Datasheet

MCP4801/4811/4821
DS22244B-page 30  2010 Microchip Technology Inc.
6.8 Designing a Double-Precision
DAC
Example 6-5 illustrates how to design a single-supply
voltage output capable of up to 24-bit resolution by
using 12-bit DACs. This design is simply a voltage
divider with a buffered output.
As an example, if an application similar to the one
developed in Section 6.6.1 “Design Example:
Design a Bipolar DAC Using Example 6-3 with 12-
bit MCP4821 or MCP4822” 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 = 4.1 x 10
6
. Since 2
22
=4.2x10
6
,
22-bit resolution is desired. Since
DNL = ±0.75 LSb, this design can be done with
the 12-bit MCP4821 or MCP4822 DAC devices.
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 shown in the
equation of Example 6-5.
EXAMPLE 6-5: SIMPLE, DOUBLE-PRECISION DAC WITH MCP4821
V
DD
R
2
V
O
V
DD
R
1
V
OUTA
for Fine Adjustment
SPI
3-wire
R
1
>> R
2
V
O
V
OUTA
R
2
V
OUTB
R
1
+
R
1
R
2
+
------------------------------------------------------=
0.1 µF
V
CC
+
V
CC
–
V
OUTA
2.048 G
A
D
A
2
12
-------  =
V
OUTB
2.048 G
B
D
B
2
12
-------  =
DAC
A
G
x
= Gain selection (1x or 2x)
D
n
= Digital value of DAC (0-4096)
DAC
B
V
OUTB
for Fine Adjustment