MT-075 datasheet(4/9 Pages) AD

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MT-075

In the case of a single-ended input signal (for example, if VIN– is grounded, and the input signal

is applied to VIN+), the input impedance becomes:

() ⎟

⎟

⎠

⎞

⎜

⎝

⎛

−

sem

Eq. 6

The circuit's single-ended input impedance is effectively higher than it would be for a

conventional op amp connected as an inverter, because a fraction of the differential output

voltage appears at the inputs as a common-mode signal, partially bootstrapping the voltage

across the input resistor RG.

Figure 3 shows some of the possible configurations for the AD813x differential amplifier. Figure

3A is the standard configuration which utilizes two feedback networks, characterized by

feedback factors β1 and β2, respectively. Note that each feedback factor can vary anywhere

between 0 and 1.

RG1 + RF1

–

RF1

RG1

RF2

RG2

(A)

β1 =

G =

2 (1 –

β1)

β1 + β2

RG1

RG2 + RF2

β2 =

RG2

–

RF2

RG2

–

RF1

RG1

(B)

(D)

VOCM

–

RF1

RG1

(C)

(E)

VOCM

β1 = 0

β2 = 1

β1 = 0

β2 = 1

β2 = 0

VOCM

V+

V–

VOUT–

VOUT+

VOUT–

VOUT+

VOUT–

VOUT+

VOUT–

VOUT+

VOUT–

VOUT+

Figure 3: Some Configurations for Differential Amplifiers

Figure 3B shows a configuration where there is no feedback from VOUT– to V+, i.e., β1 = 0. In

this case, β2 determines the amount of VOUT+ that is fed back to V–, and the circuit is similar to a

non-inverting op amp configuration, except for the presence of the additional complementary

output. Therefore, the overall gain is twice that of a non-inverting op amp, or 2

× (1 + RF2/RG2),

or 2

× (1/β2).

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