Data Sheet
MCP4725
DS22039D-page 38 © 2009 Microchip Technology Inc.
8.5.4.1 Design a Bipolar DAC using
Example 8-3
Some applications desires an output step magnitude of
1 mV with an output range of ±2.05V. The following
steps explain the design solution:
1. Calculate the range: +2.05V – (-2.05V) = 4.1V.
2. Calculate the resolution needed:
4.1V/1 mV = 4100 steps
Note that 2
12
= 4096 for 12-bit resolution.
3. The amplifier gain (R
2
/R
1
), multiplied by V
DD
,
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
DD
value must be selected first. If a V
DD
of
4.1V is used, solve for the amplifier’s gain by
setting the DAC code to 0, knowing that the out-
put 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.
R
2
–
R
1
---------
2.05–
V
DD
-------------
2.05–
4.1
-------------==
R
2
R
1
------
1
2
---=
→
If R
1
= 20 kΩ and R
2
= 10 kΩ, the gain will be 0.5.
R
4
R
3
R
4
+()
------------------------
2.05V 0.5 V
DD
⋅
()+
1.5 V
DD
⋅
-------------------------------------------------
2
3
---==
If R
4
= 20 kΩ, then R
3
= 10 kΩ