THS1206IDAR Datasheet, PDF (35/43 Pages) Texas Instruments – 12-BIT, 4 ANALOG INPUT, 6 MSPS, SIMULTANEOUS SAMPLINGANALOG TO-DIGITAL CONVERTERS

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THS1206
SLAS217H – MAY 1999 – REVISED JULY 2003#
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.
AINP
AINM
VREFP
+
Σ ^VADC
–
12-Bit
ADC
VREFM
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:
+ VADC ABS(AINP–AINM)
(2)
An advantage to single-ended operation is that the common-mode voltage
+ ⁾ VCM
AINM AINP
2
(3)
can be rejected in the differential configuration, if the following condition for the analog input voltages is true:
v v AGND AINM, AINP AVDD
(4)
v v 1 V VCM 4 V
(5)
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.
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