MCP4921 datasheet(32/50 Pages) MICROCHIP

32 / 50 page

MCP4901/4911/4921

DS22248A-page 32

 2010 Microchip Technology Inc.

6.7

Designing a Double-Precision

DAC

Example 6-5 illustrates how to design a single-supply

voltage output capable of up to 24-bit resolution by

using 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

Section 6.5.1

“Design

Example:

Design a bipolar dac using example 6-3 with 12-bit

MCP4912 or MCP4922” 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.

Calculate the resolution needed:

4.1V/1 µV = 4.1x 106. Since 222 = 4.2 x 106,

22-bit resolution is desired. Since DNL = ±0.75

LSB, this design can be done with the MCP4921

or MCP4922.

Since the 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.

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

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 MCP4921 OR MCP4922.

VREF

DACB

VDD

DACA

VDD

DACA (Fine Adjust)

DACB (Course Adjust)

SPI

R1 >> R2

OUTAR2

OUTBR1

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

G = Gain selection (1x or 2x)

D = Digital value of DAC (0-4096)

0.1 µF

VCC+

VCC–

OUTA

REFA GA

-------

OUTB

REFB GB

-------

VOUTA

VOUTB

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