AD7245A/AD7248A

REV. A

–12–

The circuit of Figure 10 can be modified to provide a program-

plies are required on the AD7245A/AD7248A). The AD7245A/

AD7248A is configured as before. The current through R1 is

mirrored with a current mirror circuit to provide the program-

mable source current (see CMOS DAC Application Guide,

Publication No. G872-30-10/84, for suitable current mirror cir-

cuit). As before the absolute value of the source current will be

affected by the

±0.2% tolerance on V

formance of the current mirror will also affect the value of the

source current.

FUNCTION GENERATOR WITH PROGRAMMABLE

FREQUENCY

Figure 11 shows how the AD7245A/AD7248A with the AD537,

voltage-to-frequency converter and the AD639, trigonometric

function generator to provide a complete function generator

with programmable frequency. The circuit provides square

wave, triwave and sine wave outputs, each output of

±10 V

amplitude.

The AD7245A/AD7248A provides a programmable voltage to

the AD537 input. Since both the AD7245A/AD7248A and

AD537 are guaranteed monotonic, the output frequency will al-

ways increase with increasing digital code. The AD537 provides

a square wave output which is conditioned for

±10 V by ampli-

fier A1. The AD537 also provides a differential triwave output.

This is conditioned by amplifiers A2 and A3 to provide the

±1.8 V triwave required at the input of the AD639. The triwave

is further scaled by amplifier A4 to provide a

±10 V output.

Adjusting the triwave applied to the AD639 adjust the distortion

performance of the sine wave output, (+10 V in configuration

shown). Amplitude, offset and symmetry of the triwave can af-

fect the distortion. By adjusting these, via VR1 and VR2, an

output sine wave with harmonic distortion of better than –50 dB

can be achieved at low and intermediate frequencies.

680 pF) the output frequency range is 0 to 100 kHz over the

digital input code range. The step size for frequency increments

is 25 Hz. The accuracy of the output frequency is limited to 8 or

9 bits by the AD537, but is guaranteed monotonic to 12 bits.

MICROPROCESSOR INTERFACING—AD7245

AD7245A—8086A INTERFACE

Figure 12 shows the 8086 16-bit processor interfacing to the

AD7245A. In the setup shown the double buffering feature of

the DAC is not used and the LDAC input is tied LOW. AD0–

AD11 of the 16-bit data bus are connected to the AD7245A

data bus (DB0-DB11). The 12-bit word is written to the

AD7245A in one MOV instruction and the analog output re-

sponds immediately. In this example the DAC address is D000.

A software routine for Figure 12 is given in Table V.

Figure 12. AD7245A to 8086 Interface

Figure 11. Programmable Function Generator

Table V. Sample Program for Loading AD7245A from 8086

ASSUME DS: DACLOAD, CS: DACLOAD

DACLOAD SEGMENT AT 000

00 8CC9

MOV CS,

: DEFINE DATA SEGMENT

CS

REGISTER

02 8ED9

MOV DS,

: EQUAL TO CODE

CX

SEGMENT REGISTER

04 BF00D0

0MOV DI,

: LOAD DI WITH D000

#D000

07 C705

MOV MEM, : DAC LOADED WITH WXYZ

“YZWX” #YZWX

0B EA00 00

: CONTROL IS RETURNED TO

0E 00 FF

THE MONITOR PROGRAM