AD830

REV. A

–8–

TRADITIONAL DIFFERENTIAL AMPLIFICATION

In the past, when differential amplification was needed to reject

common-mode signals superimposed with a desired signal; most

often the solution used was the classic op amp based difference

simply achieved, but the overall performance is poor and the cir-

cuit possesses many serious problems that make it difficult to re-

alize a robust design with moderate to high levels of

performance.

Figure 19. Op Amp Based Difference Amplifier

PROBLEMS WITH THE OP AMP BASED APPROACH

• Low Common-Mode Rejection Ratio (CMRR)

• Low Impedance Inputs

• CMRR Highly Sensitive to the Value of Source R

• Different Input Impedance for the + and – Input

• Poor High Frequency CMRR

High CMRR

• Halves the Bandwidth of the Op Amp

• High Power Dissipation in the Resistors for Large Common-

Mode Voltage

AD830 FOR DIFFERENTIAL AMPLIFICATION

The AD830 amplifier was specifically developed to solve the

listed problems with the discrete difference amplifier approach.

Its topology, discussed in detail in a later section, by design acts

as a difference amplifier. The circuit of Figure 20 shows how

simply the AD830 is configured to produce the difference of two

exactly reproduced at the output relative to a separate output

common. Any common-mode voltage present at the input is

removed by the AD830.

A=1

V

→ I

V

→ I

Figure 20. AD830 as a Difference Amplifier

ADVANTAGEOUS PROPERTIES OF THE AD830

• High Common-Mode Rejection Ratio (CMRR)

• High Impedance Inputs

• Symmetrical Dynamic Response for +1 and –1 Gain

• Low Sensitivity to the Value of Source R

• Equal Input Impedance for the + and – Input

• Excellent High Frequency CMRR

• No Halving of the Bandwidth

• Constant Power Distortion vs. Common-Mode Voltage

• Highly Matched Resistors Not Needed