AD7228

Data Sheet

Rev. D | Page 10 of 15

Table 7. Unipolar Code Table

DAC Latch Contents

MSB

Analog Output

1 1 1 1

1 1 1 1

1 0 0 0

0 0 0 1

1 0 0 0

0 0 0 0

2

256

128

REF

V

+

=













0 1 1 1

1 1 1 1

0 0 0 0

0 0 0 1

0 0 0 0

0 0 0 0

0 V

1

Bipolar Output Operation

Each of the DACs on the AD7228 can be individually configured

for bipolar output operation. This is possible using one external

amplifier and two resistors per channel. Figure 11 shows a circuit

used to implement offset binary coding (bipolar operation) with

DAC 1 of the AD7228. In this case,

)

(

)

(

1

1

REF

REF

OUT

V

R1

R2

V

D

R1

R2

V

×













−

×

×











 +

=

With R1 = R2,

Table 8. Bipolar Code Table

DAC Latch Contents

MSB

LSB

Analog Output

1 1 1 1

1 1 1 1

1 0 0 0

0 0 0 1

1 0 0 0

0 0 0 0

0 V

0 1 1 1

1 1 1 1

0 0 0 0

0 0 0 1

0 0 0 0

0 0 0 0

Mismatch between R1 and R2 causes gain and offset errors;

therefore, these resistors must match and track over temperature.

The AD7228 can be operated from a single supply or from dual

supplies. Table 8 shows the digital code vs. output voltage

relationship for the circuit of Figure 11 with R1 = R2.

AC Reference Signal

In some applications, it may be desirable to have an ac signal

applied as the reference input to the AD7228. The AD7228 has

multiplying capability within the upper (10 V) and lower (2 V)

limits of reference voltage when operated with dual supplies.

Therefore, ac signals must be ac-coupled and biased up before

being applied to the reference input. Figure 12 shows an ac

signal applied to the reference input of the AD7228. For input

frequencies up to 50 kHz, the output distortion typically remains

less than 0.1%. The typical 3 dB bandwidth for small signal

inputs is 800 kHz.

12

9

11

1

GND

*ADDITIONAL PINS OMITTED FOR CLARITY.

VOUT

R1

10kΩ

±0.1%

R2

10kΩ

±0.1%

+15V

–15V

AD7228*

10

DAC 1

Figure 11. Bipolar Output Circuit

+10V

REFERENCE

INPUT

15kΩ

+15V

–5V

10kΩ

+2V

12

9

11

1

GND

*ADDITIONAL PINS OMITTED FOR CLARITY.

+15V

AD7228*

10

DAC 1

+4V

–4V

Figure 12. Applying an AC Signal to the AD7228