Datasheet

OPA227, 2227, 4227
OPA228, 2228, 4228
14
SBOS110A
www.ti.com
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.
FIGURE 6. 0.1Hz to 10Hz Bandpass Filter Used to Test Wideband Noise of the OPA227 and OPA228 Series.
FIGURE 7. Noise Test Circuit.
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.
R
4
9.09kΩ
R
3
1kΩ
R
7
97.6kΩ
R
6
40.2kΩ
C
2
1µF
C
1
1µF
C
3
0.47µF
C
4
22nF
R
2
2MΩ
R
8
402kΩ
R
5
634kΩ
Input from
Device
Under
Test
R
1
2MΩ
(OPA227)
U1
(OPA227)
U2
6
2
3
R
10
226kΩ
R
9
178kΩ
C
5
0.47µF
C
6
10nF
R
11
178kΩ
(OPA227)
U3
6
V
OUT
2
3
100kΩ
V
OUT
6
2
3
OPA227
22pF
10Ω
Device
Under
Test