AD7225

REV. B

–7–

Only the data held in the DAC register determines the analog

output of the converter. The LDAC signal is common to all four

DACs and controls the transfer of information from the input

registers to the DAC registers. Data is latched into all four DAC

registers simultaneously on the rising edge of LDAC. The

LDAC

signal is level triggered and therefore the DAC registers

may be made transparent by tying LDAC LOW (in this case the

outputs of the converters will respond to the data held in their

respective input latches). LDAC is an asynchronous signal and

is independent of WR. This is useful in many applications.

However, in systems where the asynchronous LDAC can occur

during a write cycle (or vice versa) care must be taken to ensure

that incorrect data is not latched through to the output. In other

words, if LDAC is activated prior to the rising edge of WR (or

WR

or longer after WR goes HIGH to ensure correct data is latched

through to the output. Table II shows the truth table for AD7225

operation. Figure 9 shows the input control logic for the part

and the write cycle timing diagram is given in Figure 10.

Table II. AD7225 Truth Table

WR

LDAC

Function

H

H

No Operation. Device not selected

L

H

Input Register of Selected DAC Transparent

g

H

Input Register of Selected DAC Latched

H

L

All Four DAC Registers Transparent

(i.e. Outputs respond to data held in respective

input registers)

Input Registers are Latched

H

g

All Four DAC Registers Latched

L

L

DAC Registers and Selected Input Register

Transparent Output follows Input Data for

Selected Channel.

Figure 9. Input Control Logic

Figure 10. Write Cycle Timing Diagram

GROUND MANAGEMENT AND LAYOUT

Since the AD7225 contains four reference inputs which can be

driven from ac sources (see AC REFERENCE SIGNAL sec-

tion) careful layout and grounding is important to minimize

analog crosstalk between the four channels. The dynamic per-

formance of the four DACs depends upon the optimum choice

of board layout. Figure 11 shows the relationship between input

Figure 11. Channel-to-Channel Isolation

Figure 12. Suggested PCB Layout for AD7225.

Layout Shows Component Side (Top View)

frequency and channel-to-channel isolation. Figure 12 shows a

printed circuit board layout which is aimed at minimizing

crosstalk and feedthrough. The four input signals are screened

avoid slew rate limiting effects from the output amplifier during

measurements.