THS1009

SLAS287A – AUGUST 2000 – REVISED DECEMBER 2002

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27

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.

_

+

5 V

R1

R1

RS

3.5 V

2.5 V

1.5 V

THS1009

AINP

REFOUT

R2

R2

1.25 V

1 V

0 V

–1 V

REFIN

+

C

10

µF

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.

THS1009

AINP

AINM

REFOUT

C

C

R

R

200

Ω

49.9

Ω

Mini Circuits

T4–1

+

10

µF

Figure 36. Transformer Coupled Input