MCP47CMB02 datasheet(89/124 Pages) MICROCHIP

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DS20006089B-page 89

MCP47CXBXX

8.5

Selectable Gain and Offset Bipolar

Voltage Output

In some applications, precision digital control of the

output range is desirable. Figure 8-6 illustrates how to

use the DAC devices to achieve this in a bipolar or

single-supply application.

This circuit is typically used for linearizing a sensor

whose slope and offset varies.

The equation to design a bipolar “window” DAC would

be utilized if R3, R4 and R5 are populated.

8.5.1

BIPOLAR DAC EXAMPLE

An output step-size of 1 mV, with an output range of

±2.05V, is desired for a particular application.

The equation can be simplified to:

EQUATION 8-4:

EQUATION 8-5:

FIGURE 8-6:

Bipolar Voltage Source with

Selectable Gain and Offset.

EQUATION 8-6:

VOUT, VOA+ AND VO

CALCULATIONS

EQUATION 8-7:

BIPOLAR “WINDOW” DAC

USING R4 AND R5

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 bipo-

lar 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, solve for the amplifier’s

gain by setting the DAC to 0, knowing that

the output needs to be -2.05V.

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

DAC to 4096, knowing that the output

needs to be +2.05V.

–

---------

2.05

–

4.096V

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

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

------

---

R3 R4



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

2.05V

0.5 4.096V





1.5 4.096V



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

---

If R4 = 20 k, then R3 = 10 k.

VCC+

VCC-

VOUT

I2C

2-Wire

VREF

Optional

MCP47CVBXX

VDD

VIN

Optional

VOA+

VCC+

VCC-

C1 = 0.1 µF

Offset Adjust

Gain Adjust

VOUT = VREF • G •

DAC Register Value

VOA+ =

VOUT • R4 + VCC- • R5

R3 + R4

VO = VOA+ • (1 +

) – VIN • (

)

Thevenin

Equivalent

V45

VCC+R4 VCC-R5

R4 R5

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

VIN+

VOUTR45 V45R3

R3 R45

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

R45

R4R5

R4 R5

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

VIN+ 1

------



 V

------





–

Offset Adjust Gain Adjust

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