 7. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive
Copyright © 2009, Texas Instruments Incorporated
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