MCP47CMB02 datasheet(87/124 Pages) MICROCHIP

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DS20006089B-page 87

MCP47CXBXX

8.3

Application Examples

The MCP47CXBXX devices are rail-to-rail output

DACs designed to operate with a VDD range of 2.7V to

5.5V. The internal output amplifier is robust enough to

drive common, small signal loads directly, thus

eliminating the cost and size of the external buffers for

most applications. The user can use the gain of 1 or 2

of the output op amp by setting the Configuration

can be used. There are various user options and easy-

to-use features that make the devices suitable for

various modern DAC applications.

Application examples include:

• Decreasing Output Step-Size

• Building a “Window” DAC

• Bipolar Operation

• Selectable Gain and Offset Bipolar Voltage Output

• Designing a Double Precision DAC

• Building Programmable Current Source

• Serial Interface Communication Times

• Software I2C Interface Reset Sequence

• Power Supply Considerations

• Layout Considerations

8.3.1

DC SET POINT OR CALIBRATION

A common application for the devices is a digitally

controlled set point and/or calibration of variable

parameters, such as sensor offset or slope. For

example, the MCP47CVB2X provides 4096 output

steps. If the voltage reference is 4.096V (where Gx = 0),

the LSb size is 1 mV. If a smaller output step-size is

desired, a lower external voltage reference is needed.

8.3.1.1

Decreasing Output Step-Size

If the application is calibrating the bias voltage of a

diode or transistor, a bias voltage range of 0.8V may be

desired, with about 200 µV resolution per step. Two

common methods to achieve small step-size are to use

a lower VREF pin voltage or a voltage divider on the

DAC’s output.

Using an external Voltage Reference (VREF) is an

option if the external reference is available with the

desired output voltage range. However, when using a

low-voltage reference voltage, occasionally the noise

floor causes an SNR error that is intolerable. Using a

voltage divider method is another option, and provides

some advantages when external voltage reference

needs to be very low, or when the desired output

voltage is not available. In this case, a larger value

reference voltage is used, while two resistors scale the

output range down to the precise desired level.

Figure 8-3 illustrates this concept. A bypass capacitor

on the output of the voltage divider plays a critical

function in attenuating the output noise of the DAC and

the induced noise from the environment.

FIGURE 8-3:

Example Circuit of Set Point

or Threshold Calibration.

EQUATION 8-1:

VOUT AND VTRIP

CALCULATIONS

VCC+

VCC-

VOUT

I2C

2-Wire

VREF

Optional

MCP47CVBXX

VDD

RSENSE

Comp.

VDD

VTRIP

Vtrip VOUT

R1 R2

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







VOUT = VREF • G •

DAC Register Value

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