LT1028_15 Datasheet, PDF (15/28 Pages) Linear Technology – Ultralow Noise Precision High Speed Op Amps

LT1028/LT1128
Applications Information
Frequency Response
The LT1028’s Gain, Phase vs Frequency plot indicates that
the device is stable in closed-loop gains greater than +2 or
–1 because phase margin is about 50° at an open-loop gain
of 6dB. In the voltage follower configuration phase margin
seems inadequate. This is indeed true when the output is
shorted to the inverting input and the noninverting input
is driven from a 50Ω source impedance. However, when
feedback is through a parallel R-C network (provided C_F
< 68pF), the LT1028 will be stable because of interaction
between the input resistance and capacitance and the
feedback network. Larger source resistance at the non-
inverting input has a similar effect. The following voltage
follower configurations are stable:
33pF
2k
–
LT1028
+
50Ω
–
LT1028
500Ω ₊
50Ω
1028 F09
Figure 9
Another configuration which requires unity-gain stability
is shown below. When C_F is large enough to effectively
short the output to the input at 15MHz, oscillations can
occur. The insertion of R_S2 ≥ 500Ω will prevent the LT1028
from oscillating. When R_S1 ≥ 500Ω, the additional noise
contribution due to the presence of R_S2 will be minimal.
When R_S1 ≤ 100Ω, R_S2 is not necessary, because R_S1
represents a heavy load on the output through the C_F
short. When 100Ω < R_S1 < 500Ω, R_S2 should match R_S1.
For example, R_S1 = R_S2 = 300Ω will be stable. The noise
increase due to R_S2 is 40%.
C1
R1
R_S1
–
R_S2
LT1028
+
1028 F10
Figure 10
If C_F is only used to cut noise bandwidth, a similar effect
can be achieved using the over-compensation terminal.
The Gain, Phase plot also shows that phase margin is
about 45° at gain of 10 (20dB). The following configuration
has a high (≈70%) overshoot without the 10pF capacitor
because of additional phase shift caused by the feedback
resistor – input capacitance pole. The presence of the 10pF
capacitor cancels this pole and reduces overshoot to 5%.
10pF
10k
1.1k
–
LT1028
+
50Ω
1028 F11
Figure 11
Over-Compensation
The LT1028/LT1128 are equipped with a frequency over-
compensation terminal (Pin 5). A capacitor connected
between Pin 5 and the output will reduce noise bandwidth.
Details are shown on the Slew Rate, Gain-Bandwidth Prod-
uct vs Over-Compensation Capacitor plot. An additional
benefit is increased capacitive load handling capability.
For more information www.linear.com/LT1028
1028fc
15