OPA354_16 Datasheet, PDF (19/44 Pages) Texas Instruments – 250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers

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OPA354, OPA2354, OPA4354
SBOS233F – MARCH 2002 – REVISED JUNE 2016
Feature Description (continued)
8.3.6 Driving Analog-to-Digital converters
The OPA354 series op amps offer 60 ns of 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 input capacitance and resulting charge injection while providing signal gain.
For applications requiring high DC accuracy, the OPA350 series is recommended.
Figure 34 illustrates 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
5kW
V_IN
+2.5V
5kW
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
Copyright © 2016, Texas Instruments Incorporated
Figure 34. The OPA354 in Inverting Configuration Driving the ADS7816
8.3.7 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 device output resistance, along with any additional load
resistance, to create a pole in the small-signal response that degrades the phase margin. Refer to the typical
characteristic curve, Frequency Response for Various C_L (Figure 13) for details.
The OPA354 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 curves, Recommended R_S vs Capacitive Load
(Figure 14) and Frequency Response vs Capacitive Load (Figure 15) 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 35. This configuration significantly reduces ringing with large
capacitive loads—see the typical characteristic curve, Frequency Response vs Capacitive Load (Figure 15).
However, if there is a resistive load in parallel with the capacitive load, R_S creates a voltage divider. This voltage
division 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 approximately a 0.2% error at the output.
V+
R_S
OPA354
V_OUT
V_IN
R_L
C_L
Copyright © 2016, Texas Instruments Incorporated
Figure 35. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive
Copyright © 2002–2016, Texas Instruments Incorporated
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Product Folder Links: OPA354 OPA2354 OPA4354