LT1112 Datasheet, PDF (10/12 Pages) Linear Technology – Dual/Quad Low Power Precision, Picoamp Input Op Amps

LT1112/LT1114
APPLICATI S I FOR ATIO
800mV of the supplies. The effect of input and output
overdrive on the other amplifiers in the LT1112 or
LT1114 packages is negligible, as each amplifier is
biased independently.
Advantages of Matched Dual and Quad Op Amps
In many applications the performance of a system de-
pends on the matching between two operational amplifi-
ers rather than the individual characteristics of the two op
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 illustrates
these concepts. Output offset is a function of the differ-
ence between the offsets of the two halves of the LT1112.
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 noninverting input currents
(I_B+). The difference between these two currents (∆I_B+) 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 rejec-
tion ratio match (∆CMRR and ∆PSRR) are best demon-
strated with a numerical example:
Assume CMRR_A = + 1µV/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 LT1112/LT1114, by specifying and guarantee-
ing all of these matching parameters, can significantly
improve the performance of matching-dependent
circuits.
Typical performance of the instrumentation amplifier:
Input offset voltage = 35µV
Offset voltage drift = 0.3µV/°C
Input bias current = 80pA
Input offset current = 100pA
Input resistance = 800GΩ
Input noise = 0.42µV_P-P
Three Op Amp Instrumentation Amplifier
IN– +
1/2 LT1112
OR
1/4 LT1114
–A
–
1/2 LT1112
OR
IN⁺
1+/4 LT1114
D
R4
100Ω
0.5%
R6
10k
0.5%
R1
10k
1%
R3
2.1k
1%
R8
200Ω
–
C1
33pF
R10 ^{LT1097 OR}
1M ₊^{1/B4LOTR11C14}
OUTPUT
R2 _R5
10k _100Ω
1% _0.5%
R7
9.88k
0.5%
R9
200Ω
GAIN = 1000
TRIM R8 FOR GAIN
TRIM R9 FOR DC
COMMON-MODE REJECTION
TRIM R10 FOR AC
COMMON-MODE REJECTION
LT1112/14 • AI02
When the instrumentation amplifier is used with high
impedance sources, the LT1114 is recommended be-
cause its CMRR vs frequency performance is better than
the LT1112’s. For example, with two matched 1MΩ source
resistors, CMRR at 100Hz is 100dB with the LT1114, 76dB
with the LT1112.
This difference is explained by the fact that capacitance
between adjacent pins on an IC package is about 0.25pF
(including package, socket and PC board trace capaci-
tances).
On the dual op amp package, positive input A is next to the
V ^– pin (AC ground), while positive input B has no AC
ground pin adjacent to it, resulting in a 0.25pF input
capacitance mismatch. At 100Hz, 0.25pF represents a
6.4 × 10⁹ input impedance mismatch, which is only 76dB
higher than the 1MΩ source resistors.
On the quad package, all four inputs are adjacent to a
power supply terminal–therefore, there is no mismatch.
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