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AD7228CQ

LC2MOS Octal 8-Bit DAC

Analog Devices 

AD7228

Data Sheet

Table 7. Unipolar Code Table

DAC Latch Contents

MSB

LSB1

Analog Output

1111

1000

1111

0001

+VREF(255/256)

+VREF(129/256)

1000

0000

+VREF  128  = + VREF

 256  2

0111

1111

+VREF(127/256)

0000

0000

0001

0000

+VREF(1/256)

0V

1 1 LSB = (VREF)(2−8) = VREF (1/256).

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,

VOUT

= 1 +



R2

R1





×

(D1

×

VREF

)

−





R2

R1





×

(VREF

)

With R1 = R2,

VOUT = (2D1 − 1) × (VREF)

where D1 is a fractional representation of the digital word in

Latch 1 of the AD7228 (0 ≤ D1 ≤ 255/256).

VREF

VREF

11

VDD

1

Table 8. Bipolar Code Table

DAC Latch Contents

MSB

LSB

1111

1111

1000

0001

1000

0000

0111

1111

0000

0001

0000

0000

Analog Output

+VREF(127/128)

+VREF(1/128)

0V

−VREF(1/128)

−VREF(127/128)

−VREF(128/128) = −VREF

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.

R1

10kΩ

±0.1%

R2

10kΩ

±0.1%

+15V

DAC 1

VOUT1

9

AD7228*

–15V

VOUT

10

VSS

12

GND

*ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 11. Bipolar Output Circuit

+4V

REFERENCE

INPUT

–4V

+15V

15kΩ

10kΩ

+10V

+2V

VREF

11

+15V

VDD

1

DAC 1

9 VOUT1

AD7228*

*ADDITIONAL PINS OMITTED FOR CLARITY.

10

VSS

–5V

12

GND

Figure 12. Applying an AC Signal to the AD7228

Rev. D | Page 10 of 15

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