Datasheet

 2004 Microchip Technology Inc. DS21897A-page 25
MCP4921/4922
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 similar application to the one devel-
oped in Section 6.5.1 “Design a bipolar dac using
Example 6-3” 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.
1. Calculate the resolution needed:
4.1V/1uV = 4.1e06. Since 2
22
= 4.2e06, 22-bit
resolution is desired. Since DNL = ±0.75 LSB,
this design can be attempted with the
MCP492X.
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.
3. If R
2
is 100Ω, then R
1
needs to be 100 kΩ.
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.
V
REF
MCP492X
V
DD
R
2
V
O
MCP492X
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 select (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