THS12082

12-BIT, 8 MSPS, SIMULTANEOUS SAMPLING ANALOG-TO-DIGITAL CONVERTERS

SLAS271 – MAY 2000

28

POST OFFICE BOX 655303

analog input configuration and reference voltage (continued)

A differential operation is desired for many applications. Figure 16 shows a simplified model for the analog inputs

AINM and AINP, which are configured for differential operation. This configuration has a few advantages, which

are discussed in the following paragraphs.

12-Bit

ADC

VREFP

VREFM

AINP

Σ

VADC

AINM

+

–

Figure 16. Differential Input Stage

V

ADC +

ABS(AINP–AINM)

An advantage to single-ended operation is that the common-mode voltage

V

CM +

AINM

) AINP

2

can be rejected in the differential configuration, if the following condition for the analog input voltages is true:

AGND

v AINM, AINP v AV

DD

1V

v V

CM v

4V

In addition to the common-mode voltage rejection, the differential operation allows a dc-offset rejection, which

is common to both analog inputs. See also Figure 18.

single-ended mode of operation

The THS12082 can be configured for single-ended operation using dc or ac coupling. In either case, the input

of the THS12082 must be driven from an operational amplifier that does not degrade the ADC performance.

Because the THS12082 operates from a 5-V single supply, it is necessary to level-shift ground-based bipolar

signals to comply with its input requirements. This can be achieved with dc- and ac-coupling. An application

example is shown for dc-coupled level shifting in the following section, dc-coupling.

(2)

(3)

(4)

(5)