Datasheet

MCP4921/4922
DS21897A-page 24  2004 Microchip Technology Inc.
6.6 Selectable Gain and Offset Bipolar
Voltage Output Using A Dual DAC
In some applications, precision digital control of the
output range is desirable. Example 6-4 illustrates how
to use the MCP4922 to achieve this in a bipolar or
single-supply application.
This circuit is typically used in Multiplier mode and is
ideal for linearizing a sensor whose slope and offset
varies. Refer to Section 6.9 “Using Multiplier Mode”
for more information on Multiplier mode.
The equation to design a bipolar “window” DAC would
be utilized if R
3
, R
4
and R
5
are populated.
EXAMPLE 6-4: Bipolar Voltage Source With Selectable Gain and Offset.
V
REFA
MCP492X
V
DD
R
3
R
4
R
2
V
O
MCP492X
V
DD
R
1
DAC
A
(Gain Adjust)
DAC
B
(Offset Adjust)
SPI™
3
R
5
V
CC
+
Thevenin
Bipolar “Window” DAC using R
4
and R
5
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
0.1uF
V
CC
–
AV
SS
= GND
V
CC
+
V
CC
–
V
OUTB
V
REFB
G
B
()
D
B
2
12
-------
=
V
OUTA
V
OUTB
V
OUTA
V
REFA
G
A
()
D
A
2
12
-------
=
V
IN+
V
OUTB
R
4
V
CC-
R
3
+
R
3
R
4
+
------------------------------------------------
=
V
O
V
IN+
1
R
2
R
1
------
+


V
OUTA
R
2
R
1
------


–=
Equivalent
V
45
V
CC+
R
4
V
CC-
R
5
+
R
4
R
5
+
--------------------------------------------
= R
45
R
4
R
5
R
4
R
5
+
------------------
=
V
IN+
V
OUTB
R
45
V
45
R
3
+
R
3
R
45
+
-----------------------------------------------
= V
O
V
IN+
1
R
2
R
1
------
+


V
OUTA
R
2
R
1
------


–=
Offset Adjust Gain Adjust
Offset Adjust Gain Adjust
V
REFB