AD8401 Datasheet, PDF (11/12 Pages) Analog Devices – 8-Bit, 4-Channel Data Acquisition System

AD8401
APPLICATIONS INFORMATION
The software programming needs to format data as defined by
the transfer equations and Code Tables that follow.
DAC Transfer Equation
V _OUT
= 2.500 × ^D = 2.500 × ²⁵⁵
256
256
for
a
2.50
V
full
scale
where D is the decimal value 0 through 255 of the 8-bit data
word.
Table III. DAC Unipolar Code
DAC Register Contents
Decimal Binary
255
1111 1111
129
1000 0001
128
1000 0000
127
0111 1111
1
0000 0001
0
0000 0000
Nominal
Analog
General Transfer
Equation
Output V_OUT
2.500 × ²⁵⁵
256
2.490 V
2.500 × ¹²⁹
256
1.260 V
2.500
×
128
256
1.250 V
2.500
×
127
256
1.240 V
2.500 × ¹
256
0.010 V
2.500
×
0
256
0.000 V
The nominal output voltages listed in the Code Table are sub-
ject to the static performance specifications. The INL, Zero-
Scale and Full-Scale errors describe the total specified variation
that will be encountered from part to part. One LSB of error for
the 2.5 V FS range is 9.766 millivolts (= 2.50/256).
Although separate AGNDs exist for both the DAC and ADC to
minimize crosstalk, writing data to the DAC while the ADC is
performing a conversion may result in an incorrect conversion
from the ADC due to signal interaction between the DAC and
ADC. Therefore, to ensure correct operation of the ADC, the
DAC register should not be updated while the ADC is converting.
The AD8401 is configured for an input range of +3.0 volts Full
Scale. The nominal transfer characteristic for this range is plot-
ted in Figure 23. The output coding is natural binary with one
LSB equal to 11.72 millivolts. Note that the first code transition
between 0 LSB and 1 LSB occurs at 5.8 mV, one half of the
11.72 mV LSB step size. The last code transition occurs at Full
Scale minus 1.5 LSBs, which is a 2.982 V input.
The AD8401 is easily interfaced to most microprocessors by us-
ing either address bits or address decode to select the appropri-
ate multiplexer channel. Figure 24 shows how easily the AD8401
interfaces to the AD899. No additional hardware is required.
OUTPUT
CODE
11111111
11111110
11111101
FULL SCALE
TRANSITION
00000011
1LSB = FS
256
00000010
00000001
FS – 1LSB
00000000
123
FS
VIN INPUT VOLTAGE – LSBs
Figure 23. ADC 0 V to +3 V Input Transfer Characteristic
REV. 0
–11–