Datasheet
MCP4706/4716/4726
DS22272C-page 68 © 2011-2012 Microchip Technology Inc.
8.6 Designing a Double-Precision
DAC
Figure 8-7 shows an example design of a single-supply
voltage output capable of up to 24-bit resolution. This
requires two 12-bit DACs. This design is simply a
voltage divider with a buffered output.
As an example, if a similar application to the one
developed in Section 8.5.1 “Bipolar DAC Example
Using MCP4726” required a resolution of 1 µV instead
of 1 mV, and a range of 0V to 4.1V, then 12-bit
resolution would not be adequate.
Step 1: Calculate the resolution needed:
4.1V/1 µV = 4.1 x 10
6
. Since 2
22
=4.2x10
6
,
22-bit resolution is desired. Since
DNL = ±0.75 LSb, this design can be attempted
with the 12-bit DAC.
Step 2: Since DAC
B
’s V
OUTB
has a resolution of 1 mV,
its output only needs to be “pulled” 1/1000 to meet
the 1 µV target. Dividing V
OUTA
by 1000 would
allow the application to compensate for DAC
B
’s
DNL error.
Step 3: If R
2
is 100Ω, then R
1
needs to be 100 kΩ.
Step 4: The resulting transfer function is shown in the
equation of Example 8-6.
FIGURE 8-7: Simple Double Precision
DAC using MCP4726.
EQUATION 8-6: V
OUT
CALCULATION
8.7 Building Programmable Current
Source
Figure 8-8 shows an example of building a
programmable current source using a voltage follower.
The current sensor resistor is used to convert the DAC
voltage output into a digitally-selectable current source.
The smaller R
SENSE
is, the less power dissipated
across it. However, this also reduces the resolution that
the current can be controlled.
FIGURE 8-8: Digitally-Controlled Current
Source.
R
1
V
CC
+
V
CC
–
V
OUT
I
2
C™
2-wire
V
REF
Optional
MCP4726 (A)
V
DD
I
2
C™
2-wire
V
REF
Optional
MCP4726 (B)
V
DD
R
2
0.1 µF
V
OA
V
OB
V
OUT
=
G = Selected Op Amp Gain
V
OA
*
R
2
+ V
OB
*
R
1
R
1
+ R
2
V
OA
= (V
REF
* G * DAC A Register Value)/4096
V
OB
= (V
REF
* G * DAC B Register Value)/4096
Where:
R
SENSE
I
b
Load
I
L
V
CC
+
V
CC
–
V
OUT
I
L
V
OUT
R
sense
---------------
β
β
1+
-------------
×
=
I
b
I
L
β
----=
β = Common-Emitter Current Gain. where
V
DD
I
2
C™
2-wire
V
REF
Optional
MCP47X6
V
DD
(or V
REF
)