MCP48FVB21 datasheet(65/84 Pages) MICROCHIP

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DS20005466A-page 65

MCP48FVBXX

8.5

Designing a Double-Precision

DAC

Figure 8-6 shows an example design of a single-supply

voltage output capable of up to 24-bit resolution. This

requires two 12-bit DACs. This design is simply a

voltage divider with a buffered output.

Double-Precision DAC Example

If a similar application to the one developed in Bipolar

DAC Example required a resolution of 1 µV instead of

1 mV and a range of 0V to 4.1V, then 12-bit resolution

would not be adequate.

FIGURE 8-6:

Simple Double-Precision

DAC using MCP48FVBX2.

EQUATION 8-8:

VOUT CALCULATION

8.6

Building Programmable Current

Source

Figure 8-7

shows

example

building

programmable current source using a voltage follower.

The current sensor resistor is used to convert the DAC

voltage output into a digitally-selectable current source.

The smaller RSENSE is, the less power dissipated

across it. However, this also reduces the resolution that

the current can be controlled.

FIGURE 8-7:

Digitally-Controlled Current

Source.

Step 1:

Calculate the resolution needed:

4.1V/1 µV = 4.1 x 106.

Since 222 =4.2 x 106, 22-bit resolution is

desired. Since DNL = ±1.0 LSb, this design

can be attempted with the 12-bit DAC.

Step 2:

Since DAC1’s VOUT1 has a resolution of

1 mV, its output only needs to be “pulled”

1/1000 to meet the 1 µV target. Dividing

VOUT0 by 1000 would allow the application

to compensate for DAC1’s DNL error.

Step 3:

If R2 is 100, then R1 needs to be 100 k.

Step 4:

The resulting transfer function is shown in

Equation 8-8.

VCC+

VCC–

VOUT

SPI

4-wire

VREF

Optional

MCP48FVBX2

VDD

SPI

4-wire

VREF

Optional

MCP48FVBX2

VDD

0.1 µF

VOUT0

VOUT1

(DAC0)

(DAC1)

VOUT =

Gx = Selected Op Amp Gain

VOUT0 • R2 + VOUT1 • R1

R1 + R2

VOUT0 = (VREF • G • DAC0 Register Value)/4096

VOUT1 = (VREF • G • DAC1 Register Value)/4096

Where:

RSENSE

Load

VCC+

VCC–

VOUT



----

Common-Emitter Current Gain

where

VDD

SPI

4-wire

VREF

Optional

MCP48FVBXX

VDD

(or VREF)

OUT

SEN SE

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



 1

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



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