MCP4821 datasheet(30/48 Pages) MICROCHIP

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MCP4801/4811/4821

DS22244B-page 30

 2010 Microchip Technology Inc.

6.8

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 an application similar to the one

developed

Section 6.6.1

“Design

Example:

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

bit MCP4821 or MCP4822” 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 106.

Since

222 =4.2 x 106,

22-bit

resolution

desired.

Since

DNL = ±0.75 LSb, this design can be done with

the 12-bit MCP4821 or MCP4822 DAC devices.

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 shown in the

equation of Example 6-5.

EXAMPLE 6-5:

SIMPLE, DOUBLE-PRECISION DAC WITH MCP4821

VDD

VOUTA for Fine Adjustment

SPI

3-wire

R1 >> R2

OUTAR2

OUTBR1

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

0.1 µF

VCC+

VCC–

OUTA

2.048 G

-------



OUTB

2.048 G

-------



DACA

Gx = Gain selection (1x or 2x)

Dn = Digital value of DAC (0-4096)

DACB

VOUTB for Fine Adjustment

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