THS1206 Datasheet, PDF (30/41 Pages) Texas Instruments – 12-BIT 6 MSPS, SIMULTANEOUS SAMPLING ANALOG-TO-DIGITAL CONVERTERS

THS1206
12-BIT 6 MSPS, SIMULTANEOUS SAMPLING
ANALOG-TO-DIGITAL CONVERTERS
SLAS217D – MAY 1999 – REVISED APRIL 2000
analog input configuration and reference voltage (continued)
A differential operation is desired for many applications. Figure 18 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 18. 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 20.
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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