MCP48CMB21 datasheet(80/106 Pages) MICROCHIP

80 / 106 page

MCP48CXBXX

DS20006160A-page 80

 2019 Microchip Technology Inc.

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.

As an example, if a similar application to the one

developed in Section 8.4.1 “Bipolar DAC Example”

required a resolution of 1 µV instead of 1 mV, and a

range of 0V to 4.1V, then a 12-bit resolution would not

be adequate.

FIGURE 8-6:

Simple Double Precision

DAC Using MCP48CVBX2.

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 is dissipated

across it. However, this also reduces the resolution that

the current can be controlled at.

FIGURE 8-7:

Digitally-Controlled Current

Source.

8.7

Serial Interface Communication

Times

Table 7-2 shows the time/frequency of the supported

operations of the SPI Serial Interface for the different

serial interface operational frequencies. This, along

with the VOUT output performance (such as slew rate),

would be used to determine your application’s volatile

DAC register update rate.

Step 1: Calculate the resolution needed:

4.1V/1 µV = 4.1 x 106

Since 222 =4.2 x106, a 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

2-wire

VREF

Optional

MCP48CVBX2

VDD

SPI

2-wire

VREF

Optional

MCP48CVBX2

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

Rsense

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



 1

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





----

Common-Emitter Current Gain

where

VDD

SPI

2-wire

VREF

Optional

MCP48CVBXX

VDD

(or VREF)

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