參數(shù)資料
型號: LT1128
廠商: Linear Technology Corporation
英文描述: Ultra Low Noise Precision High Speed Op Amps
中文描述: 超低噪聲精密高速運算放大器
文件頁數(shù): 13/20頁
文件大小: 439K
代理商: LT1128
13
LT1028/LT1128
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
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
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
F
is large enough to effectively
short the output to the input at 15MHz, oscillations can
occur. The insertion of R
S2
500
will prevent the
LT1028 from oscillating. When R
S1
500
, the additional
noise contribution due to the presence of R
S2
will be
minimal. When R
S1
100
, R
S2
is not necessary, be-
cause R
S1
represents a heavy load on the output through
the C
F
short. When 100
< R
S1
< 500
, R
S2
should match
R
S1
. For example, R
S1
= R
S2
= 300
will be stable. The
noise increase due to R
S2
is 40%.
U
S
A
O
PPLICATI
U
U
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.
1028/1128 AI09
+
33pF
2k
LT1028
50
+
LT1028
50
500
1028/1128 AI10
C1
R1
R
S1
R
S2
LT1028
+
Unity-Gain Buffer Applications (LT1128 Only)
When R
F
100
and the input is driven with a fast, large-
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
F
500
, the output is capable of handling the current
requirements (I
L
20mA at 10V) and the amplifier stays
in its active mode and a smooth transition will occur.
As with all operational amplifiers when R
F
> 2k, a pole will
be created with R
F
and the amplifier’s input capacitance,
creating additional phase shift and reducing the phase
margin. A small capacitor (20pF to 50pF) in parallel with R
F
will eliminate this problem.
Test Circuit for Offset Voltage
and Offset Voltage Drift with Temperature
+
–15V
10k*
200
*
LT1028
1028/1128 AI08
10k*
V
= 100V
* RESISTORS MUST HAVE LOW
THERMOELECTRIC POTENTIAL
V
O
6
7
2
4
3
15V
+
R
F
1028/1128 AI07
OUTPUT
6V/
μ
s
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