THS1009 Datasheet, PDF (27/31 Pages) Texas Instruments – 10-BIT, 2 ANALOG INPUT, 8 MSPS, SIMULTANEOUS SAMPLING ANALOG-TO-DIGITAL CONVERTER

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THS1009
SLAS287A – AUGUST 2000 – REVISED DECEMBER 2002
SINGLE-ENDED MODE OF OPERATION
The THS1009 can be configured for single-ended operation using dc or ac coupling. In either case, the input of the
THS1009 should be driven from an operational amplifier that does not degrade the ADC performance. Because the
THS1009 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.
DC COUPLING
An operational amplifier can be configured to shift the signal level according to the analog input voltage range of the
THS1009. The analog input voltage range of the THS1009 goes from 1.5 V to 3.5 V. An operational amplifier can
be used as shown in Figure 35.
Figure 35 shows an example with the analog input signal in the range between –1 V and 1 V. The signal is shifted
by an operational amplifier to the analog input range of the THS1009 (1.5 V to 3.5 V). The operational amplifier is
configured as an inverting amplifier with a gain of –1. The required dc voltage of 1.25 V at the noninverting input is
derived from the 2.5-V output reference REFOUT of the THS1009 by using a resistor divider. Therefore, the
operational amplifier output voltage is centered at 2.5 V. The 10 µF tantalum capacitor is required for bypassing
REFOUT. REFIN of the THS1009 must be connected directly to REFOUT in single-ended mode. The use of ratio
matched, thin-film resistor networks minimizes gain and offset errors.
1V
0V
–1 V
R1
1.25 V
R1
5V
_
+
3.5 V
2.5 V
1.5 V
RS
C
THS1009
AINP
REFIN
REFOUT
+
R2
10 µF
R2
Figure 35. Level-Shift for DC-Coupled Input
DIFFERENTIAL MODE OF OPERATION
For the differential mode of operation, a conversion from single-ended to differential is required. A conversion to
differential signals can be achieved by using an RF-transformer, which provides a center tap. Best performance is
achieved in differential mode.
Mini Circuits
49.9 Ω ^T4–1
R
200 Ω ^C
R
C
THS1009
AINP
AINM
+
10 µF
REFOUT
Figure 36. Transformer Coupled Input
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