Datasheet

MCP4921/4922
DS21897A-page 22  2004 Microchip Technology Inc.
6.4.1.2 Building a “Window” DAC
When calibrating a set point or threshold of a sensor,
rarely does the sensor utilize the entire output range of
the DAC. If the LSB size is adequate to meet the appli-
cation’s accuracy needs, then the resolution is sacri-
ficed 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
or AV
SS
, then creating
a “window” around the threshold has several advan-
tages. 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 and Example 6-4
illustrates this concept.
The MCP492X’s low, ±0.75 (max.) DNL performance
is critical to meet calibration accuracy in production.
EXAMPLE 6-2: Single-Supply “Window” DAC.
V
REF
MCP492X
V
DD
SPI™
3
V
trip
R
1
R
2
0.1 µF
Comparator
R
3
V
CC-
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
V
CC+
V
CC+
V
CC-
V
OUT
V
OUT
V
REF
G
D
2
12
-------
=
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
2
R
23
+
--------------------------------------------
=
R
1
R
23
V
23
V
OUT
V
O
Thevenin
Equivalent
R
sense