THS1209_14 Datasheet, PDF (28/34 Pages) Texas Instruments – 12-BIT, 2 ANALOG INPUT, 8 MSPS, SIMULTANEOUS SAMPLING ANALOG-TO-DIGITAL CONVERTER

THS1209
SLAS288B – JULY 2000 – REVISED DECEMBER 2002
www.ti.com
ANALOG INPUT CONFIGURATION AND REFERENCE VOLTAGE
The THS1209 features two analog input channels. These can be configured for either single-ended or differential
operation. Figure 33 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 goes from 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. The resulting analog input voltage swing of 2 V can be expressed
by:
VREFM v AINP v VREFP
(1)
VREFP
AINP
12-Bit
ADC
VREFM
Figure 33. Single-Ended Input Stage
A differential operation is desired for many applications due to a better signal-to-noise ratio. Figure 34 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 THS1209 offers 1 differential analog input and in the single-ended mode
2 analog inputs. If the analog input architecture id differential, common-mode noise and common-mode voltages can
be rejected. Additional details for both modes are given below.
AINP
AINM
VREFP
+
Σ ^VADC
–
12-Bit
ADC
VREFM
Figure 34. 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 THS1209 can be configured for single-ended operation using dc or ac coupling. In every case, the input of the
THS1209 should be driven from an operational amplifier that does not degrade the ADC performance. Because the
THS1209 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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