MCP4921 datasheet(28/50 Pages) MICROCHIP

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MCP4901/4911/4921

DS22248A-page 28

 2010 Microchip Technology Inc.

6.4

Single-Supply Operation

The MCP4901/4911/4921 devices are rail-to-rail volt-

age 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 an external output buffer for most

applications.

6.4.1

DC SET POINT OR CALIBRATION

common

application

for

DAC

devices

digitally-controlled set points and/or calibration of

variable parameters, such as sensor offset or slope.

For example, the MCP4921 and MCP4922 provide

4096 output steps. If the external voltage reference

(VREF) is 4.096V, the LSb size is 1 mV. If a smaller

output step size is desired, a lower external voltage

reference is needed.

6.4.1.1

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 is to either

reduce VREF to 0.82V 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 an 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

environment.

EXAMPLE 6-1:

EXAMPLE CIRCUIT OF SET POINT OR THRESHOLD CALIBRATION.

VDD

SPI

3-wire

VTRIP

0.1 uF

Comparator

G = Gain selection (1x or 2x)

Dn = Digital value of DAC (0-255) for MCP4901/MCP4902

OUT

REF

------



VCC+

VCC–

VOUT

trip

OUT

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







VDD

RSENSE

DAC

= Digital value of DAC (0-1023) for MCP4911/MCP4912

= Digital value of DAC (0-4095) for MCP4921/MCP4922

N = DAC Bit Resolution

VREF

MCP4901

MCP4911

MCP4921

(a) Single Output DAC:

(b) Dual Output DAC:

MCP4902

MCP4912

MCP4922

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