Datasheet

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
V
REFP
V
REFM
AINP
Σ
V
ADC
AINM
+
–
Figure 41. Differential Input Stage
In comparison to the single-ended configuration it can be seen that the voltage, V
ADC
, which is applied at the input of the
ADC is the difference between the input AINP and AINM. This means that V
REFM
defines the minimum voltage (V
ADC
)
which can be applied to the ADC. V
REFP
defines the maximum voltage (VADC), which can be applied to the ADC. The
voltage V
ADC
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 AINM, AINP AV
DD
1V V
CM
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)