MCP4821 **MCP4822**

6.8

Designing A Double-Precision

1.

Calculate the resolution needed:

4.1V/1 µV = 4.1e06. Since 2

22

= 4.2e06, 22-bit

resolution is desired. Since DNL = ±0.75 LSb,

this design can be attempted with the MCP482X

family.

Since DAC

B

‘s V

OUTB

has a resolution of 1 mV,

its output only needs to be “pulled” 1/1000 to

meet the 1 µV target. Dividing V

OUTA

by 1000

would allow the application to compensate for

B

‘s DNL error.

If R

2

is 100Ω, then R

1

needs to be 100 kΩ.

The resulting transfer function is shown in the

equation of Example 6-5.

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

developed in

Section 6.6.1 “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.

2.

3.

4.

V

DD

A

(Fine Adjust)

V

OUTA

R

1

>> R

2

V

OUTB

R

2

0.1 µF

B

(Course Adjust)

SPI™

3

D

A

V

OUTA

=

2.048V

⋅

G

A

-------

12

2

V

OUTA

R

2

+

V

OUTB

R

1

V

O

= -----------------------------------------------------

R

1

+

R

2

D

B

V

OUTB

=

2.048V

⋅

G

B

-------

12

2

V

CC

+

V

O

R

1

MCP482X

V

DD

MCP482X

V

CC

–

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

EXAMPLE 6-5:

Simple, Double-Precision DAC

DS21953A-page 24

©

2005 Microchip Technology Inc.