Datasheet
MCP4706/4716/4726
DS22272C-page 66 © 2011-2012 Microchip Technology Inc.
8.3.1.2 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
, 2 • V
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. Figure 8-4 and Figure 8-6 illustrate
this concept.
FIGURE 8-4: Single-Supply “Window”
DAC.
EQUATION 8-2: V
OUT
AND V
TRIP
CALCULATIONS
8.4 Bipolar Operation
Bipolar operation is achievable by utilizing an external
operational amplifier. This configuration is desirable
due to the wide variety and availability of op amps. This
allows a general purpose DAC, with its cost and
availability advantages, to meet almost any desired
output voltage range, power and noise performance.
Figure 8-5 illustrates a simple bipolar voltage source
configuration. R
1
and R
2
allow the gain to be selected,
while R
3
and R
4
shift the DAC's output to a selected
offset. Note that R4 can be tied to V
DD
, instead of V
SS
,
if a higher offset is desired.
FIGURE 8-5: Digitally-Controlled Bipolar
Voltage Source Example Circuit.
EQUATION 8-3: V
OUT
, V
OA+
, AND V
O
CALCULATIONS
R
1
V
CC
+
V
CC
–
V
O
I
2
C™
2-wire
V
REF
Optional
MCP47X6
V
DD
V
OUT
R
2
C
1
R
3
V
CC
+
V
CC
–
R
SENSE
Comp.
V
TRIP
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
+
---------------------------------------------=
Thevenin
Equivalent
R
1
R
23
V
23
V
OUT
V
TRIP
V
OUT
= V
REF
• G •
DAC Register Value
2
N
R
3
V
CC
+
V
CC
–
V
O
I
2
C™
2-wire
V
REF
Optional
MCP47X6
V
DD
R
2
V
OUT
V
IN
R
1
R
4
C
1
V
OA+
V
OA+
=
V
OUT
• R
4
R
3
+ R
4
V
OUT
= V
REF
• G •
DAC Register Value
2
N
V
O
= V
OA+
• ( 1 + ) - V
DD
• ( )
R
2
R
1
R
2
R
1