LT1114 datasheet(11/16 Pages) LINEAR

11 / 16 page

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 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.

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

–

1/2 LT1112

1/4 LT1114

1/2 LT1112

1/4 LT1114

IN–

IN+

10k

2.1k

200

Ω

10k

100

Ω

0.5%

100

Ω

0.5%

33pF

R10

10k

0.5%

9.88k

0.5%

200

Ω

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 = 35

µV

Offset voltage drift = 0.3

µV/°C

Input bias current = 80pA

Input offset current = 100pA

Input resistance = 800G

Ω

Input noise = 0.42

µVP-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 1M

Ω 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 1M

Ω 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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