MCP47FVB01 datasheet(65/92 Pages) MICROCHIP

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DS20005405A-page 65

MCP47FVBXX

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.

------

---



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

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

MCP47FVBXX

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

CC+R4

CC-R5

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

IN+

OUTR45

45R3

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

4R5

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

IN+ 1

------





------





–

Offset Adjust Gain Adjust

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