MCP4802 datasheet(26/46 Pages) MICROCHIP

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MCP4802/4812/4822

DS22249A-page 26

 2010 Microchip Technology Inc.

6.5

Single-Supply Operation

The MCP4802/4812/4822 family of devices are rail-to-

rail voltage output DAC devices designed to operate

with a VDD range of 2.7V to 5.5V. Its output amplifier is

robust enough to drive small-signal loads directly.

Therefore, it does not require any external output buffer

for most applications.

6.5.1

DC SET POINT OR CALIBRATION

A common application for the devices is a digitally-

controlled set point and/or calibration of variable

parameters, such as sensor offset or slope. For

example, the MCP4822 provides 4096 output steps. If

G = 1 is selected, the internal 2.048V VREF would

produce 500 µV of resolution. If G = 2 is selected, the

internal 2.048 VREF would produce 1 mV of resolution.

6.5.1.1

Decreasing Output Step Size

If the application is calibrating the bias voltage of a

diode or transistor, a bias voltage range of 0.8V may be

desired with about 200 µV resolution per step. Two

common methods to achieve a 0.8V range are to either

reduce VREF to 0.82V (using the MCP49XX family

device that uses external reference) or use a voltage

divider on the DAC’s output.

Using a VREF is an option if the VREF is available with

the

desired

output

voltage

range.

However,

occasionally, when using a low-voltage VREF, the noise

floor causes SNR error that is intolerable. Using a

voltage divider method is another option and provides

some advantages when VREF needs to be very low or

when the desired output voltage is not available. In this

case, a larger value VREF is used while two resistors

scale the output range down to the precise desired

level.

Example 6-1 illustrates this concept. Note that the

bypass capacitor on the output of the voltage divider

plays a critical function in attenuating the output noise

of the DAC and the induced noise from the environ-

ment.

EXAMPLE 6-1:

EXAMPLE CIRCUIT OF SET POINT OR THRESHOLD CALIBRATION

VDD

SPI

3-wire

VTRIP

0.1 µF

Comparator

VOUT

2.048 G

------



VCC+

VCC–

VOUT

Vtrip

VOUT

R1 R2

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







VDD

RSENSE

DAC

(a) Single Output DAC:

MCP4801

MCP4811

MCP4821

(b) Dual Output DAC:

MCP4802

MCP4812

MCP4822

= Gain selection (1x or 2x)

Dn = Digital value of DAC (0-255) for MCP4801/MCP4802

= Digital value of DAC (0-1023) for MCP4811/MCP4812

= Digital value of DAC (0-4095) for MCP4821/MCP4822

= DAC bit resolution

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