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.
Figures 8 and 11 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. Fig-
ures 9 and 10 show the large- and small-signal step re-
sponses for the G = +2 configuration with 100pF load
capacitance. Figures 12 and 13 show the large- and small-
signal step responses for the G = –2 configuration with
100pF load capacitance.
22pF
15pF
2kΩ
2kΩ
OPA228
2kΩ
100pF
1kΩ
2kΩ
OPA228
2kΩ
100pF
FIGURE 8. Compensation of the OPA228 for G =+2.
FIGURE 11. Compensation for OPA228 for G = –2.
5mV/div
5mV/div
OPA228
400ns/div
OPA228
400ns/div
FIGURE 9. Large-Signal Step Response, G = +2, C
LOAD
=
100pF, Input Signal = 5Vp-p.
FIGURE 12. Large-Signal Step Response, G = –2, C
LOAD
=
100pF, Input Signal = 5Vp-p.
25mV/div
25mV/div
OPA228
200ns/div
OPA228
200ns/div
FIGURE 10. Small-Signal Step Response, G = +2, C
LOAD
=
100pF, Input Signal = 50mVp-p.
OPA227, 2227, 4227
OPA228, 2228, 4228
SBOS110A
FIGURE 13. Small-Signal Step Response, G = –2, C
LOAD
=
100pF, Input Signal = 50mVp-p.
15
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