10
LT1002
1002fb
APPLICATIONS INFORMATION
WUU
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Advantages of Matched Dual Op Amps In many applica-
tions the performance of a system depends on the match-
ing between two operational amplifiers rather than the
individual characteristics of the two op amps. Two or three
op amp instrumentation amplifiers, tracking voltage refer-
ences and low drift active filters are some of the circuits
requiring matching between two op amps.
The well-known triple op amp configuration illustrates
these concepts. Output offset is a function of the differ-
ence between the offsets of the two halves of the LT1002.
This error cancellation principle holds for a considerable
number of input referred parameters in addition to offset
voltage and its drift with temperature. Input bias current
will be the average of the two non-inverting input currents
(I
B
+
). The difference between these two currents (I
OS
+
) is
the offset current of the instrumentation amplifier. The
difference between the inverting input currents (I
OS
–
) will
cause errors flowing through R1, R2, and R3. Common-
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 rejec-
tion ratio match (∆CMRR and ∆PSRR) are best demon-
strated with a numerical example:
Assume CMRR
A
= +1.0µV/V or 120dB,
and CMRR
B
= +0.75µV/V or 122.5dB,
then ∆CMRR = 0.25µV/V or 132dB;
if CMRR
B
= –0.75µV/V which is still 122.5dB,
then ∆CMRR = 1.75µV/V or 115dB.
Clearly, the LT1002, by specifying and guaranteeing all of
these matching parameters, can significantly improve the
performance of matching dependent circuits.
1002 TA07
R1
10k
1%
R4
100Ω
1%
R6
10k
1%
R5
100Ω
1%
INPUT
OUTPUT
–
+
GAIN = 1000
–
+
A
1/2
LT1002
–
+
B
1/2
LT1002
LT1037
–
INPUT
+
R2
10k
1%
R7
9.76k
1%
R3
2.1k
1%
C1
100pF
R10
100k
R8
200Ω
R9
200Ω
Three Op Amp Instrumentation Amplifier
Trim R8 for gain
Trim R9 for DC common mode rejection
Trim R10 for AC common mode rejection
Typical performance of the instrumentation amplifier:
Input offset voltage = 25µV
Input bias current = 0.7nA
Input resistance = 200 GΩ
Input offset current = 0.6nA
Input noise = 0.5µV p-p
Power bandwidth (V
0
= ±10V) = 80kHz
