MCP4921T-ESN MICROCHIP [Microchip Technology], MCP4921T-ESN Datasheet - Page 23

• Manufacturer Part Number: MCP4921T-ESN
• Manufacturer: MICROCHIP [Microchip Technology]

6.5
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:
6.5.1
An output step magnitude of 1 mV with an output range
of ±2.05V is desired for a particular application.
1.
2.
3.
 2004 Microchip Technology Inc.
If R
Calculate the range: +2.05V – (-2.05V) = 4.1V.
Calculate the resolution needed:
4.1V/1 mV = 4100
Since 2
The amplifier gain (R
must be equal to the desired minimum output to
achieve bipolar operation. Since any gain can
be realized by choosing resistor values (R
the V
a V
setting the DAC to 0, knowing that the output
needs to be -2.05V.
simplified to:
1
= 20 k and R
–
-------- -
REF
R
Bipolar Operation
R
REF
1
2
DESIGN A BIPOLAR DAC USING
EXAMPLE 6-3
12
of 4.1V is used, solve for the gain by
=
source needs to be determined first. If
= 4096, 12-bit resolution is desired.
–
------------ -
V
2.05
REF
2
SPI™
=
V
V
V
Digitally-Controlled Bipolar Voltage Source.
= 10 k , the gain will be 0.5.
OUT
IN+
O
–
------------ -
V
4.1
2.05
=
2
REF
/R
=
=
3
V
The equation can be
1
IN+
V
------------------- -
R
), multiplied by V
V
OUT
3
MCP492X
REF
V


+
1
R
----- -
R
DD
R
R
G
+
2
1
4
4
-------
2
R
----- -
R
=
D
12
2
1


1
-- -
2
–
V
REF
1
V
+R
OUT
REF


R
----- -
R
2
),
2
1
,


R
3
R
4
V
REF
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
Example 6-3 illustrates a simple bipolar voltage source
configuration. R
while R
offset. Note that R4 can be tied to V
AV
V
desired.
4.
If R
REF
SS
Next, solve for R
4096, knowing that the output needs to be
+2.05V.
R
4
, if a higher offset is desired. Note that a pull-up to
could be used, instead of R
1
= 20 k , then R
0.1 µF
3
V
and R
IN
---------------------- -
R
+
3
R
V
V
+
R
4
CC
4
CC
MCP4921/4922
1
R
2
and R
shift the DAC's output to a selected
4
+
–
=
3
3
and R
2.05V
-----------------------------------------
2
= 10 k
allow the gain to be selected,
V
O
1.5V
4
+
by setting the DAC to
0.5V
REF
4
, if a higher offset is
REF
DS21897A-page 23
REF
=
2
-- -
3
, instead of