Datasheet

ADS803
7
SBAS074B
www.ti.com
TYPICAL CHARACTERISTICS (Cont.)
At T
A
= full specified temperature range, V
S
= +5V, specified input range = 1.5V to 3.5V, and single-ended input and sampling rate = 5MHz, unless otherwise specified.
800k
600k
400k
200k
0
OUTPUT NOISE HISTOGRAM
(DC Input, V
IN
= 2Vp-p)
Counts
N 2N 1 N N + 1 N + 2
Code
800k
600k
400k
200k
0
OUTPUT NOISE HISTOGRAM
(DC Input, V
IN
= 5Vp-p Range)
Counts
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
interface configuration for the ADS803. With the V
REF
pin
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 amplifiers
zero-centered analog signal to comply with the A/D converters
input range requirements. Using a DC blocking capacitor
between the output of the driving amplifier and the converters
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) 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
R
G
402
ADS803
R
S
24.9
2k2k
2k
2k
100pF
0.1µF
0.1µF2Vp-p
IN
IN
(+2V)
REFB
(+1V)
V
REF
SEL
REFT
(+3V)
+5V 5V
+2.5V
DC