AD8303

REV. 0

–12–

BIPOLAR OUTPUT OPERATION

Although the AD8303 has been designed for single-supply

operation, the output can also be configured for bipolar

operation. A typical circuit is shown in Figure 31. This circuit

uses the AD8303’s internal voltage reference to generate a

application, one half of an OP293 dual op amp is used as a

buffer. The other op amp then amplifies the DAC output

voltage to produce a bipolar output swing. The output voltage is

coded in offset binary and is given by:

VO = 0.5 mV × Digital Code ×

R4

R3

+ R4









× 1+

R2

R1









–1.0 V

×

R2

R1

where 0.5 mV represents the pretrimmed value for one LSB of

the AD8303, Digital Code is the digital code sent to the DAC,

and 1.0 V is the AD8303 reference voltage.

SDI

CLK

CS

LDA

LDB

RS

MSB

SHDN

AD8303

13

2

14

4

1

+3V

AGND DGND

OPTIONAL

FULL-SCALE

TRIM

3

+3V

–3V

1/2

OP293

OPTIONAL

ZERO TRIM

R4

19.08k

Ω

R1

10k

Ω

R2

20.48k

Ω

R3

10k

Ω

7

6

5

8

10

9

11

12

1/2

OP293

Figure 31. Bipolar Output Operation

For a

±2.048 V full scale using the circuit values shown, the

transfer function becomes:

VO = 1 mV × Digital Code – 2.048 V

Note that the full-scale span has increased from 2.048 V to

4.096 V (

±2.048 V). Therefore, although each AD8303 LSB

represents 0.5 mV, each output LSB of the bipolar circuit has

been scaled to 1 mV. The code table for this circuit is shown in

Table IV.

Table IV. Bipolar Code Table

Hexadecimal Number Decimal Number

Analog Output

in DAC Register

in DAC Register

Voltage (V)

FFF

4095

2.047

801

2049

0.001

800

2048

0

7FF

2047

–0.001

000

0

–2.048

As with the false-ground generator circuit, resistor matching is

important to maintain accuracy. Resistor pairs R1-R2 and

R3-R4 should be selected to match within 0.01%. In addition,

these resistors must be of the same type (preferably metal film)

to insure temperature coefficient matching. Mismatching

between R1 and R2 causes offset and gain errors while an R3 to

R4 mismatch yields gain errors.

GENERATING A NEGATIVE SUPPLY VOLTAGE

Some applications may require a bipolar output configuration,

as shown in Figure 31, but only have a single power supply rail

available. This is very common in data acquisition systems using

microprocessor-based systems. In these systems, +12 V, +15 V,

and/or +5 V only are available. Single supply rails are, of course,

common in battery-powered systems. Shown in Figure 32 is a

method of generating a negative supply using a single IC and

two capacitors. The ADM8660 employs a charge pump

technique to invert supply voltages as low as 1.5 V. A shutdown

feature on the ADM8660 complements the shutdown of the

AD8303. Note, however, that the ADM8660 requires about

500

µs to turn on after exiting the shutdown state.

+3V

10µF

ADM8660

V+

GND

SHUTDOWN

LV

OSC

FC

CAP+

CAP–

2

4

61

3

7

5

8

10µF

1/6

74HC04

SHDN FROM AD8303

–3V

Figure 32. Generating a Negative Supply Voltage

MICROCOMPUTER INTERFACES

The AD8303 serial data input provides an easy interface to a

variety of single-chip microcomputers (

µCs). Many µCs have a

built-in serial data capability which can be used for communi-

cating with the DAC. In cases where no serial port is provided,

or it is being used for some other purpose (such as an RS-232

communications interface), the AD8303 can easily be addressed

in software.

Twelve data bits are required to load a value into the AD8303.

If more than 12 bits are transmitted before the Chip Select

input goes high, the extra (i.e., the most significant) bits are

ignored. This feature is valuable because most

µCs only transmit

data in 8-bit increments. Thus, the

µC sends 16 bits to the DAC

instead of 12 bits. The AD8303 will only respond to the last 12

bits clocked into the SDI input, however, so the serial data

interface is not affected.