Datasheet

MCP4902/4912/4922
DS22250A-page 32  2010 Microchip Technology Inc.
6.7 Designing a Double-Precision
DAC Using a Dual DAC
Example 6-5 illustrates how to design a single-supply
voltage output capable of up to 24-bit resolution from a
dual 12-bit DAC. This design is simply a voltage divider
with a buffered output.
As an example, if a application similar to the one
developed in Section 6.5.1 “Design Example: Design
a Bipolar DAC Using Example 6-3 with 12-bit
MCP4922 or MCP4921” required a resolution of 1 µV
instead of 1 mV and a range of 0V to 4.1V, then 12-bit
resolution would not be adequate.
Step 1: Calculate the resolution needed:
4.1V/1 µV = 4.1x10
6
. Since 2
22
= 4.2x10
6
, 22-
bit resolution is desired. Since DNL = ±0.75
LSb, this design can be attempted with the
MCP4922.
Step 2: Since DAC
B
’s V
OUTB
has a resolution of 1 mV,
its output only needs to be “pulled” 1/1000 to
meet the 1 µV target. Dividing V
OUTA
by 1000
would allow the application to compensate for
DAC
B
’s DNL error.
Step 3: If R
2
is 100, then R
1
needs to be 100 k.
Step 4:The resulting transfer function is not perfectly
linear, as shown in the equation of
Example 6-5.
EXAMPLE 6-5: SIMPLE, DOUBLE-PRECISION DAC WITH MCP4922
V
REF
MCP4922
V
DD
R
2
MCP4922
V
DD
R
1
DAC
A
(Fine Adjust)
DAC
B
(Course Adjust)
SPI
3
R
1
>> R
2
V
O
V
OUTA
R
2
V
OUTB
R
1
+
R
1
R
2
+
-----------------------------------------------------=
G = Gain selection (1x or 2x)
D = Digital value of DAC (0- 4096)
0.1 µF
V
CC
+
V
CC
–
V
OUTA
V
REFA
G
A
D
A
2
12
-------=
V
OUTB
V
REFB
G
B
D
B
2
12
-------=
V
OUTA
V
OUTB
V
O