MCP4802-ESLASHMS datasheet(31/46 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS22249A-page 31

MCP4802/4812/4822

6.9

Building Programmable Current

Source

Example 6-6 shows an example of building a

programmable current source using a voltage follower.

The current sensor (sensor resistor) is used to convert

the DAC voltage output into a digitally-selectable

current source.

Adding the resistor network from Example 6-2 would

be advantageous in this application. The smaller

RSENSE is, the less power dissipated across it.

However, this also reduces the resolution that the

current can be controlled with. The voltage divider, or

“window”, DAC configuration would allow the range to

be reduced, thus increasing resolution around the

range of interest. When working with very small sensor

voltages, plan on eliminating the amplifier’s offset error

by storing the DAC’s setting under known sensor

conditions.

EXAMPLE 6-6:

DIGITALLY-CONTROLLED CURRENT SOURCE

DAC

RSENSE

Load

VDD

SPI

3-wire

VCC+

VCC–

VOUT

Rsense

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



 1

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





----

Common-Emitter Current Gain

where

VDD or VREF

(a) Single Output DAC:

MCP4801

MCP4811

MCP4821

(b) Dual Output DAC:

MCP4802

MCP4812

MCP4822

= Gain selection (1x or 2x)

Dn = Digital value of DAC (0-255) for MCP4801/MCP4802

= Digital value of DAC (0-1023) for MCP4811/MCP4812

= Digital value of DAC (0-4095) for MCP4821/MCP4822

N = DAC bit resolution

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