MCP4716 datasheet(68/86 Pages) MICROCHIP

68 / 86 page

MCP4706/4716/4726

DS22272A-page 68

8.6

Designing a Double-Precision

DAC

Figure 8-7 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.

As an example, if a similar application to the one

developed in Section 8.5.1 “Bipolar DAC Example

Using MCP4726” 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.

Step 1: Calculate the resolution needed:

4.1V/1 µV = 4.1 x 106.

Since

222 =4.2 x106,

22-bit

resolution

desired.

Since

DNL = ±0.75 LSb, this design can be attempted

with the 12-bit DAC.

Step 2: Since DACB‘s VOUTB has a resolution of 1 mV,

its output only needs to be “pulled” 1/1000 to meet

the 1 µV target. Dividing VOUTA by 1000 would

allow the application to compensate for DACB‘s

DNL error.

Step 3: If R2 is 100Ω, then R1 needs to be 100 kΩ.

Step 4: The resulting transfer function is shown in the

equation of Example 8-6.

FIGURE 8-7:

Simple Double Precision

DAC using MCP4726.

EQUATION 8-6:

VOUT CALCULATION

8.7

Building Programmable Current

Source

Example 8-8

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 8-8:

Digitally-Controlled Current

Source.

VCC+

VCC–

VOUT

I2C™

2-wire

VREF

Optional

MCP4726 (A)

VDD

I2C™

2-wire

VREF

Optional

MCP4726 (B)

VDD

0.1 µF

VOA

VOB

VOUT =

G = Selected Op Amp Gain

VOA * R2 + VOB * R1

R1 + R2

VOA = (VREF * G * DAC A Register Value)/4096

VOB = (VREF * G * DAC B Register Value)/4096

Where:

RSENSE

Load

VCC+

VCC–

VOUT

OUT

sense

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

β 1

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

----

β = Common-Emitter Current Gain.

where

VDD

I2C™

2-wire

VREF

Optional

MCP47X6

VDD

(or VREF)

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