MCP4921 datasheet(25/40 Pages) MICROCHIP

25 / 40 page

 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.

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

attempted

with

the

MCP492X.

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.

If R2 is 100Ω, then R1 needs to be 100 kΩ.

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

MCP492X

VDD

DACA (Fine Adjust)

DACB (Course Adjust)

SPI™

R1 >> R2

OUTAR2

OUTBR1

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

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

0.1 µF

VCC+

VCC–

OUTA

REFAGA

-------

OUTB

REFBGB

-------

VOUTA

VOUTB

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