LT1124/LT1125
APPLICATI
S I FOR ATIO
High Speed Operation
When the feedback around the op amp is resistive (R
F
),
a pole will be created with R
F
, the source resistance and
capacitance (R
S
, C
S
), and the amplifier input capacitance
(C
IN
≈
2pF). In low closed loop gain configurations and
with R
S
and R
F
in the kilohm range, this pole can create
excess phase shift and even oscillation. A small capacitor
(C
F
) in parallel with R
F
eliminates this problem (see
Figure 2). With R
S
(C
S
+ C
IN
) = R
F
C
F
, the effect of the
feedback pole is completely removed.
C
F
R
F
–
R
S
C
S
C
IN
OUTPUT
+
1124/25 F02
Figure 2. High Speed Operation
Unity Gain Buffer Applications
When R
F
≤
100Ω and the input is driven with a fast, large
signal pulse (>1V), the output waveform will look as
shown in Figure 3.
R
F
–
+
OUTPUT
4.5V/µs
1124/25 F03
Figure 3. Unity-Gain Buffer Applications
U
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
≥500Ω,
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.
Noise Testing
Each individual amplifier is tested to 4.2nV/√Hz voltage
noise; i.e., for the LT1124 two tests, for the LT1125 four
tests are performed. Noise testing for competing multiple
op amps, if done at all, may be sample tested or tested
using the circuit shown in Figure 4.
e
n OUT
=
√
(e
nA
)
2
+ (e
nB
)
2
+ (e
nC
)
2
+ (e
nD
)
2
If the LT1125 were tested this way, the noise limit would
be
√
4 • (4.2nV/√Hz)
2
= 8.4nV/√Hz. But is this an effective
screen? What if three of the four amplifiers are at a typical
2.7nV/√Hz, and the fourth one was contaminated and has
6.9nV/√Hz noise?
RMS Sum =
√
(2.7)
2
+ (2.7)
2
+ (2.7)
2
+ (6.9)
2
= 8.33nV/√Hz
This passes an 8.4nV/√Hz spec, yet one of the amplifiers
is 64% over the LT1125 spec limit. Clearly, for proper
noise measurement, the op amps have to be tested
individually.
–
+
A
W
U
UO
–
+
B
–
+
C
–
+
D
OUT
1124/25 F04
Figure 4. Competing Quad Op Amp Noise Test Method
9
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