THS1007_14 Datasheet, PDF (28/34 Pages) Texas Instruments – 10-BIT, 4 ANALOG INPUT, 6-MSPS, SIMULTANEOUS SAMPLING ANALOG-TO-DIGITAL CONVERTER

THS1007
SLAS286B − AUGUST 2000− REVISED DECEMBER 2010
www.ti.com
ANALOG INPUT CONFIGURATION AND REFERENCE VOLTAGE
The THS1007 features four analog input channels. These can be configured for either single-ended or
differential operation. Figure 36 shows a simplified model, where a single-ended configuration for channel AINP
is selected. The reference voltages for the ADC itself are V_REFP and V_REFM (either internal or external reference
voltage). The analog input voltage range is between V_REFM to V_REFP. This means that V_REFM defines the
minimum voltage, and V_REFP defines the maximum voltage, which can be applied to the ADC. The internal
reference source provides the voltage V_REFM of 1.5 V and the voltage V_REFP of 3.5 V (see also section
reference voltage). The resulting analog input voltage swing of 2 V can be expressed by:
VREFM v AINP v VREFP
(1)
V_REFP
AINP
10-Bit
ADC
V_REFM
Figure 36. Single-Ended Input Stage
A differential operation is desired in many applications due to a better signal-to-noise ration. Figure 37 shows
a simplified model for the analog inputs AINM and AINP, which are configured for differential operation. The
differential operation mode provides in terms of performance benefits over the single-ended mode and is
therefore recommended for best performance. The THS1007 offers 2 differential analog inputs and in the
single-ended mode4 analog inputs. If the analog input architecture is different, common-mode voltages can be
rejected. Additional details for both modes are given below.
AINP
AINM
+
Σ ^VADC
−
V_REFP
10-Bit
ADC
V_REFM
Figure 37. 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. 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
)
2
AINP
(3)
can be rejected in the differential configuration, if the following condition for the analog input voltages is true:
AGND v AINM, AINP v AVDD
(4)
1 V v VCM v 4 V
(5)
SINGLE-ENDED MODE OF OPERATION
The THS1007 can be configured for single-ended operation using dc or ac coupling. In either case, the input
of the THS1007 must be driven from an operational amplifier that does not degrade the ADC performance.
Because the THS1007 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.
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