THS1209_14 Datasheet, PDF (29/34 Pages) Texas Instruments – 12-BIT, 2 ANALOG INPUT, 8 MSPS, SIMULTANEOUS SAMPLING ANALOG-TO-DIGITAL CONVERTER

www.ti.com
THS1209
SLAS288B – JULY 2000 – REVISED DECEMBER 2002
DC COUPLING
An operational amplifier can be configured to shift the signal level according to the analog input voltage range of the
THS1209. The analog input voltage range of the THS1209 goes from 1.5 V to 3.5 V. An op-amp can be used as
shown in Figure 35.
Figure 35 shows an example where the analog input signal in the range from –1 V up to 1 V is shifted by an
operational amplifier to the analog input range of the THS1209 (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 THS1209 by using a resistor divider. Therefore, the op-amp output voltage
is centered at 2.5 V. The 10 µF tantalum capacitor is required for bypassing REFOUT. REFIN of the THS1209 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
THS1209
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
T4–1
49.9 Ω
R
THS1209
AINP
200 Ω ^C
R
AINM
C
+
10 µF
REFOUT
Figure 36. Transformer Coupled Input
29