MCP4921 datasheet(21/40 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21897A-page 21

MCP4921/4922

6.4

Single-Supply Operation

The MCP492X is a rail-to-rail (R-R) input and output

DAC designed to operate with a VDD range of 2.7V to

5.5V. Its output amplifier is robust enough to drive com-

mon, small-signal loads directly, thus eliminating the

cost and size of an external buffer for most applications.

6.4.1

DC SET POINT OR CALIBRATION

A common application for a DAC with the MCP492X’s

performance is digitally-controlled set points and/or

calibration of variable parameters, such as sensor off-

set or slope. 12-bit resolution provides 4096 output

steps. If a 4.096V VREF is provided, an LSB would

represent 1 mV of resolution. If a smaller output step

size is desired, the output range would need to be

reduced.

6.4.1.1

Decreasing The Output Step Size

If the output range is reduced relative to AVSS, simply

reducing VREF will reduce the magnitude of each out-

put step. If the application is calibrating the threshold

of a diode, transistor or resistor tied to AVSS or VREF,

a theshold range of 0.8V may be desired to provide

200 µV resolution. 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. If a VREF is avail-

able with the desired output value, using that VREF is an

option. Occasionally, when using a low-voltage VREF,

the noise floor causes SNR error that is intolerable.

The voltage divider method 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. Using a com-

mon VREF output has availability and cost advantages.

Example 6-1 illustrates this concept. Note that the volt-

age divider can be connected to AVSS or VREF,

depending on the application’s requirements.

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

is critical to meeting calibration accuracy in production.

EXAMPLE 6-1:

Set Point or Threshold Calibration.

VREF

MCP492X

VDD

SPI™

Vtrip

0.1 uF

Comparator

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

OUT

REFG

-------

VCC+

VCC–

VOUT

trip

OUT

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





VDD

Rsense

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