–2–

AN-732

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

RR

RR

error

VD

R

RR

error

OUT

=

−

+

+









×

















+

+

×





































7

2

1

22 7

27

2

7

27

VV

V

DD

IN

IN

=−

21

follows:

error

{1 – [(R2+2R7/R2+R7)

error/2]}

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 performance

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 se-

lected, 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 very

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. Current can be converted to a voltage

with a resistor and then amplified or injected directly

into a summing node.

R7

R6

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.

R7

R9

R4

C9

V–

V+

Figure 4. Bistable Multivibrator

L+

L–

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