
RM0453 Rev 2
603/1454
RM0453
Digital-to-analog converter (DAC)
617
19.4.12 DAC channel buffer calibration
The transfer function for an N-bit digital-to-analog converter (DAC) is:
Where V
OUT
is the analog output, D is the digital input, G is the gain, V
ref
is the nominal full-
scale voltage, and V
os
is the offset voltage. For an ideal DAC channel, G = 1 and V
os
= 0.
Due to output buffer characteristics, the voltage offset may differ from part-to-part and
introduce an absolute offset error on the analog output. To compensate the V
os
, a calibration
is required by a trimming technique.
The calibration is only valid when the DAC channel is operating with buffer enabled
(MODE1[2:0] = 000b or 001b or 100b or 101b). if applied in other modes when the buffer is
off, it has no effect. During the calibration:
•
The buffer output is disconnected from the pin internal/external connections and put in
tristate mode (HiZ).
•
The buffer acts as a comparator to sense the middle-code value 0x800 and compare it
to VREF+/2 signal through an internal bridge, then toggle its output signal to 0 or 1
depending on the comparison result (CAL_FLAG1 bit).
Two calibration techniques are provided:
•
Factory trimming (default setting)
The DAC buffer offset is factory trimmed. The default value of OTRIM1[4:0] bits in
DAC_CCR register is the factory trimming value and it is loaded once DAC digital
interface is reset.
•
User trimming
The user trimming can be done when the operating conditions differs from nominal
factory trimming conditions and in particular when V
DDA
voltage, temperature, VREF+
values change and can be done at any point during application by software.
Note:
Refer to the datasheet for more details of the Nominal factory trimming conditions
In addition, when V
DD
is removed (example the device enters in STANDBY or VBAT modes)
the calibration is required.
The steps to perform a user trimming calibration are as below:
1
0
0
Sample and
hold mode
Enabled
Connected to external pin
1
0
1
Connected to external pin and to on chip peripherals (such as
comparators)
1
1
0
Disabled
Connected to external pin and to on chip peripherals (such as
comparators)
1
1
1
Connected to on chip peripherals (such as comparators)
Table 116. Channel output modes summary (continued)
MODE
1
[2:0] Mode
Buffer
Output
connections
Vout
D 2N
⁄
(
)
G Vref
×
×
(
)
VOS
+
=