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MCP4922T-EMS Datasheet(HTML) 25 Page - Microchip Technology

Part No. MCP4922T-EMS
Description  12-Bit DAC with SPI™ Interface
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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MCP4922T-EMS Datasheet(HTML) 25 Page - Microchip Technology

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 2004 Microchip Technology Inc.
DS21897A-page 25
MCP4921/4922
6.7
Designing A Double-Precision
DAC Using A Dual DAC
Example 6-5 illustrates how to design a single-supply
voltage output capable of up to 24-bit resolution from a
dual 12-bit DAC. This design is simply a voltage divider
with a buffered output.
As an example, if a similar application to the one devel-
oped in Section 6.5.1 “Design a bipolar dac using
Example 6-3” 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.
1.
Calculate the resolution needed:
4.1V/1uV = 4.1e06. Since 222 = 4.2e06, 22-bit
resolution is desired. Since DNL = ±0.75 LSB,
this
design
can
be
attempted
with
the
MCP492X.
2.
Since DACB‘s VOUTB has a resolution of 1 mV,
its output only needs to be “pulled” 1/1000 to
meet the 1 µV target. Dividing VOUTA by 1000
would allow the application to compensate for
DACB‘s DNL error.
3.
If R2 is 100Ω, then R1 needs to be 100 kΩ.
4.
The resulting transfer function is not perfectly
linear, as shown in the equation of Example 6-5.
EXAMPLE 6-5:
Simple, Double-Precision DAC.
VREF
MCP492X
VDD
R2
VO
MCP492X
VDD
R1
DACA (Fine Adjust)
DACB (Course Adjust)
SPI™
3
R1 >> R2
V
O
V
OUTAR2
V
OUTBR1
+
R
1
R
2
+
-----------------------------------------------------
=
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
0.1 µF
VCC+
VCC
V
OUTA
V
REFAGA
D
A
2
12
-------
=
V
OUTB
V
REFBGB
D
B
2
12
-------
=
VOUTA
VOUTB


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