MCP4921 datasheet(22/40 Pages) MICROCHIP

22 / 40 page

MCP4921/4922

DS21897A-page 22

 2004 Microchip Technology Inc.

6.4.1.2

Building a “Window” DAC

When calibrating a set point or threshold of a sensor,

rarely does the sensor utilize the entire output range of

the DAC. If the LSB size is adequate to meet the appli-

cation’s accuracy needs, then the resolution is sacri-

ficed without consequences. If greater accuracy is

needed, then the output range will need to be reduced

to increase the resolution around the desired threshold.

If the threshold is not near VREF or AVSS, then creating

a “window” around the threshold has several advan-

tages. One simple method to create this “window” is to

use a voltage divider network with a pull-up and pull-

down

resistor.

Example 6-2

and

Example 6-4

illustrates this concept.

The MCP492X’s low, ±0.75 (max.) DNL performance

is critical to meet calibration accuracy in production.

EXAMPLE 6-2:

Single-Supply “Window” DAC.

VREF

MCP492X

VDD

SPI™

Vtrip

0.1 µF

Comparator

VCC-

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

VCC+

VCC-

VOUT

OUT

REFG

-------

2R3

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

CC+R2

()

CC-R3

()

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

tr ip

OUTR23

23R1

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

R23

V23

VOUT

Thevenin

Equivalent

Rsense

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