9
LT1169
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
–
+
R2
OUTPUT
RB
CB
R1
CS
RS
CB CS
RB = RS
RS > R1 OR R2
TRANSDUCER
–
+
OUTPUT
CF
CB
RB
CB = CFCS
RB = RFRS
RF
CS
RS
TRANSDUCER
LT1169 F03
Q = CV;
dQ
dt
= I = C
dV
dt
capacitor CB, is used to cancel the phase shift caused by
the op amp input capacitance and RB.
Reduced Power Supply Operation
To take full advantage of a wide input common-mode
range, the LT1169 was designed to eliminate phase rever-
sal. Referring to the photographs in Figure 4, the LT1169
is shown operating in the follower mode (AV = 1) at ±5V
supplies with the input swinging
±5.2V. The output of the
LT1169 clips cleanly and recovers with no phase reversal,
unlike the competition as shown by the last photograph.
This has the benefit of preventing lockup in servo systems
and minimizing distortion components. The effect of input
and output overdrive on one amplifier has no effect on the
other, as each amplifier is biased independently.
Advantages of Matched Dual Op Amps
In many applications the performance of a system
depends on the matching between two operational ampli-
fiers rather than the individual characteristics of the two op
LT1169 Output
Input:
±5.2 Sine Wave
OPA2111 Output
LT1169 F04a
LT1169 F04b
LT1169 F04c
Figure 4. Voltage Follower with Input Exceeding the Common Mode Range (VS = ±5V)
Figure 3. Inverting and Noninverting Gain Configurations
amps. Two or three op amp instrumentation amplifiers,
tracking voltage references and low drift active filters
are some of the circuits requiring matching between two
op amps.
The well-known triple op amp configuration in Figure 5
illustrates these concepts. Output offset is a function of the
difference between the two halves of the LT1169. This error
cancellation principle holds for a considerable
number of input referred parameters in addition to
offset voltage and bias current. Input bias current will
be the average of the two noninverting input currents (IB+).
The difference between these two currents (
IB+)
is the offset current of the instrumentation amplifier. Com-
mon-mode and power supply rejections will be
dependent only on the match between the two amplifiers
(assuming perfect resistor matching).
The concepts of common mode and power supply
rejection ratio match (
CMRR and PSRR) are best dem-
onstrated with a numerical example: