–2–

AN-733

The algebraic sum of these two components should be

= R2 and R7 = R6, then:

Difference amplifiers are commonly used in high

accuracy circuits to improve the common-mode rejec-

tion ratio, typically known as CMRR.

For this type of application, CMRR depends upon how

tightly matched resistors are used; poorly matched resis-

tors result in a low value of CMRR.

To see how this works, consider a hypothetical source

of error for resistor R7 (1 – error). Using the superposi-

tion principle and letting R4 = R2 and R7 = R6, the output

voltage would be as follows:

V

R

R

R

R

R

R

error

VD

R

R

R

error

−

+

+









×

















+

+

×





































7

2

1

2 2 7

2

7

2

7

2

7

V

V

V

DD

IN

IN

=

−

2

1

follows:

These equations demonstrate that when there is not an

responds only to the differential voltage being applied to

its inputs; under these conditions, the CMRR of the circuit

becomes highly dependent on the CMRR of the amplifier

selected for this job.

As mentioned above, errors introduced by resistor

mismatch can be a big drawback of discrete differential

amplifiers, but there are different ways to optimize this

circuit configuration:

1. The differential gain is directly related to the ratio

R7/R2; therefore, one way to optimize the perfor-

mance of this circuit is to place the amplifier in a high

gain configuration. When larger values for resistors

R7 and R6 and smaller values for resistors R2 and R4

are selected, the higher the gain, the higher the CMRR.

For example, when R7 = R6 = 10 k, and R2 = R4 = 1 k,

and error = 0.1%, CMRR improves to better than 80 dB.

For high gain configuration, select amplifiers with

AD8571, AD8603, and AD8605) to reduce errors.

2. Select resistors that have much tighter tolerance and

accuracy. The more closely they are matched, the better

the CMRR. For example, if a CMRR of 90 dB is needed,

then match resistors to approximately 0.02%.

CURRENT-TO-VOLTAGE CONVERTER

Current may be measured in two ways with an opera-

tional amplifier: It can be converted to a voltage with a

resistor and then amplified, or injected directly into a

summing node.

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Figure 3. Current-to-Voltage Converter

Figure 3 is a typical representation of a current-to-voltage

transducer. The input current is fed directly into the sum-

ming node and the amplifier output voltage changes to

exactly the same current from the summing node through

R7. The scale factor of this circuit is R7 volts per amps.

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Figure 5. Output Response

GENERATION OF SQUARE WAVEFORMS USING A

BISTABLE MULTIVIBRATOR

A square waveform can be simply generated by arrang-

ing the amplifier for a bistable multivibrator to switch

states periodically as Figure 5 shows.

Once the output of the amplifier reaches one of two pos-

sible levels, such as L+, capacitor C9 charges toward this

level through resistor R7. The voltage across C9, which

is applied to the negative input terminal of the ampli-

fier denoted as V–, then rises exponentially toward L+

with a time constant  = C9R7. Meanwhile, the voltage

REV. A