Datasheet
2004 Microchip Technology Inc. DS21897A-page 23
MCP4921/4922
6.5 Bipolar Operation
Bipolar operation is achievable using the MCP492X 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
AV
SS
, if a higher offset is desired. 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 A BIPOLAR DAC USING
EXAMPLE 6-3
An output step magnitude of 1 mV with an output range
of ±2.05V is desired for a particular application.
1. Calculate the range: +2.05V – (-2.05V) = 4.1V.
2. Calculate the resolution needed:
4.1V/1 mV = 4100
Since 2
12
= 4096, 12-bit resolution is desired.
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 determined 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:
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
MCP492X
V
REF
V
DD
SPI™
3
V
OUT
R
3
R
4
R
2
R
1
V
IN
+
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
0.1 µF
V
CC
+
V
CC
–
V
OUT
V
REF
G
D
2
12
-------
=
V
IN+
V
OUT
R
4
R
3
R
4
+
--------------------
=
V
O
V
O
V
IN+
1
R
2
R
1
------
+
V
REF
R
2
R
1
------
–=
R
2
–
R
1
---------
2.05–
V
REF
-------------
2.05–
4.1
-------------
==
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 0.5V
REF
+
1.5V
REF
-----------------------------------------
2
3
---
==
If R
4
= 20 kΩ, then R
3
= 10 kΩ