Datasheet

11
LT1112/LT1114
111214fb
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.
Three Op Amp Instrumentation Amplifier
APPLICATIO S I FOR ATIO
WUUU
–
+
–
+
1/2 LT1112
OR
1/4 LT1114
A
1/2 LT1112
OR
1/4 LT1114
D
IN
–
IN
+
R1
10k
1%
R3
2.1k
1%
R8
200Ω
R2
10k
1%
R4
100Ω
0.5%
R5
100Ω
0.5%
C1
33pF
R10
1M
R6
10k
0.5%
R7
9.88k
0.5%
R9
200Ω
OUTPUT
–
+
LT1097 OR
1/4LT1114
B OR C
GAIN = 1000
TRIM R8 FOR GAIN
TRIM R9 FOR DC
COMMON MODE REJECTION
TRIM R10 FOR AC
COMMON MODE REJECTION
LT1112/14 • AI02
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
When the instrumentation amplifier is used with high
impedance sources, the LT1114 is recommended because
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 • 10
9
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.