MCP4017 datasheet(49/66 Pages) MICROCHIP

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DS22147A-page 49

MCP4017/18/19

8.0

APPLICATIONS EXAMPLES

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 MCP4017/18/19 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

applications, 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 is not

necessary to create the voltage divider, but its

presence is useful when the desired threshold has

limited range. It is “windowed” because R1 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 MCP4018’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 a resolution of 2 mV or

better is adequate to accurately detect the presence of

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

the MCP4018, would be a potential solution. Figure 8-

2 illustrates this example application.

FIGURE 8-2:

Set Point or Threshold

Calibration.

VTRIP

VDD

RWB

R1 R2

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

⎝⎠

⎛⎞

D = Digital Potentiometer Wiper Setting (0-127)

RAB = RNominal

RWB = RAB • D

127

D =

• (R1 + RAB ) • 127

VTRIP

VDD

VOUT

MCP4018

SDA

SCL

VTRIP

0.1 µF

Comparator

VCC+

VCC–

VDD

Rsense

VDD

MCP4018

SDA

SCL

MCP6021

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