OPA354-Q1_15 Datasheet, PDF (16/36 Pages) Texas Instruments – OPAx354-Q1 250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers

OPA354-Q1, OPA2354-Q1, OPA4354-Q1
SBOS492B – JUNE 2009 – REVISED DECEMBER 2014
Feature Description (continued)
5kW
V_IN
+2.5V
330pF
5kW
OPA354-Q1
+5V
+In
-In
V+
V_REF
ADS7816, ADS7861,
or ADS7864
12−Bit A/D Converter
GND
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V_IN = 0V to -5V for 0V to 5V output.
NOTE: A/D Converter Input = 0V to V_REF
Figure 34. OPA354-Q1 Inverting Configuration Driving the ADS7816
7.3.7 Capacitive Load and Stability
The OPAx354-Q1 family op-amps can drive a wide range of capacitive loads. However, all op-amps under
certain conditions can 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 output resistance of the op-amp, along with any additional
load resistance, to create a pole in the small-signal response that degrades the phase margin. For details see
Figure 15, Frequency Response vs Capacitive Load.
The OPAx354-Q1 topology enhances the ability of the device to drive capacitive loads. In unity gain, these op-
amps perform well with large capacitive loads. For details see Figure 14, Recommended R_S vs Capacitive Load,
and Figure 15, Frequency Response 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 35. This configuration significantly reduces ringing with large
capacitive loads—see Figure 15, Frequency Response vs Capacitive Load. However, if a resistive load is in
parallel with the capacitive load, R_S creates a voltage divider. This configuration 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, the error at the output is only about a 0.2%.
V+
R_S
OPA354-Q1
V_IN
V_OUT
R_L
C_L
Figure 35. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive
7.3.8 Wideband Transimpedance Amplifier
Wide bandwidth, low-input bias current, and low input voltage and current noise make the OPAx354-Q1 family of
devices an ideal wideband photodiode transimpedance amplifier for low-voltage single-supply applications. Low-
voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to
increase at high frequency.
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