DAC8408
SINGLE SUPPLY, VOLTAGE OUTPUT OPERATION
The DAC8408 can be connected with a single +5 V supply to
produce DAC output voltages from 0 V to +1.5 V. In Figure 8,
the DAC8408 R-2R ladder is inverted from its normal connec-
tion. A +1.500 V reference is connected to the current output pin
4 (IOUT 1A), and the normal VREF input pin becomes the DAC
output. Instead of a normal current output, the R-2R ladder out-
puts a voltage. The OP-490, consisting of four precision low
power op amps that can operate its inputs and outputs to zero
volts, buffers the DAC to produce a low impedance output volt-
age from 0 V to +1.5 V full-scale. Table III shows the code table.
With the supply and reference voltages as shown, better than 1/2
LSB differential and integral nonlinearity can be expected. To
maintain this performance level, the +5 V supply must not drop
below 4.75 V. Similarly, the reference voltage must be no higher
than 1.5 V. This is because the CMOS switches require a mini-
mum level of bias in order to maintain the linearity performance.
Table III. Single Supply Binary Code Table (Refer to Figure 8)
DAC Data Input
MSB
LSB
Analog Output
11111111
10000001
1 0 0 0 0 00 0
01111111
00000001
00000000
255
VREF 256 , +1.4941 V
129
VREF 256 , +0.7559 V
128
VREF 256 , +0.7500 V
127
VREF 256 , +0.7441 V
1
VREF 256 , +0.0059 V
0
VREF 256 , 0.0000 V
Figure 8. Unipolar Supply, Voltage Output DAC Operation
–12–
REV. A