AD8139

Rev. A | Page 20 of 24

Using the β notation, Equation 15 can be written as

( )

ICM

OCM

ACM

V

V

V

β

−

+

β

=

1

(16)

or equivalently,

(

)

ICM

OCM

ICM

ACM

V

V

V

V

−

β

+

=

(17)

2

IN

IP

ICM

V

V

V

+

=

.

within their respective linear ranges.

Calculating Input Impedance

The input impedance of the circuit in Figure 57 will depend on

whether the amplifier is being driven by a single-ended or a

differential signal source. For balanced differential input signals,

G

dm

IN,

R

R

2

=

(18)

)

(

2

1

F

G

F

G

IN

R

R

R

R

R

+

−

=

(19)

The input impedance of a conventional inverting op amp

because a fraction of the differential output voltage appears at

the increased input resistance.

Input Common-Mode Swing Considerations

In some single-ended-to-differential applications, when using a

single-supply voltage attention must be paid to the swing of the

DGND AGND REFGND REF REFBUFIN PDBUF

ADR431

2.5V

REFERENCE

47

µF

AD7674

IN+

IN–

AVDD

DVDD

AD8139

+

–

8

2.5V

2

1

6

3

4

5

200

Ω

200

Ω

324

Ω

324

Ω

5V

20

Ω

15

Ω

15

Ω

2.7nF

2.7nF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

+1.7V

+0.95V

+0.2V

+2.5V

GND

–2.5V

Figure 59. AD8139 Driving AD7674, 18-Bit, 800 kSPS A/D Converter