AD7245A/AD7248A

REV. B

–12–

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

dual supplies 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 pro-

grammable source current (see CMOS DAC Application Guide,

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

circuit). As before the absolute value of the source current will

be affected by the

±0.2% tolerance on V

mance 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

always increase with increasing digital code. The AD537 pro-

vides a square wave output which is conditioned for

±10 V by

amplifier 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 affect

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.

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—8086 INTERFACE

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

AD7245A. In the setup shown in Figure 12, the double buffer-

ing 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

responds immediately. In this example the DAC address is

D000. A software routine for Figure 12 is given in Table V.

8086

ALE

WR

AD15

AD0

*LINEAR CIRCUITRY OMITTED FOR CLARITY

ADDRESS/DATA BUS

AD7245A*

16-BIT

LATCH

ADDRESS

DECODE

ADDRESS BUS

CS

LDAC

WR

DB11

DB0

Figure 12. AD7245A to 8086 Interface

AD537

GND

DEC

C

O/P

C

20k

+

+15V

AD7245A/

AD7248A

DGND

AGND

REF

OUT

10V

SQUARE

WAVE

+15V

33k

82k

+15V

4.7k

+15V

56k

56k

5.6k

5k

VR2

4.12k

4.12k

10k

VR1

3.9k

3.9k

22k

22k

A1, A2, A3, A4 – 2

AD712

680pF

A3

A4

–15V

–15V

+15V

1V

SINE WAVE

20k

15k

X1

X2

U1

U2

COM

Y2

W

Z1

Z2

UP

AD639

10V

TRI WAVE

10

0.1 F10 F

+

A2

A1

Figure 11. Programmable Function Generator

Since the tolerance value on the reference voltage of the AD7245A/

AD7248A is

±0.2% from device to device for a fixed value of R1.

Because the input bias current of the AD7245A/AD7248A’s op

amp is only of the order of picoamps, its effect on the sink cur-

effect even further and prevents noise pickup which could occur