AD7245A/AD7248A
The circuit of Figure 10 can be modified to provide a program-
mable current source to AGND or –V
SINK
(for –V
SINK
, dual sup-
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
REF
. In this case the per-
formance of the current mirror will also affect the value of the
source current.
FUNCTION GENERATOR WITH PROGRAMMABLE
FREQUENCY
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 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.
Using the capacitor value shown in Figure 11 for C
F
(i.e.,
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.
Figure 12. AD7245A to 8086 Interface
Table V. Sample Program for Loading AD7245A from 8086
ASSUME DS: DACLOAD, CS: DACLOAD
DACLOAD SEGMENT AT 000
00 8CC9
02 8ED9
04 BF00D0
MOV CS,
CS
MOV DS,
CX
0MOV DI,
#D000
: DEFINE DATA SEGMENT
REGISTER
: EQUAL TO CODE
SEGMENT REGISTER
: LOAD DI WITH D000
07 C705
MOV MEM, : DAC LOADED WITH WXYZ
“YZWX” #YZWX
0B EA00 00
0E 00 FF
: CONTROL IS RETURNED TO
THE MONITOR PROGRAM
Figure 11. Programmable Function Generator
–12–
REV. A
AD [ ANALOG DEVICES ]
