DAC1220
SBAS082G –FEBRUARY 1998–REVISED SEPTEMBER 2009...................................................................................................................................... www.ti.com
Setting the Output Voltage
The code may be given in either straight binary or
offset two's complement format. This is controlled by
the DF bit in the Command Register (see the register
description in Table 13 for details). The two data
format options and the 16- or 20-bit option give rise to
four transfer functions, which are shown in Table 8.
For reference, several ideal output voltages for given
input codes are shown in Table 9.
To set the DAC1220 output voltage, write a code to
the Data Input Register (DIR). A write to any of the
bytes in the DIR causes the voltage to change at the
completion of the write command.
The DAC1220 operates in either 16- or 20-bit mode.
The DIR is 24 bits wide, and the code stored in it is
left justified, with the least significant bits ignored.
Therefore, in 16-bit mode, only the upper 16 bits of
the DIR are significant, and in 20-bit mode, only the
upper 20 bits of the DIR are significant.
Note that the DIR code can also be considered a
24-bit number. This may be convenient in software. In
this case the transfer functions for 16- and 20-bit
modes are the same, except that in 16-bit mode the
code is truncated by eight bits, and in 20-bit mode the
code is truncated by four bits.
In 20-bit mode, all three bytes of the DIR must be
written to in order to completely update the code. In
16-bit mode, it is only necessary to write to the two
upper bytes; a write to the lower byte has no effect on
the output.
Table 8. Transfer Functions
DATA FORMAT
Offset two's complement
20-BIT MODE
16-BIT MODE
code)219
code)215
VOUT + 2VREF
VOUT + 2VREF
VOUT + 2VREF
220
216
Straight binary
code
220
code
VOUT + 2VREF
216
Table 9. Example Output Voltages
APPROXIMATE
OUTPUT
VOLTAGE
(1)
RESOLUTION
DATA FORMAT
Two's complement
Straight binary
CODE
8000h
0000h
8000h
0000h
0000h
8000h
0000h
8000h
7FFFh
FFFFh
7FFFFh
FFFFFh
DIR CONTENT
8000xxh
0000xxh
80000xh
00000xh
0000xxh
8000xxh
00000xh
80000xh
7FFFxxh
FFFFxxh
7FFFFxh
FFFFFxh
16-bit
0V
Two's complement
Straight binary
20-bit
16-bit
20-bit
16-bit
20-bit
Two's complement
Straight binary
2.5V
Two's complement
Straight binary
Two's complement
Straight binary
5V
Two's complement
Straight binary
(1) x = Do not care
14
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