MCP4021 datasheet(48/64 Pages) MICROCHIP

48 / 64 page

MCP4021/2/3/4

DS21945E-page 48

8.0

APPLICATIONS EXAMPLES

Non-volatile digital potentiometers have a multitude of

practical uses in modern electronic circuits. The most

popular uses include precision calibration of set point

thresholds, sensor trimming, LCD bias trimming, audio

attenuation, adjustable power supplies, motor control

overcurrent trip setting, adjustable gain amplifiers and

offset trimming. The MCP4021/2/3/4 devices can be

used to replace the common mechanical trim pot in

applications where the operating and terminal voltages

are within CMOS process limitations (VDD = 2.7V to

5.5V).

8.1

Set Point Threshold Trimming

Applications that need accurate detection of an input

threshold event often need several sources of error

eliminated. Use of comparators and operational

amplifiers (op amps) with low offset and gain error can

help achieve the desired accuracy, but in many applica-

tions, the input source variation is beyond the

designer’s control. If the entire system can be

calibrated after assembly in a controlled environment

(like factory test), these sources of error are minimized,

if not entirely eliminated.

Figure 8-1 illustrates a common digital potentiometer

configuration. This configuration is often referred to as

a “windowed voltage divider”. Note that R1 and R2 are

not necessary to create the voltage divider, but their

presence is useful when the desired threshold has

limited range. It is “windowed” because R1 and R2 can

narrow the adjustable range of VTRIP to a value much

less than VDD – VSS. If the output range is reduced, the

magnitude of each output step is reduced. This

effectively increases the trimming resolution for a fixed

digital potentiometer resolution. This technique may

allow a lower-cost digital potentiometer to be utilized

(64 steps instead of 256 steps).

The

MCP4021’s

and

MCP4023’s

low

DNL

performance is critical to meeting calibration accuracy

in production without having to use a higher precision

digital potentiometer.

EQUATION 8-1:

CALCULATING THE

WIPER SETTING FROM

THE DESIRED VTRIP

FIGURE 8-1:

Using the Digital

Potentiometer to Set a Precise Output Voltage.

8.1.1

TRIMMING A THRESHOLD FOR AN

OPTICAL SENSOR

If the application has to calibrate the threshold of a

diode, transistor or resistor, a variation range of 0.1V is

common. Often, the desired resolution of 2 mV or

better is adequate to accurately detect the presence of

a precise signal. A “windowed” voltage divider, utilizing

the MCP4021 or

MCP4023, would be a potential

solution as shown in Figure 8-2.

FIGURE 8-2:

Set Point or Threshold

Calibration.

VTRIP

VDD

R2 RWB

R1 RAB R2

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

⎝⎠

⎛⎞

RAB

RNominal

RWB

RAB

------

⎝⎠

⎛⎞

•

VTRIP

VDD

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

⎝⎠

⎛⎞

R1 RAB R2

() R

–

()

•

⎝⎠

⎛⎞ 63

•

Where:

D = Digital Potentiometer Wiper Setting (0-63)

VDD

VOUT

MCP4021

U/D

VTRIP

0.1 µF

Comparator

VCC+

VCC–

VDD

Rsense

VDD

MCP4021

U/D

MCP6021

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