MCP48CMB02 datasheet(77/106 Pages) MICROCHIP

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DS20006160A-page 77

MCP48CXBXX

8.2

Application Examples

The MCP48CXBXX 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 select the gain of 1 or

2 of the output op amp by setting the Configuration

can be used. 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

• Development Support

• Power Supply Considerations

• Layout Considerations

8.2.1

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 MCP48CVB2X provides 4096 output

steps. If 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.

8.2.1.1

Decreasing Output Step Size

If the application calibrates 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 the 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 8-2 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 8-2:

Example Circuit Of Set Point

or Threshold Calibration.

EQUATION 8-1:

VOUT AND VTRIP

CALCULATIONS

VCC+

VCC–

VOUT

SPI

2-wire

VREF

Optional

MCP48CVBXX

VDD

RSENSE

Comp.

VDD

VTRIP

Vtrip VOUT

R1 R2

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







VOUT = VREF • G •

DAC Register Value

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