Datasheet
LT1028/LT1128
15
1028fb
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:
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%.
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%.
1028 F09
–
+
33pF
2k
LT1028
50Ω
–
+
LT1028
50Ω
500Ω
1028 F10
C1
R1
R
S1
R
S2
LT1028
–
+
1028 F11
10pF
10k
50Ω
1.1k
–
+
LT1028
Figure 9
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.
Figure 10
Figure 11