MCP48FVB21 datasheet(63/84 Pages) MICROCHIP

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 2015 Microchip Technology Inc.

DS20005466A-page 63

MCP48FVBXX

8.2.1.2

Building a “Window” DAC

When calibrating a set point or threshold of a sensor,

typically only a small portion of the DAC output range is

utilized. If the LSb size is adequate enough to meet the

application’s accuracy needs, the unused range is

sacrificed without consequences. If greater accuracy is

needed, then the output range will need to be reduced

to increase the resolution around the desired threshold.

If the threshold is not near VREF, 2  VREF, or VSS, then

creating a “window” around the threshold has several

advantages. One simple method to create this “window”

is to use a voltage divider network with a pull-up and

pull-down resistor. Figures 8-3 and 8-5 illustrate this

concept.

FIGURE 8-3:

Single-Supply “Window”

DAC.

EQUATION 8-2:

VOUT AND VTRIP

CALCULATIONS

8.3

Bipolar Operation

Bipolar operation is achievable by utilizing an external

operational amplifier. 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.

Figure 8-4 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.

FIGURE 8-4:

Digitally-Controlled Bipolar

Voltage Source Example Circuit.

EQUATION 8-3:

VOUT, VOA+, AND VO

CALCULATIONS

VCC+

VCC–

SPI

4-wire

VREF

Optional

MCP48FVBXX

VDD

VOUT

VCC+

VCC–

RSENSE

Comp.

VTRIP

2R3

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

CC+R2



CC-R3



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

TRIP

OUTR23

23R1

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

Thevenin

Equivalent

R23

V23

VOUT

VTRIP

VOUT = VREF • G •

DAC Register Value

VCC+

VCC–

SPI

4-wire

VREF

Optional

MCP48FVBXX

VDD

VOUT

VIN

VOA+

VOA+ =

VOUT • R4

R3 + R4

VOUT = VREF • G •

DAC Register Value

VO = VOA+ • (1 +

) - VDD • (

)

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