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ADS820

13

SBAS037B

FIGURE 8. Single-Ended Input Connection.

set halfway between the two references. This feature can be

used to adjust the gain error, improve gain drift, or to change

the full-scale input range of the ADS820. Changing the full-

scale range to a lower value has the benefit of easing the

swing requirements of external input amplifiers. The external

references can vary as long as the value of the external top

equal to +1.1V, and the difference between the external

references are greater than or equal to 800mV.

For the differential configuration, the full-scale input range

will be set to the external reference values that are

selected. For the single-ended mode, the input range is

performance curves for expected performance versus full-

scale input range.

The circuit in Figure 9 works completely on a single +5V

supply. As a reference element, it uses micro-power refer-

ence REF1004-2.5, which is set to a quiescent current of

+1.25V generated from the resistor divider. To provide the

a range of approximately 1 to 1.32. The pull-up resistor again

relieves the op amp from providing the full current drive. The

value of the pull-up and pull-down resistors is not critical and

can be varied to optimize power consumption. The need for

pull-up/down resistors depends only on the drive capability of

the selected drive amplifiers and thus can be omitted.

The ADS820 can also be configured with a single-ended input

full-scale range of +0.25V to +4.25V by tying the complemen-

tary input to the common-mode reference voltage, as shown

in Figure 8. This configuration will result in increased even-

order harmonics, especially at higher input frequencies. How-

ever, this trade-off may be quite acceptable for time-domain

applications. The driving amplifier must give adequate perfor-

mance with a +0.25V to +4.25V output swing in this case.

EXTERNAL REFERENCES AND

ADJUSTMENT OF FULL-SCALE RANGE

The internal reference buffers are limited to approximately

1mA of output current. As a result, these internal +1.25V and

+3.25V references may be overridden by external references

that have at least 18mA (at room temperature) of output drive

capability. In this instance, the common-mode voltage will be

22

26

27

CM

IN

IN

ADS820

0.1

µF

Single-Ended

Input Signal

Full Scale = +0.25V to +4.25V with internal references.

22pF

FIGURE 9. Optional External Reference to Set the Full-Scale Range Utilizing a Dual, Single-Supply Op Amp.

2k

Ω

+2.5V to +3.25V

+5V

+5V

R

P

220

Ω

R

P

220

Ω

10k

Ω

6.2k

Ω

0.1

µF

+2.5V

10k

Ω

1/2

OPA2234

1/2

OPA2234

A

1

A

2

Bottom

Reference

Top

Reference

REF1004

+1.25V

10k

Ω

10k

10k

NOTE: (1) Use parts alternatively for adjustment capability.