ADS803

7

SBAS074B

www.ti.com

800k

600k

400k

200k

0

OUTPUT NOISE HISTOGRAM

(DC Input, V

N – 2N – 1

N

N + 1

N + 2

Code

800k

600k

400k

200k

0

OUTPUT NOISE HISTOGRAM

(DC Input, V

N – 2N – 1

N

N + 1

N + 2

Code

APPLICATION INFORMATION

DRIVING THE ANALOG INPUT

The ADS803 allows its analog inputs to be driven either

single-ended or differentially. The focus of the following

discussion is on the single-ended configuration. Typically, its

implementation is easier to achieve and the rated specifica-

tions for the ADS803 are characterized using the single-

ended mode of operation.

AC-COUPLED INPUT CONFIGURATION

Given in Figure 1 is the circuit example of the most common

connected to the SEL pin, the full-scale input range is defined

to be 2Vp-p. This signal is ac-coupled in single-ended form

to the ADS803 using the low-distortion voltage-feedback

amplifier OPA642. As is generally necessary for single-

supply components, operating the ADS803 with a full-scale

input signal swing requires a level-shift of the amplifier’s

zero-centered analog signal to comply with the A/D converter’s

input range requirements. Using a DC blocking capacitor

between the output of the driving amplifier and the converter’s

input, a simple level-shifting scheme can be implemented. In

this configuration, the top and bottom references (REFT and

REFB) provide an output voltage of +3V and +2V, respec-

tively. Here, two resistor pairs (2 • 2k

Ω) are used to create a

common-mode voltage of approximately +2.5V to bias the

inputs of the ADS803 (IN, IN) to the required DC voltage.

FIGURE 1. AC-Coupled Input Configuration for 2Vp-p Input Swing and Common-Mode Voltage at +2.5V Derived from Internal

Top and Bottom Reference.

OPA642

V

IN

+V

IN

0V

–V

IN

R

F

402

Ω

R

G

402

Ω

ADS803

R

S

24.9

Ω

2k

Ω

2k

Ω

2k

Ω

2k

Ω

100pF

0.1

µF

0.1

µF

2Vp-p

IN

IN

(+2V)

REFB

(+1V)

V

REF

SEL

REFT

(+3V)

+5V –5V

+2.5V

DC