Datasheet
2010 Microchip Technology Inc. DS22249A-page 27
MCP4802/4812/4822
6.5.1.2 Building a “Window” DAC
When calibrating a set point or threshold of a sensor,
typically only a small portion of the DAC output range is
utilized. If the LSb size is adequate enough to meet the
application’s accuracy needs, the unused range is
sacrificed without consequences. If greater accuracy is
needed, then the output range will need to be reduced
to increase the resolution around the desired threshold.
If the threshold is not near V
REF
, 2V
REF
or V
SS
, then
creating a “window” around the threshold has several
advantages. One simple method to create this
“window” is to use a voltage divider network with a pull-
up and pull-down resistor. Example 6-2 shows this
concept.
EXAMPLE 6-2: SINGLE-SUPPLY “WINDOW” DAC
DAC
V
DD
SPI
3-wire
V
TRIP
R
1
R
2
0.1 µF
Comparator
R
3
V
CC-
V
CC+
V
CC+
V
CC-
V
OUT
R
23
R
2
R
3
R
2
R
3
+
-------------------=
V
23
V
CC+
R
2
V
CC-
R
3
+
R
2
R
3
+
------------------------------------------------------=
V
trip
V
OUT
R
23
V
23
R
1
+
R
1
R
23
+
---------------------------------------------=
R
1
R
23
V
23
V
OUT
V
O
Thevenin
Equivalent
R
SENSE
V
OUT
2.048 G
D
n
2
N
------
=
(a) Single Output DAC:
MCP4801
MCP4811
MCP4821
(b) Dual Output DAC:
MCP4802
MCP4812
MCP4822
G = Gain selection (1x or 2x)
D
n
= Digital value of DAC (0-255) for MCP4801/MCP4802
= Digital value of DAC (0-1023) for MCP4811/MCP4812
= Digital value of DAC (0-4095) for MCP4821/MCP4822
N = DAC bit resolution