MCP48FVB21 datasheet(64/84 Pages) MICROCHIP

64 / 84 page

MCP48FVBXX

DS20005466A-page 64

 2015 Microchip Technology Inc.

8.4

Selectable Gain and Offset Bipolar

Voltage Output

In some applications, precision digital control of the

output range is desirable. Figure 8-5 illustrates how to

use 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.

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 8-5:

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

bipolar operation. Since any gain can be

realized

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

SPI

4-wire

VREF

Optional

MCP48FVBXX

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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