THS1207 Datasheet, PDF (28/33 Pages) Texas Instruments – 12 bit 4 ANALOG INPUT 6 MSPS SIMULTANEOUS SAMPLING

THS1207
SLAS284A – AUGUST 2000 – REVISED DECEMBER 2002
www.ti.com
SINGLE-ENDED MODE OF OPERATION
The THS1207 can be configured for single-ended operation using dc or ac-coupling. In either case, the input
of the THS1207 must be driven from an operational amplifier that does not degrade the ADC performance.
Because the THS1207 operates from a single supply 5 V, 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 THS1207. The analog input voltage range of the THS1207 is between 1.5 V and 3.5 V. An operational
amplifier can be used as shown in Figure 38.
Figure 38 shows an example where the analog input signal in the range between –1 V up to 1 V. This signal
is shifted by an operational amplifier to the analog input range of the THS1207 (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 THS1207 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 THS1207 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
THS1207
AINP
REFIN
REFOUT
+
R2
10 µF
R2
Figure 38. 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
THS1207
AINP
200 Ω ^C
R
AINM
C
+
10 µF
REFOUT
Figure 39. Transformer Coupled Input
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