ADS800

12

SBAS035B

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using two operational amplifiers, one in the noninverting mode

for the input and the other amplifier in the inverting mode for the

complementary input. The low distortion circuit in Figure 7 will

provide the necessary input shifting required for signals cen-

tered around ground. It also employs a diode for output level

shifting to ensure a low distortion +3.25V output swing. Other

amplifiers can be used in place of the OPA842s if the lowest

distortion is not necessary. If output level shifting circuits are

not used, care must be taken to select operational amplifiers

that give the necessary performance when swinging to +3.25V

with a

±5V supply operational amplifier.

The ADS800 can also be configured with a single-ended

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

complementary input to the common-mode reference volt-

age, as shown in Figure 8. This configuration will result in

increased even-order harmonics, especially at higher input

frequencies. However, this tradeoff may be quite acceptable

for time-domain applications. The driving amplifier must give

adequate performance 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

FIGURE 8. Single-Ended Input Connection.

22

26

27

CM

IN

IN

ADS800

0.1µF

Single-Ended

Input Signal

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

22pF

FIGURE 7. A Low Distortion DC-Coupled, Single-Ended to Differential Input Driver Circuit.

604

Ω

301

Ω

301

Ω

301

Ω

604

Ω

49.9

Ω

301

Ω

604

Ω

2.49k

Ω

2.49k

Ω +2.25V

OPA842

OPA130

301

Ω

0.1

µF

OPA842

OPA842

+5V

–5V

+5V(2)

+5V

–5V

+5V

+5V

+5V

–5V

BAS16(1)

BAS16(1)

301

Ω

24.9

Ω

Input Level

Shift Buffer

Optional

High Impedance

Input Amplifier

DC-Coupled

Input Signal

26 IN

22 CM

27 IN

ADS800

NOTES: (1) A Philips BAS16 diode or equivalent

may be used. (2) Supply bypassing not shown.

22pF

22pF

604

Ω

0.1

µF

0.1

µF