MT-075

Eq. 4

~

R

G

V

OUT–

+

–

GAIN =

V

IN+

V

IN–

(B) EQUIVALENT CIRCUIT:

+

–

+

–

–

+

+

–

R

G

R

G

V

IN+

V

IN–

V

OUT+

V

OUT–

V+

V–

(A) FUNCTIONAL DIAGRAM

V

OCM

R

R

G

1 –

R

F

2 × (R

Figure 1: AD813x, AD493x Differential ADC Driver

Functional Diagram and Equivalent Circuit

The AD813x and ADA493x uses two feedback loops to separately control the differential and

common-mode output voltages. The differential feedback, set with external resistors, controls

only the differential output voltage. The common-mode feedback controls only the common-

mode output voltage. This architecture makes it easy to arbitrarily set the output common-mode

level in level shifting applications. It is forced, by internal common-mode feedback, to be equal

result is nearly perfectly balanced differential outputs of identical amplitude and exactly 180°

apart in phase over a wide frequency range. The circuit can be used with either a differential or a

The circuit can be analyzed using the assumptions and procedures summarized in Figure 2. As in

the case of op amp circuit dc analysis, one can first make the assumption that the currents into

the inverting and non-inverting input are zero (i.e., the input impedances are high relative to the

values of the feedback resistors). The second assumption is that feedback forces the non-

inverting and inverting input voltages to be equal. The third assumption is that the output

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