REV. A

OP221

–9–

SPECIAL NOTES ON THE APPLICATION OF DUAL

MATCHED OPERATIONAL AMPLIFIERS

Advantages of Dual Monolithic Operational Amplifiers

Dual matched operational amplifiers provide the engineer with a

powerful tool for designing instrumentation amplifiers and many

other differential-input circuits. These designs are based on the

principle that careful matching between two operational amplifiers

can minimize the effect of dc errors in the individual amplifiers.

Reference to the circuit shown in Figure 6, a differential-in,

differential-out amplifier, shows how the reductions in error can

be accomplished. Assuming the resistors used are ideally matched,

the gain of each side will be identical. If the offset voltages of

each amplifier are perfectly matched, then the net differential

voltage at the amplifier’s output will be zero. Note that the output

offset error of this amplifier is not a function of the offset voltage

of the individual amplifiers, but only a function of the difference

(degree of matching) between the amplifiers’ offset voltages. This

error-cancellation principle holds for a considerable number of

input referred error parameters—offset voltage, offset voltage

drift, inverting and noninverting bias currents, common mode

and power supply rejection ratios. Note also that the impedances

of each input, both common-mode and differential-mode, are

high and tightly matched, an important feature not practical with

single operation amplifier circuits.

INPUT

OUTPUT

SIDE

‘A’

SIDE

‘R’

R1

R2

R3

R4

OP221

+

–

+

–

Figure 6. Differential-In, Differential-Out Amplifier

INSTRUMENTATION AMPLIFIER APPLICATIONS

Two-Op Amp Configuration

The two-op amp circuit (Figure 7) is recommended where the

common-mode input voltage range is relatively limited; the

common-mode and differential voltage both appear at V1. The

high open-loop gain of the OP221 is very important in achieving

good CMRR in this configuration. Finite open-loop gain of A1

(Ao1) causes undesired feedthrough of the common-mode input.

For Ad/Ao, << 1, the common-mode error (CME) at the out-

put due to this effect is approximately (2 Ad/Ao1) x VCM. This

circuit features independent adjustment of CMRR and differ-

ential gain.

Three-Op Amp Configuration

The three-op amp circuit (Figure 8) has increased common-

mode voltage range because the common-mode voltage is not

amplified as it is in Figure 7. The CMR of this amplifier is directly

proportional to the match of the CMR of the input op amps. CMRR

can be raised even further by trimming the output stage resistors.

R1

R2

GAIN

ADJ

R0

V1

R3

A1

A2

1/2

OP221

1/2

OP221

R1

R0

R4

1 +

1

2

R2

R1

+

R2 + R3

R0

+

–

VCM

R3

R4

R4

R3

R4

R3

R3

R4

R2

R1

R1

R0

–

+

Figure 7. Two-Op Amp Circuit

A1

A2

R1

R2

R0

1/2

OP221

2R1

R0

A3

1/2

OP221

OP221

R2

R2

V–

–

–

V+

V+

V–

R2

R1

V2

V1

Figure 8. Three-Op Amp Circuit