AD7840
PIN CONFIGURATIONS
DIP/SSOP
PLCC
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.
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 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
REV. B
–5–
Figure 3. AD586 Driving AD7840 REF IN
AD [ ANALOG DEVICES ]
