R1
2MΩ
R2
2MΩ
R8
402kΩ
R11
178kΩ
C6
10nF
C4
22nF
R3
1kΩ
R4
9.09kΩ
R6
R7
R9
R10
40.2kΩ
97.6kΩ
178kΩ
226kΩ
2
3
2
3
C1
1µF
C2
1µF
6
6
U2
VOUT
U3
U1
C3
C5
0.47µF
0.47µF
(OPA227)
(OPA227)
(OPA227)
Input from
Device
Under
R5
634kΩ
Test
FIGURE 6. 0.1Hz to 10Hz Bandpass Filter Used to Test Wideband Noise of the OPA227 and OPA228 Series.
USING THE OPA228 IN LOW GAINS
The OPA228 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 compen-
sation, 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 op amp.
22pF
100kΩ
10Ω
2
3
6
VOUT
A variety of compensation techniques have been evaluated
specifically for use with the OPA228. The recommended
configuration consists of an additional capacitor (CF) in
parallel with the feedback resistance, as shown in Figures
8 and 11. This feedback capacitor serves two purposes in
compensating the circuit. The op amp’s input capacitance
and the feedback resistors interact to cause phase shift that
can result in instability. CF compensates the input capaci-
tance, minimizing peaking. Additionally, at high frequen-
cies, the closed-loop gain of the amplifier is strongly
influenced by the ratio of the input capacitance and the
feedback capacitor. Thus, CF can be selected to yield good
stability while maintaining high speed.
OPA227
Device
Under
Test
FIGURE 7. Noise Test Circuit.
Figure 6 shows the 0.1Hz 10Hz bandpass filter used to test
the noise of the OPA227 and OPA228. The filter circuit was
designed using Texas Instruments’FilterPro software (avail-
able at www.ti.com). Figure 7 shows the configuration of
the OPA227 and OPA228 for noise testing.
OPA227, 2227, 4227
OPA228, 2228, 4228
14
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