AD7840

REV. B

–5–

PIN CONFIGURATIONS

DIP/SSOP

PLCC

for external use, it should he decoupled to AGND with a 200

Ω

resistor in series with a parallel combination of a 10

µF tantalum

capacitor and a 0.1

µF ceramic capacitor.

Figure 2. Internal Reference

EXTERNAL REFERENCE

In some applications, the user may require a system reference or

some other external reference to drive the AD7840 reference in-

put. Figure 3 shows how the AD586 5 V reference can be con-

ditioned to provide the 3 V reference required by the AD7840

REF IN. An alternate source of reference voltage for the

AD7840 in systems which use both a DAC and an ADC is to

use the REF OUT voltage of ADCs such as the AD7870 and

AD7871. A circuit showing this arrangement is shown in

Figure 20.

Figure 3. AD586 Driving AD7840 REF IN

D/A SECTION

The AD7840 contains a 14-bit voltage mode D/A converter

consisting of highly stable thin film resistors and high speed

NMOS single-pole, double-throw switches. The simplified cir-

cuit diagram for the DAC section is shown in Figure 1. The

three MSBs of the data word are decoded to drive the seven

switches A–G. The 11 LSBs switch an 11-bit R-2R ladder struc-

ture. The output voltage from this converter has the same polar-

ity as the reference voltage, REF IN.

The REF IN voltage is internally buffered by a unity gain ampli-

fier before being applied to the D/A converter and the bipolar

bias circuitry. The D/A converter is configured and sealed for a

3 V reference and the device is tested with 3 V applied to REF

IN. Operating the AD7840 at reference voltages outside the

±5% tolerance range may result in degraded performance from

the part.

Figure 1. DAC Ladder Structure

INTERNAL REFERENCE

The AD7840 has an on-chip temperature compensated buried

Zener reference (see Figure 2) which is factory trimmed to 3 V

±10 mV. The reference voltage is provided at the REF OUT

pin. This reference can be used to provide both the reference

voltage for the D/A converter and the bipolar bias circuitry. This

is achieved by connecting the REF OUT pin to the REF IN pin

of the device.

The reference voltage can also be used as a reference for other

components and is capable of providing up to 500

µA to an ex-

ternal load. The maximum recommended capacitance on REF

OUT for normal operation is 50 pF. If the reference is required