Datasheet

 2010 Microchip Technology Inc. DS22250A-page 31
MCP4902/4912/4922
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 MCP4902/4912/4922 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
DAC
B
V
DD
R
3
R
4
R
2
DAC
A
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
0.1uF
V
CC
–
V
CC
+
V
CC
–
V
OUTB
V
REFB
G
B

D
B
2
N
-------=
V
OUTA
V
OUTB
V
OUTA
V
REFA
G
A

D
A
2
N
-------=
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
GX = Gain selection (1x or 2x)
D
A
, D
B
= Digital value of DAC (0-255) for MCP4902
= Digital value of DAC (0-1023) for MCP4912
= Digital value of DAC (0-4095) for MCP4922
N = DAC Bit Resolution
V
O
Dual Output DAC:
MCP4902
MCP4912
MCP4922