OPA227 Datasheet, PDF (14/25 Pages) Burr-Brown (TI) – High Precision, Low Noise OPERATIONAL AMPLIFIERS

R₁
2MΩ
R₂
2MΩ
R₈
402kΩ
R₁₁
178kΩ
R₃
1kΩ
C₁
C₂
1µF
1µF
Input from
Device
Under
Test
R₄
9.09kΩ
R₆
R₇
40.2kΩ 97.6kΩ
U1
(OPA227)
C₃
0.47µF
R₅
634kΩ
C₄
22nF
C₆
10nF
2
6
U2
3
(OPA227)
R₉
178kΩ
R₁₀
226kΩ
2
6
U3
C₅
3
0.47µF
(OPA227)
V_OUT
FIGURE 6. 0.1Hz to 10Hz Bandpass Filter Used to Test Wideband Noise of the OPA227 and OPA228 Series.
22pF
100kΩ
10Ω
2
6
OPA227
3
V_OUT
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.
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.
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 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. C_F 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, C_F can be selected to yield good
stability while maintaining high speed.
OPA227, 2227, 4227
14
OPA228, 2228, 4228
www.ti.com
SBOS110A