AD7228A

REV. A

–7–

DACs for correct timing relationships during the calibration

cycle of the instrument.

Figure 10b. AD7228A Timing Deskew Circuit

COARSE/FINE ADJUST

The DACs on the AD7228A can be paired together to form a

coarse/fine adjust function as indicated in Figure 11. The func-

tion is achieved using one external op amp and a few resistors

per pair of DACs.

DAC1 is the most significant or coarse DAC. Data is first

loaded to this DAC to coarsely set the output voltage. DAC2 is

then used to fine tune this output voltage. Varying the ratio of

R1 to R2 varies the relative effect of the coarse and fine DACs

on the output voltage. For the resistor values shown, DAC2 has

a resolution of 150

µV in a 10 V output range. Since each DAC

on the AD7228A is guaranteed monotonic, the coarse adjust-

ment and fine adjustment are each monotonic. One application

for this is as a set-point controller (see “Circuit Applications of

the AD7226 Quad CMOS DAC” available from Analog Devices,

Publication Number E873–15–11/84).

Figure 11. Coarse/Fine Adjust Circuit

SELF-PROGRAMMABLE REFERENCE

The circuit of Figure 12 shows how one DAC of the AD7228,

in this case DAC1, may be used in a feedback configuration to

provide a programmable reference for itself and the other seven

V

1

+ G

()

1

+ G • D

1

()

•V

IN

where G = R2/R1

voltage at the output varies between 2.5 V and 10 V giving an

effective 10-bit dynamic range to the other seven converters. For

greater than 6.8 k

Ω.

Figure 12. Self-Programmable Reference

MICROPROCESSOR INTERFACING

Figure 14. AD7228A to 8085A/Z80 Interface

Figure 15. AD7228A to 6809/6502 Interface