LT1112/LT1114
W U U
APPLICATIO S I FOR ATIO
U
Three Op Amp Instrumentation Amplifier
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 = 0.42µVP-P
R4
100Ω
0.5%
R6
10k
–
IN
+
0.5%
1/2 LT1112
OR
1/4 LT1114
–
R1
10k
1%
A
–
R3
2.1k
1%
LT1097 OR
1/4LT1114
R10
1M
OUTPUT
When the instrumentation amplifier is used with high
impedancesources,theLT1114isrecommendedbecause
its CMRR vs frequency performance is better than the
LT1112’s. For example, with two matched 1MΩ source
resistors, CMRRat100Hzis100dBwiththeLT1114, 76dB
with the LT1112.
C1
33pF
+
B OR C
R8
200Ω
R2
10k
1%
R5
100Ω
0.5%
GAIN = 1000
–
TRIM R8 FOR GAIN
TRIM R9 FOR DC
COMMON MODE REJECTION
TRIM R10 FOR AC
COMMON MODE REJECTION
1/2 LT1112
OR
R7
9.88k
0.5%
1/4 LT1114
+
+
IN
D
This difference is explained by the fact that capacitance
betweenadjacentpinsonanICpackageisabout0.25pF
(including package, socket and PC board trace capaci-
tances).
R9
200Ω
LT1112/14 • AI02
The concepts of common mode and power supply rejec-
tion ratio match (∆CMRR and ∆PSRR) are best demon-
strated with a numerical example:
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.
Assume CMRRA = +1µV/V or 120dB,
and CMRRB = +0.75µV/V or 122.5dB,
then ∆CMRR = 0.25µV/V or 132dB;
if CMRRB = –0.75µV/V which is still 122.5dB,
then ∆CMRR = 1.75µV/V or 115dB.
On the quad package, all four inputs are adjacent to a
power supply terminal—therefore, there is no mismatch.
ClearlytheLT1112/LT1114,byspecifyingandguarantee-
ing all of these matching parameters, can significantly
improve the performance of matching-dependent
circuits.
111214fb
11
Linear Systems [ Linear Systems ]
