10

®

ADS804

In addition, the common-mode voltage (CMV) may be used

as a reference level to provide the appropriate offset for the

driving circuitry. However, care must be taken not to appre-

ciably load this node, which is not buffered and has a high

impedance. An alternate method of generating a common-

mode voltage is given in Figure 6. Here, two external

precision resistors (tolerance 1% or better) are located be-

tween the top and bottom reference pins. The common-

mode level will appear at the midpoint. The output buffers

of the top and bottom reference are designed to supply

approximately 2mA of output current.

ADS804

CM

REFB

0.1µF

0.1µF

VREF

0.1µF

10µF

+

0.1µF

REFT

0.1µF

10µF

+

FIGURE 5. Recommended Reference Bypassing Scheme.

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 which supply the

necessary current to the internal nodes. As shown, the output

according to the equation (1):

(1)

Note that the current drive capability of this amplifier is

limited to about 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.

The top reference (REFT) and the bottom reference (REFB)

are brought out mainly for external bypassing. For proper

operation with all reference configurations, it is necessary to

provide solid bypassing to the reference pins in order to keep

the clock feedthrough to a minimum. Figure 5 shows the

recommended decoupling network.

FIGURE 6. Alternative Circuit to Generate Common-Mode

Voltage.

FIGURE 4. Equivalent Reference Circuit.

IN

IN

REFT

R

1

CMV

R

2

0.1µF

0.1µF

ADS804

REFB

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

ADS804