MCP4822-ESLASHMS datasheet(22/36 Pages) MICROCHIP

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MCP4821/MCP4822

DS21953A-page 22

6.6

Bipolar Operation

Bipolar operation is achievable using the MCP482X

devices by using an external operational amplifier (op

amp). This configuration is desirable due to the wide

variety and availability of op amps. This allows a gen-

eral purpose DAC, with its cost and availability advan-

tages, 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 AVSS,

if a higher offset is desired. 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 A BIPOLAR DAC USING

EXAMPLE 6-3

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

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

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

Calculate the resolution needed:

4.1V/1 mV = 4100

Since 212 = 4096, 12-bit resolution is desired.

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:

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

4096, knowing that the output needs to be

+2.05V.

MCP482X

VDD

SPI™

VOUT

VIN+

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

0.1 µF

VCC+

VCC–

OUT

2.048 G

-------

⋅

IN+

OUTR4

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

IN+ 1

------

⎝⎠

⎛⎞

------

⎝⎠

⎛⎞

–

---------

2.05

–

4.096V

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

If R1 = 20 kΩ and R2 = 10 kΩ, the gain will be 0.5

------

---

()

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

2.05V

0.5 4.096V

⋅

()

1.5 4.096V

⋅

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

---

If R4 = 20 kΩ, then R3 = 10 kΩ

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