OPA227 Datasheet, PDF (27/46 Pages) Burr-Brown (TI) – High Precision, Low Noise OPERATIONAL AMPLIFIERS

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OPA227, OPA2227, OPA4227
OPA228, OPA2228, OPA4228
SBOS110B – MAY 1998 – REVISED JUNE 2015
Typical Application (continued)
8.2.2 Detailed Design Procedure
8.2.2.1 Using the OPAx228 in Low Gains
The OPAx228 family is intended for applications with signal gains of 5 or greater, but it is possible to take
advantage of their high-speed in lower gains. Without external compensation, the OPA228 has sufficient phase
margin to maintain stability in unity gain with purely resistive loads. However, the addition of load capacitance
can reduce the phase margin and destabilize the operational amplifier.
A variety of compensation techniques have been evaluated specifically for use with the OPA228. The
recommended configuration consists of an additional capacitor (C_F) in parallel with the feedback resistance, as
shown in Figure 51 and Figure 52. This feedback capacitor serves two purposes in compensating the circuit. The
operational amplifier’s input capacitance and the feedback resistors interact to cause phase shift that can result
in instability. C_F compensates the input capacitance, minimizing peaking. Additionally, at high frequencies, the
closed-loop gain of the amplifier is strongly influenced by the ratio of the input capacitance and the feedback
capacitor. Thus, C_F can be selected to yield good stability while maintaining high-speed.
Without external compensation, the noise specification of the OPA228 is the same as that for the OPA227 in
gains of 5 or greater. With the additional external compensation, the output noise of the of the OPA228 will be
higher. The amount of noise increase is directly related to the increase in high-frequency closed-loop gain
established by the C_IN/C_F ratio.
Figure 51 and Figure 52 show the recommended circuit for gains of 2 and –2, respectively. The figures suggest
approximate values for C_F. Because compensation is highly dependent on circuit design, board layout, and load
conditions, C_F should be optimized experimentally for best results. Figure 53 and Figure 55 show the large- and
small-signal step responses for the G = 2 configuration with 100-pF load capacitance.Figure 54 and Figure 56
show the large- and small-signal step responses for the G = –2 configuration with 100-pF load capacitance.
22pF
15pF
2kΩ
2kΩ
OPA228
2kΩ
100pF
1kΩ
2kΩ
OPA228
2kΩ
100pF
Figure 51. Compensation of the OPA228 for G = 2
8.2.3 Application Curves
Figure 52. Compensation for OPA228 for G = –2
OPA228
400ns/div
Figure 53. Large-Signal Step Response, G = 2,
C_LOAD = 100 pF, Input Signal = 5 Vp-p
OPA228
400ns/div
Figure 54. Large-Signal Step Response, G = –2, CLOAD =
100 pF, Input Signal = 5 Vp-p
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Product Folder Links: OPA227 OPA2227 OPA4227 OPA228 OPA2228 OPA4228