LT1128 Linear Technology Corporation, LT1128 Datasheet - Page 13

• Manufacturer Part Number: LT1128
• Description: Ultra Low Noise Precision High Speed Op Amps
• Manufacturer: Linear Technology Corporation

Unity-Gain Buffer Applications (LT1128 Only)
When R
signal pulse (>1V), the output waveform will look as
shown in the pulsed operation diagram.
During the fast feedthrough-like portion of the output, the
input protection diodes effectively short the output to the
input and a current, limited only by the output short-circuit
protection, will be drawn by the signal generator. With R
requirements (I
in its active mode and a smooth transition will occur.
As with all operational amplifiers when R
be created with R
creating additional phase shift and reducing the phase
margin. A small capacitor (20pF to 50pF) in parallel with R
will eliminate this problem.
A
terminals, can exceed the inherent drift of the amplifier
unless proper care is exercised. Air currents should be
minimized, package leads should be short, the two input
leads should be close together and maintained at the same
temperature.
The circuit shown to measure offset voltage is also used
as the burn-in configuration for the LT1028/LT1128.
PPLICATI
500 , the output is capable of handling the current
and Offset Voltage Drift with Temperature
F
+
–
100 and the input is driven with a fast, large-
Test Circuit for Offset Voltage
10k*
200 *
R
L
F
* RESISTORS MUST HAVE LOW
O
V
THERMOELECTRIC POTENTIAL
F
2
3
O
and the amplifier’s input capacitance,
20mA at 10V) and the amplifier stays
= 100V
10k*
U
–
+
LT1028
LT1128
S
–15V
15V
OS
4
7
I FOR ATIO
OUTPUT
U
6
W
V
O
F
1028/1128 AI08
> 2k, a pole will
1028/1128 AI07
6V/ s
U
F
F
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 noninvert-
ing input is driven from a 50 source impedance. How-
ever, when feedback is through a parallel R-C network
(provided C
interaction between the input resistance and capacitance
and the feedback network. Larger source resistance at the
noninverting input has a similar effect. The following
voltage follower configurations are stable:
Another configuration which requires unity-gain stability
is shown below. When C
short the output to the input at 15MHz, oscillations can
occur. The insertion of R
LT1028 from oscillating. When R
noise contribution due to the presence of R
minimal. When R
cause R
the C
R
noise increase due to R
S1
. For example, R
F
short. When 100 < R
50
S1
–
+
LT1028
33pF
represents a heavy load on the output through
F
< 68pF), the LT1028 will be stable because of
2k
R
S1
R
S2
S1
S1
100 , R
S2
= R
F
–
+
LT1028
is 40%.
is large enough to effectively
S2
S2
C1
S1
LT1028/LT1128
< 500 , R
= 300 will be stable. The
R1
S2
S1
500
1028/1128 AI10
50
500
is not necessary, be-
500 , the additional
–
+
will prevent the
S2
LT1028
should match
S2
1028/1128 AI09
13
will be