Datasheet
© 2011-2012 Microchip Technology Inc. DS22272C-page 67
MCP4706/4716/4726
8.5 Selectable Gain and Offset Bipolar
Voltage Output
In some applications, precision digital control of the
output range is desirable. Figure 8-6 illustrates how to
use the DAC devices to achieve this in a bipolar or
single-supply application.
This circuit is typically used for linearizing a sensor
whose slope and offset varies.
The equation to design a bipolar “window” DAC would
be utilized if R
3
, R
4
and R
5
are populated.
8.5.1 BIPOLAR DAC EXAMPLE USING
MCP4726
An output step size of 1 mV, with an output range of
±2.05V, is desired for a particular application.
Step 1: Calculate the range: +2.05V – (-2.05V) = 4.1V.
Step 2: Calculate the resolution needed:
4.1V/1 mV = 4100
Since 2
12
= 4096, 12-bit resolution is desired.
Step 3: The amplifier gain (R
2
/R
1
), multiplied by
full-scale V
OUT
(4.096V), must be equal to the
desired minimum output to achieve bipolar
operation. Since any gain can be realized by
choosing resistor values (R
1
+R
2
), the V
REF
value
must be selected first. If a V
REF
of 4.096V is used,
solve for the amplifier’s gain by setting the DAC to
0, knowing that the output needs to be -2.05V.
The equation can be simplified to:
Step 4: Next, solve for R
3
and R
4
by setting the DAC to
4096, knowing that the output needs to be +2.05V.
FIGURE 8-6: Bipolar Voltage Source with
Selectable Gain and Offset.
EQUATION 8-4: V
OUT
, V
OA+
, AND V
O
CALCULATIONS
EQUATION 8-5:
BIPOLAR “WINDOW” DAC
USING R
4
AND R
5
R
2
–
R
1
---------
2.05–
4.096V
-----------------=
If R
1
= 20 kΩ and R
2
= 10 kΩ, the gain will be 0.5.
R
2
R
1
------
1
2
---=
R
4
R
3
R
4
+()
------------------------
2.05V 0.5 4.096V
⋅
()+
1.5 4.096V
⋅
-------------------------------------------------------
2
3
---==
If R
4
= 20 kΩ, then R
3
= 10 kΩ
R
3
V
CC
+
V
CC
–
V
OUT
I
2
C™
2-wire
V
REF
Optional
MCP4726
V
DD
R
2
V
O
V
IN
R
1
R
4
C
1
R
5
Optional
V
OA+
V
CC
+
V
CC
–
C
1
= 0.1 µF
Offset Adjust Gain Adjust
V
OUT
= V
REF
• G •
DAC Register Value
2
N
V
OA+
=
V
OUT
• R
4
+ V
CC-
• R
5
R
3
+ R
4
V
O
= V
OA+
• ( 1 + ) - V
IN
• ( )
R
2
R
1
R
2
R
1
Thevenin
Equivalent
V
45
V
CC+
R
4
V
CC-
R
5
+
R
4
R
5
+
---------------------------------------------=
V
IN+
V
OUT
R
45
V
45
R
3
+
R
3
R
45
+
---------------------------------------------=
R
45
R
4
R
5
R
4
R
5
+
-------------------=
V
O
V
IN+
1
R
2
R
1
------+
⎝⎠
⎛⎞
V
A
R
2
R
1
------
⎝⎠
⎛⎞
–=
Offset Adjust Gain Adjust