MCP4902-ESLASHST datasheet(32/48 Pages) MICROCHIP

32 / 48 page

MCP4902/4912/4922

DS22250A-page 32

 2010 Microchip Technology Inc.

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 application similar to the one

developed in Section 6.5.1 “Design Example: Design

a Bipolar DAC Using Example 6-3 with 12-bit

MCP4922 or MCP4921” 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 = 4.1x106. Since 222 = 4.2x106, 22-

bit resolution is desired. Since DNL = ±0.75

LSb, this design can be attempted with the

MCP4922.

Step 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.

Step 3: If R2 is 100, then R1 needs to be 100 k.

Step 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 WITH MCP4922

VREF

MCP4922

VDD

MCP4922

VDD

DACA (Fine Adjust)

DACB (Course Adjust)

SPI

R1 >> R2

VOUTAR2 VOUTBR1

R1 R2

-----------------------------------------------------

G = Gain selection (1x or 2x)

D = Digital value of DAC (0- 4096)

0.1 µF

VCC+

VCC–

VOUTA

VREFAGA

-------

VOUTB

VREFBGB

-------

VOUTA

VOUTB

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