LT1126/LT1127
11267fa
7
–
+
OUTPUT
1126-7 AI01
R
S
C
S
R
F
C
F
C
IN
FREQUENCY (Hz)
GAIN ERROR (%)
0.01
0.1
1.0
0.1 10 100
1126-7 TA03
0.001
1
GAIN ERROR =
CLOSED LOOP GAIN
OPEN LOOP GAIN
TYPICAL
PRECISION
OP AMP
LT1126/LT1127
–
+
OUTPUT
1126-7 TA02
365Ω
1%
INPUT
15k
5%
340k
1%
20k
TRIM
+15V
–15V
RN60C FILM RESISTORS
THE HIGH GAIN AND WIDE BANDWIDTH OF THE LT1126/LT1127 IS USEFUL IN LOW FREQUENCY HIGH CLOSED
LOOP GAIN AMPLIFIER APPLICATIONS. A TYPICAL PRECISION OP AMP MAY HAVE AN OPEN LOOP GAIN OF ONE
MILLION WITH 500kHz BANDWIDTH. AS THE GAIN ERROR PLOT SHOWS, THIS DEVICE IS CAPABLE OF 0.1%
AMPLIFYING ACCURACY UP TO 0.3Hz ONLY. EVEN INSTRUMENTATION RANGE SIGNALS CAN VARY AT A FASTER
RATE. THE LT1126/LT1127 “GAIN PRECISION — BANDWIDTH PRODUCT” IS 330 TIMES HIGHER, AS SHOWN.
LT1126
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
Gain Error vs Frequency
Closed Loop Gain = 1000
U
SA
O
PP
L
IC
AT
ITY
P
I
CA
L
Gain 1000 Amplifier with 0.01% Accuracy, DC to 5Hz
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. With
R
S
(C
S
+ C
IN
) = R
F
C
F
, the effect of the feedback pole is
completely removed.
APPLICATIO S I FOR ATIO
WUU
U
