THS1206

SLAS217H – MAY 1999 – REVISED JULY 2003#

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35

A differential operation is desired for many applications. Figure 41 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 41. Differential Input Stage

In comparison to the single-ended configuration it can be seen that the voltage, VADC, which is applied at the input of the

ADC is the difference between the input AINP and AINM. This means that VREFM defines the minimum voltage (VADC)

which can be applied to the ADC. VREFP defines the maximum voltage (VADC), which can be applied to the ADC. The

voltage VADC can be calculated as follows:

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 45.

SINGLE-ENDED MODE OF OPERATION

The THS1206 can be configured for single-ended operation using dc- or ac-coupling. In either case, the input of the

THS1206 must be driven from an operational amplifier that does not degrade the ADC performance. Because the THS1206

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)