MCP4821 datasheet(28/50 Pages) MICROCHIP

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MCP4802/4812/4822

DS20002249B-page 28

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6.6

Bipolar Operation

Bipolar

operation

achievable

using

the

MCP4802/4812/4822 family of devices by utilizing an

external

operational

amplifier

(op

amp).

This

configuration is desirable due to the wide variety and

availability of op amps. This allows a general purpose

DAC, with its cost and availability advantages, to meet

almost any desired output voltage range, power and

noise performance.

Example 6-3 illustrates a simple bipolar voltage source

configuration. R1 and R2 allow the gain to be selected,

while R3 and R4 shift the DAC's output to a selected

offset. Note that R4 can be tied to VDD, instead of VSS,

if a higher offset is desired. Also note that a pull-up to

VDD could be used instead of R4, or in addition to R4, if

a higher offset is desired.

EXAMPLE 6-3:

DIGITALLY-CONTROLLED BIPOLAR VOLTAGE SOURCE

6.6.1

DESIGN EXAMPLE: DESIGN A

BIPOLAR DAC USING Example 6-3

WITH 12-BIT MCP4822 OR

MCP4821

An output step magnitude of 1 mV, with an output range

of ±2.05V, is desired for a particular application.

Step 1: Calculate the range: +2.05V – (-2.05V) = 4.1V.

Step 2: Calculate the resolution needed:

4.1V/1 mV = 4100

Since 212 = 4096, 12-bit resolution is

desired.

Step 3:The amplifier gain (R2/R1), multiplied by full-

scale VOUT (4.096V), must be equal to the

desired minimum output to achieve bipolar

operation. Since any gain can be realized by

choosing resistor values (R1+R2), the VREF

value must be selected first. If a VREF of 4.096V

is used (G=2), solve for the amplifier’s gain by

setting the DAC to 0, knowing that the output

needs to be -2.05V.

The equation can be simplified to:

Step 4: Next, solve for R3 and R4 by setting the DAC to

4096, knowing that the output needs to be

+2.05V.

DAC

VDD

SPI

3-wire

VOUT

VIN+

0.1 µF

VCC+

VCC–

IN+

OUTR4

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

IN+ 1

------





------





–

OUT

2.048 G

------



(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

–

---------

2.05

–

4.096V

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

If R1 = 20 k and R2 = 10 k, the gain will be 0.5.

------

---



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

2.05V

0.5 4.096V





1.5 4.096V



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

---

If R4 = 20 k, then R3 = 10 k

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