AD7569/AD7669
an RD pulse for the AD7569/AD7669. This RD pulse accesses
data from the ADC and places the conversion result into a regis-
ter on the 74646. The rising edge of this pulse generates an in-
terrupt request to the processor. The conversion result is read
from the 74646 register by performing an I/O read to the
decoded address of the 74646. Writing data to the relevant
AD7569/AD7669 DAC involves an I/O write to the 74646,
which transfers the data to the data inputs of the AD7569/
AD7669. Data is latched into the selected DAC register on the
rising edge of IOW.
UNIPOLAR (0 V to +2.5 V) CONFIGURATION
The 0 V to +2.5 V output voltage range is achieved by tying VSS
to AGNDDAC(= 0 V) and the RANGE input to VDD. The table
for output voltage versus digital code is as in Table IV with
2.VREF replacing VREF. Note that for this range
1
1 LSB = 2.VREF (2−8 ) = V
REF 128
BIPOLAR (–1.25 V to +1.25 V) CONFIGURATION
The first of the bipolar configurations is achieved by tying the
RANGE input to AGNDDAC(= 0 V) and VSS to –5 V. The VSS
voltage level at which the AD7569/AD7669 changes to bipolar
operation is approximately –1 V. When the part is configured
for bipolar outputs, the input coding becomes twos comple-
ment. The table for output voltage versus the digital code in the
DAC register is shown in Table V. Note as with the unipolar
configuration, a digital input code of all 0s produces an output
of 0 V. It should be noted, however, that a low pulse on the
RESET line for the bipolar ranges sets the output voltage to
negative full scale.
APPLYING THE AD7569/AD7669 DAC
An internal gain/offset network on the AD7569/AD7669 allows
several output voltage ranges. The part can produce unipolar
output ranges of 0 V to +1.25 V or 0 V to +2.5 V and bipolar
output ranges of –1.25 V to +1.25 V or –2.5 V to +2.5 V. Con-
nections for these various output ranges are outlined below.
UNIPOLAR (0 V to +1.25 V) CONFIGURATION
The first of the configurations provides an output voltage range
of 0 V to +1.25 V. This is achieved by tying the VSS and
RANGE inputs to AGNDDAC(= 0 V). Figure 21 shows the con-
figuration of the AD7569 to achieve this output range. A similar
configuration of the AD7669 gives the same output range. The
table for output voltage versus the digital code in the DAC regis-
ter is shown in Table IV.
Table V. Bipolar (–1.25 V to +1.25 V) Code Table
DAC Register Contents
MSB LSB
0111 1111
0000 0001
0000 0000
1111 1111
1000 0001
1000 0000
Analog Output, VOUT
127
+VREF
128
1
+VREF
128
0 V
1
–VREF
128
127
–VREF
128
Figure 21. AD7569 Unipolar (0 V to +1.25 V) Operation
128
–VREF
= –VREF
128
Table IV. Unipolar (0 V to +1.25 V) Code Table
DAC Register Contents
NOTE: 1 LSB = (VREF)(2–7) = VREF (1/128)
MSB LSB
Analog Output, VOUT
BIPOLAR (–2.5 V to +2.5 V) CONFIGURATION
The –2.5 V to +2.5 V bipolar output range is achieved by tying
the RANGE input to VDD and the VSS input to –5 V. Once
again, the input coding is 2s complement. The table for output
voltage versus digital code is as in Table V with 2.VREF replacing
255
1111 1111
1000 0001
1000 0000
0111 1111
0000 0001
0000 0000
+VREF
256
129
VREF. Note that for this range
+VREF
256
1
64
1 LSB = 4.VREF (2−8 ) = VREF
128
+VREF
+VREF
+VREF
0 V
= +VREF/2
256
127
256
1
256
NOTE: 1 LSB = (VREF) (2–8) = VREF (1/256); VREF = +1.25 V Nominal
REV. B
–15–
ADI [ ADI ]
