10

LT1002

Advantages of Matched Dual Op Amps In many applica-

tions the performance of a system depends on the match-

ing between two operational amplifiers rather than the

individual characteristics of the two op amps. Two or three

op amp instrumentation amplifiers, tracking voltage refer-

ences 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 LT1002.

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 non-inverting input currents

the offset current of the instrumentation amplifier. The

cause errors flowing through R1, R2, and R3. Common-

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 LT1002, by specifying and guaranteeing all of

these matching parameters, can significantly improve the

performance of matching dependent circuits.

1002 TA07

R1

10k

1%

R4

100

Ω

1%

R6

10k

1%

R5

100

Ω

1%

INPUT

OUTPUT

–

+

Gain = 1000

–

+

A

–

+

B

1

LT1037

–

R2

10k

1%

R7

R9

9.76k

1%

R3

R8

2.1k

1%

C1

100pF

R10

100k

200

Ω

200

Ω

Three Op Amp Instrumentation Amplifier

Trim R8 for gain

Trim R9 for DC common mode rejection

Trim R10 for AC common mode rejection

Typical performance of the instrumentation amplifier:

Input offset voltage = 25

µV

Input bias current = 0.7nA

Input resistance = 200 G

Ω

Input offset current = 0.6nA

Input noise = 0.5

µV p-p