MCP47FVB01 datasheet(63/92 Pages) MICROCHIP

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DS20005405A-page 63

MCP47FVBXX

8.3

Application Examples

The MCP47FVBXX devices are rail-to-rail output DACs

designed to operate with a VDD range of 2.7V to 5.5V.

The internal output op amplifier is robust enough to

drive common, small-signal loads directly, thus

eliminating the cost and size of external buffers for

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

of the output op amplifier by setting the Configuration

or use an external reference. Various user options and

easy-to-use features make the devices suitable for var-

ious 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 MCP47FVB2X 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.

8.3.1.1

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-volt-

age reference voltage, occasionally the noise floor

causes a 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

MCP47FVBXX

VDD

RSENSE

Comp.

VDD

VTRIP

trip

OUT

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







VOUT = VREF • G •

DAC Register Value

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