Datasheet
MCP4801/4811/4821
DS22244B-page 28 2010 Microchip Technology Inc.
6.6 Bipolar Operation
Bipolar operation is achievable using the
MCP4801/4811/4821 family of devices by utilizing 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
DD
, instead of V
SS
,
if a higher offset is desired. Also note that a pull-up to
V
DD
could be used instead of R
4
, or in addition to R
4
, if
a higher offset is desired.
EXAMPLE 6-3: DIGITALLY-CONTROLLED BIPOLAR VOLTAGE SOURCE
6.6.1 DESIGN EXAMPLE: DESIGN A
BIPOLAR DAC USING EXAMPLE 6-3
WITH 12-BIT MCP4821 OR
MCP4822
An output step magnitude 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 (G=2),
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.
DAC
V
DD
V
DD
SPI
3-wire
V
OUT
R
3
R
4
R
2
R
1
V
IN
+
0.1 µF
V
CC
+
V
CC
–
V
IN+
V
OUT
R
4
R
3
R
4
+
--------------------=
V
O
V
O
V
IN+
1
R
2
R
1
----- -+
V
DD
R
2
R
1
------
–=
V
OUT
2.048 G
D
n
2
N
------
=
G = Gain selection (1x or 2x)
D
n
= Digital value of DAC (0-255) for MCP4801/MCP4802
= Digital value of DAC (0-1023) for MCP4811/MCP4812
= Digital value of DAC (0-4095) for MCP4821/MCP4822
N = DAC bit resolution
(a) Single Output DAC:
MCP4801
MCP4811
MCP4821
(b) Dual Output DAC:
MCP4802
MCP4812
MCP4822
R
2
–
R
1
---------
2.05–
4.096V
-----------------=
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 0.5 4.096V
+
1.5 4.096V
-------------------------------------------------------
2
3
-- -==
If R
4
= 20 k, then R
3
= 10 k