OPA358 Datasheet, PDF (11/14 Pages) Burr-Brown (TI) – 3V Single-Supply 80MHz High-Speed Op Amp in SC70

OPA358
www.ti.com
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 10Ω 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, 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 = 10kΩ and R_S = 20Ω, there is only about a 0.2%
error at the output.
V+
R_S
OPA358
V_IN
V_OUT
R_L
C_L
To enable,
connect to V+
or drive with logic.
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.
SB0S296C − MARCH 2004 − REVISED FEBRUARY 2005
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 (R_F), and the
Gain Bandwidth Product (GBW) for the OPA358 (80MHz).
With these 3 variables set, the feedback capacitor value
(C_F) may be set to control the frequency response.
C_F
<1pF
(prevents gain peaking)
R_F
10MΩ
+V
λ
C_D OPA358
V_OUT
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:
Ǹ ¹
2pR_FC_F
+
GBW
4pR_FC_D
(1)
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:
Ǹ f_*3dB +
GBW
2pR_FC
D
Hz
(2)
For even higher transimpedance bandwidth, the CMOS
OPA380 (90MHz GBW), OPA355 (200MHz GBW), or the
OPA655 (400MHz GBW) may be used.
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