10

LT1112/LT1114

S

APPLICATI

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

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:

then

∆CMRR = 0.25µV/V or 132dB;

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 = 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

Three Op Amp Instrumentation Amplifier

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 1M

Ωsource

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

ground pin adjacent to it, resulting in a 0.25pF input

capacitance mismatch. At 100Hz, 0.25pF represents a

6.4

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.

–

+

1/2 LT1112

OR

1/4 LT1114

A

1/2 LT1112

OR

1/4 LT1114

D

R1

10k

1%

R3

2.1k

1%

R8

200

Ω

R2

10k

1%

R4

100

Ω

0.5%

R5

100

Ω

0.5%

C1

33pF

R10

1M

R6

10k

0.5%

R7

9.88k

0.5%

R9

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