Datasheet
MCP4902/4912/4922
DS22250A-page 30 2010 Microchip Technology Inc.
6.5 Bipolar Operation
Bipolar operation is achievable using the MCP4902/
4912/4922 family of devices by using an external
operational amplifier (op amp). 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.
Example 6-3 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
REF
instead of V
SS
,
if a higher offset is desired. Also note that a pull-up to
V
REF
could be used instead of R
4
, if a higher offset is
desired.
EXAMPLE 6-3: DIGITALLY-CONTROLLED BIPOLAR VOLTAGE SOURCE
6.5.1 DESIGN EXAMPLE: DESIGN A BIPOLAR
DAC USING EXAMPLE 6-3 WITH 12-BIT
MCP4922 OR MCP4921
An output step magnitude of 1 mV with an output range
of ±2.05V is desired for a particular application.
The following steps show the details:
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 V
REF
,
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
source needs to be deter-
mined first. If a V
REF
of 4.1V is used, solve for
the 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.
V
REF
V
REF
V
DD
SPI
3
V
OUT
R
3
R
4
2
R
1
V
IN
+
0.1 µF
V
CC
+
V
CC
–
V
O
V
IN+
V
OUT
R
4
R
3
R
4
+
--------------------=
V
O
V
IN+
1
R
2
R
1
----- -+
V
DD
R
2
R
1
------
–=
G = Gain selection (1x or 2x)
D
n
= Digital value of DAC (0-255) for MCP4901/MCP4902
V
OUT
V
REF
G
D
n
2
N
------
=
= Digital value of DAC (0-1023) for MCP4911/MCP4912
= Digital value of DAC (0-4095) for MCP4921/MCP4922
N = DAC Bit Resolution
DAC
MCP4901
MCP4911
MCP4921
(a) Single Output DAC:
(b) Dual Output DAC:
MCP4902
MCP4912
MCP4922
R
2
–
R
1
---------
2.05–
V
REF
-------------
2.05–
4.1
-------------==
If R
1
= 20 k and R
2
= 10 k, the gain will be 0.5.
R
2
R
1
------
1
2
-- -=
R
4
R
3
R
4
+
-----------------------
2.05V 0.5V
REF
+
1.5V
REF
-----------------------------------------
2
3
-- -
==
If R
4
= 20 k, then R
3
= 10 k