LT1125 Linear Technology Corporation, LT1125 Datasheet - Page 9

• Manufacturer Part Number: LT1125
• Description: Dual/quad Low Noise, High Speed Precision Op Amps
• Manufacturer: Linear Technology Corporation

High Speed Operation
When the feedback around the op amp is resistive (R
a pole will be created with R
capacitance (R
(C
with R
excess phase shift and even oscillation. A small capacitor
(C
Figure 2). With R
feedback pole is completely removed.
A
Unity Gain Buffer Applications
When R
signal pulse (>1V), the output waveform will look as
shown in Figure 3.
IN
F
PPLICATI
) in parallel with R
≈ 2pF). In low closed loop gain configurations and
S
F
and R
–
+
≤ 100Ω and the input is driven with a fast, large
R
Figure 3. Unity-Gain Buffer Applications
S
F
R
S
Figure 2. High Speed Operation
F
in the kilohm range, this pole can create
, C
O
C
S
S
S
), and the amplifier input capacitance
U
(C
S
S
F
+ C
–
+
I FOR ATIO
eliminates this problem (see
U
OUTPUT
IN
F
C
, the source resistance and
C
R
) = R
IN
F
F
F
C
W
F
, the effect of the
OUTPUT
1124/25 F02
4.5V/µs
1124/25 F03
U
F
),
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
≥500Ω, the output is capable of handling the current
requirements (I
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
If the LT1125 were tested this way, the noise limit would
be
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 =
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.
n OUT
√
Figure 4. Competing Quad Op Amp Noise Test Method
4 • (4.2nV/√Hz)
+
–
=
A
√
(e
√
nA
(2.7)
)
2
L
+ (e
≤ 20mA at 10V) and the amplifier stays
–
+
2
B
+ (2.7)
2
nB
= 8.4nV/√Hz. But is this an effective
)
2
+ (e
2
+ (2.7)
–
+
nC
C
)
LT1124/LT1125
2
2
+ (e
+ (6.9)
nD
–
+
)
2
D
2
= 8.33nV/√Hz
1124/25 F04
OUT
11245fb
9
F