5962-88766013A Datasheet, PDF (12/16 Pages) Analog Devices – LC2MOS 12-Bit DACPORTs

AD7245A/AD7248A
Since the tolerance value on the reference voltage of the AD7245A/
AD7248A is ± 0.2%, then the absolute value of I_SINK can vary by
± 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-
rent is negligible. Tying the R_OFS input to R_FB input reduces this
effect even further and prevents noise pickup which could occur
if the R_OFS pin was left unconnected.
The circuit of Figure 10 can be modified to provide a pro-
grammable current source to AGND or –V_SINK (for –V_SINK,
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_REF. In this case the perfor-
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.
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.
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
ADDRESS BUS
16-BIT
LATCH
ADDRESS
DECODE
WR
CS
LDAC
AD7245A*
WR
DB11
DB0
AD15
AD0
ADDRESS/DATA BUS
*LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 12. AD7245A to 8086 Interface
10⍀
0.1␮F
؎10V
SQUARE
WAVE
+15V
33k⍀
A1 15k⍀
82k⍀
+15V
+V_S
V_DD
+
AD7245A/
AD7248A
R_FB
V_OUT
REF
OUT
R_OFS
V_SS
DGND
AGND
20k⍀
GND O/P
DEC +VS
C
AD537
C
V_OS
–V_S
+
10␮F
+15V +15V
A2
4.7k⍀
56k⍀
10k⍀
VR1
56k⍀
C_F
680pF
5k⍀
VR2
4.12k⍀
5.6k⍀
4.12k⍀
A3
20k⍀
3.9k⍀
3.9k⍀
22k⍀
A4
22k⍀
؎10V
TRI WAVE
X1
X2
U1
U2
COM
+15V
+V_S
W
Z1
Z2
؎1V
SINE WAVE
A1, A2, A3, A4 – 2 ؋ AD712
AD639
UP
Y2
–V_S
–15V
–15V
Figure 11. Programmable Function Generator
–12–
REV. B