ADS803

9

SBAS074B

www.ti.com

V

IN

IN

IN

CM

22

Ω

22

Ω

100pF

R

T

100pF

+

4.7

µF

0.1

µF

ADS803

1:n

0.1

µF

R

G

REFERENCE OPERATION

Integrated into the ADS803 is a bandgap reference circuit

including logic that provides either a +1V or +2.5V reference

output by simply selecting the corresponding pin-strap con-

figuration. Different reference voltages can be generated by

the use of two external resistors, which will set a different

gain for the internal reference buffer. For more design flexibil-

ity, the internal reference can be shut off and an external

reference voltage used. Table I provides an overview of the

possible reference options and pin configurations.

FIGURE 3. Transformer-Coupled Input

INPUT

FULL-SCALE

REQUIRED

MODE

RANGE

CONNECT

TO

Internal

2Vp-p

+1V

SEL

Internal

5Vp-p

+2.5V

SEL

GND

Internal

2V

≤ FSR < 5V

SEL and GND

External

1V < FSR < 5V

SEL

TABLE I. Selected Reference Configuration Examples.

A simple model of the internal reference circuit is shown in

Figure 4. The internal blocks are a 1V-bandgap voltage

reference, buffer, the resistive reference ladder, and the

drivers for the top and bottom reference that supply the

necessary current to the internal nodes. As shown, the

input span of the ADS803 is determined by the voltage at

(1)

Note that the current drive capability of this amplifier is limited to

approximately 1mA and should not be used to drive low loads.

The programmable reference circuit is controlled by the voltage

applied to the select pin (SEL). Refer to Table I for an overview.

FIGURE 4. Equivalent Reference Circuit.

800

Ω

800

Ω

REFB

CM

Reference

Driver

Bandgap

and Logic

REFT

V

REF

SEL

1V

DC

Resistor Network

and Switches

Disable

Switch

to A/D

to A/D

ADS803