Datasheet

REV. A–34–
ADuC834
DAC
The ADuC834 incorporates a 12-bit, voltage output DAC
on-chip. It has a rail-to-rail voltage output buffer capable of driving
10 k/100 pF. It has two selectable ranges, 0 V to V
REF
(the inter-
nal bandgap 2.5 V reference) and 0 V to AV
DD
. It can operate in
12-bit or 8-bit mode. The DAC has a control register, DACCON,
and two data registers, DACH/L. The DAC output can be
programmed to appear at Pin 3 or Pin 12. It should be noted
that in 12-bit mode, the DAC voltage output will be updated as
soon as the DACL data SFR has been written; therefore, the
DAC data registers should be updated as DACH first, followed
by DACL. The 12-bit DAC data should be written into DACH/L
right-justified such that DACL contains the lower eight bits,
and the lower nibble of DACH contains the upper four bits.
Table XV. DACCON SFR Bit Designations
Bit Name Description
7 ––– Reserved for Future Use
6 ––– Reserved for Future Use
5 ––– Reserved for Future Use
4 DACPIN DAC Output Pin Select.
Set by the user to direct the DAC output to Pin 12 (P1.7/AIN4/DAC).
Cleared by user to direct the DAC output to Pin 3 (P1.2/DAC/IEXC1).
3 DAC8 DAC 8-bit Mode Bit.
Set by user to enable 8-bit DAC operation. In this mode, the 8-bits in DACL SFR are routed
to the 8 MSBs of the DAC, and the 4 LSBs of the DAC are set to zero.
Cleared by user to operate the DAC in its normal 12-bit mode of operation.
2 DACRN DAC Output Range Bit.
Set by user to configure DAC range of 0–AV
DD
.
Cleared by user to configure DAC range of 0 V–2.5 V (V
REF
).
1 DACCLR DAC Clear Bit.
Set to 1 by user to enable normal DAC operation.
Cleared to 0 by user to reset DAC data registers DACL/H to zero.
0 DACEN DAC Enable Bit.
Set to 1 by user to enable normal DAC operation.
Cleared to 0 by user to power down the DAC.
DACH/L DAC Data Registers
Function DAC Data Registers, written by user to update the DAC output.
SFR Address DACL (DAC Data Low Byte) FBH
DACH (DAC Data High Byte) FCH
Power-On Default Value 00H Both Registers
Bit Addressable No Both Registers
Using the D/A Converter
The on-chip D/A converter architecture consists of a resistor
string DAC followed by an output buffer amplifier, the func-
tional equivalent of which is illustrated in Figure 21.
R
R
R
R
R
ADuC834
AV
DD
V
REF
OUTPUT
BUFFER
DAC
12
HIGH-Z
DISABLE
(FROM MCU)
Figure 21. Resistor String DAC Functional Equivalent
Features of this architecture include inherent guaranteed mono-
tonicity and excellent differential linearity. As illustrated in
Figure 21, the reference source for the DAC is user selectable in
software. It can be either AV
DD
or V
REF
. In 0-to-AV
DD
mode,
the DAC output transfer function spans from 0 V to the voltage
at the AV
DD
pin. In 0-to-V
REF
mode, the DAC output transfer
function spans from 0 V to the internal V
REF
(2.5 V). The DAC
output buffer amplifier features a true rail-to-rail output stage
implementation. This means that, unloaded, each output is
capable of swinging to within less than 100 mV of both AV
DD
and ground. Moreover, the DAC’s linearity specification (when
driving a 10 k resistive load to ground) is guaranteed through
the full transfer function except codes 0 to 48 in 0-to-V
REF
mode and 0 to 100 and 3950 to 4095 in 0-to-V
DD
mode.
Linearity degradation near ground and V
DD
is caused by saturation
of the output amplifier, and a general representation of its effects
(neglecting offset and gain error) is illustrated in Figure 22. The
dotted line in Figure 22 indicates the ideal transfer function, and
the solid line represents what the transfer function might look
like with endpoint nonlinearities due to saturation of the output
amplifier.