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ADuC7060
DAC0DAT Register
Code 4095. Linearity degradation near ground and AVDD is
Name:
Address:
Default value:
Access:
Function:
DAC0DAT
0xFFFF0604
0x00000000
Read and write
This 32-bit MMR contains the DAC output
value.
caused by saturation of the output amplifier, and a general
representation of its effects (neglecting offset and gain error) is
illustrated in Figure 20. The dotted line in Figure 20 indicates the
ideal transfer function, and the solid line represents what the
transfer function may look like with endpoint nonlinearities due
to saturation of the output amplifier. Note that Figure 20
represents a transfer function in 0-to-AVDD mode only. In 0-to-
V REF or, VREF±, and ADCx/EXT_REF2IN± modes (with V REF <
AVDD or ADCx/EXT_REF2IN± < AVDD), the lower
Table 62. DAC0DAT MMR Bit Designations
nonlinearity is similar. However, the upper portion of the
Bit
31:28
27:16
15:12
11:0
Description
Reserved.
12-bit data for DAC0.
Extra four bits used in interpolation mode.
Reserved.
transfer function follows the ideal line right to the end (V REF in this
case, not AVDD), showing no signs of endpoint linearity errors.
AVDD
AVDD – 100mV
Using the DAC
The on-chip DAC architecture consists of a resistor string DAC
followed by an output buffer amplifier. The functional equivalent
is shown in Figure 20.
The reference source for the DAC is user selectable in software. It
can be AVDD, VREF±, or ADCx/EXT_REF2IN±.
100mV
?
In 0-to- AVDD mode, the DAC output transfer function
0x00000000
0x0FFF0000
?
?
spans from 0 V to the voltage at the AVDD pin.
In VREF± and ADCx/EXT_REF2IN± modes, the DAC
output transfer function spans from negative input voltage
to the voltage positive input pin. Note that these voltages
must never go below 0 V or above AVDD.
In 0-to-V REF mode, the DAC output transfer function spans
from 0 V to the internal 1.2 V reference, V REF .
Figure 20. Endpoint Nonlinearities Due to Amplifier Saturation
The endpoint nonlinearities conceptually illustrated in Figure 20
worsen as a function of output loading. Most of the ADuC7060
data sheet specifications in normal mode assume a 5 kΩ
resistive load to ground at the DAC output. As the output is
forced to source or sink more current, the nonlinear regions at
the top or bottom (respectively) of Figure 20 become larger.
The DAC can be configured in three different user modes:
normal mode, DAC interpolation mode, and op amp mode.
Normal DAC Mode
In this mode of operation, the DAC is configured as a 12-bit
voltage output DAC. By default, the DAC buffer is enabled, but
the output buffer can be disabled. If the DAC output buffer is
disabled, the DAC is capable of driving a capacitive load of only
20 pF. The DAC buffer is disabled by setting the DACBUFBYPASS
bit in DAC0CON.
The DAC output buffer amplifier features a true, rail-to-rail
output stage implementation. This means that when unloaded,
each output is capable of swinging to within less than 5 mV of
both AVDD and ground. Moreover, the linearity specification of
the DAC (when driving a 5 kΩ resistive load to ground) is
guaranteed through the full transfer function except Code 0
With larger current demands, this can significantly limit output
voltage swing.
DAC Interpolation Mode
In interpolation mode, a higher DAC output resolution of 16 bits
is achieved with a longer update rate than normal mode. The
update rate is controlled by the interpolation clock rate selected
in the DAC0CON register. In this mode, an external RC filter is
required to create a constant voltage.
Op Amp Mode
In op amp mode, the DAC output buffer is used as an op amp
with the DAC itself disabled.
ADC6 is the positive input to the op amp, ADC7 is the negative
input, and ADC8 is the output. In this mode, the DAC should
be powered down by setting Bit 9 of DAC0CON.
to Code 100 and, in 0-to- AVDD mode only, Code 3995 to
Rev. 0 | Page 52 of 100
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