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OPA358 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA358 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 18 page  OPA358 SB0S296C − MARCH 2004 − REVISED FEBRUARY 2005 www.ti.com 11 CAPACITIVE LOAD AND STABILITY The OPA358 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 output resistance, along with any additional load resistance, to create a pole in the small-signal response that degrades the phase margin. One method of improving capacitive load drive in the unity-gain configuration is to insert a 10 Ω to 20Ω resistor in series with the output, as shown in Figure 10. This significantly reduces ringing with large capacitive loads. However, if there is a resistive load in parallel with the capacitive load, RS 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 RL = 10k Ω and RS =20Ω, there is only about a 0.2% error at the output. OPA358 V+ V IN V OUT C L R L R S To enable, connect to V+ or drivewithlogic. Figure 10. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive WIDEBAND TRANSIMPEDANCE AMPLIFIER Wide bandwidth, low input bias current, and low input voltage and current noise make the OPA358 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. The key elements to a transimpedance design, as shown in Figure 11, are the expected diode capacitance (including the parasitic input common-mode and differential-mode input capacitance (1.5 + 1.5)pF for the OPA358), the desired transimpedance gain (RF), and the Gain Bandwidth Product (GBW) for the OPA358 (80MHz). With these 3 variables set, the feedback capacitor value (CF) may be set to control the frequency response. OPA358 V OUT R F 10M Ω C F <1pF (prevents gain peaking) +V λ C D To enable, connect to V+ or drive with logic. Figure 11. Transimpedance Amplifier To achieve a maximally flat 2nd-order Butterworth frequency response, the feedback pole should be set to: 1 2 pR FCF + GBW 4 pR FCD Typical surface-mount resistors have a parasitic capacitance of around 0.2pF that must be deducted from the calculated feedback capacitance value. Bandwidth is calculated by: f*3dB + GBW 2 pR FCD Hz For even higher transimpedance bandwidth, the CMOS OPA380 (90MHz GBW), OPA355 (200MHz GBW), or the OPA655 (400MHz GBW) may be used. (1) (2) |
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