MCP4921 datasheet(33/50 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS22248A-page 33

MCP4901/4911/4921

6.8

Building Programmable Current

Source

Example 6-6 shows an example for 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 is 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 the 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

OUT

sense

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



 1

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





----

G = Gain select (1x or 2x)

Dn = Digital value of DAC (0-255) for MCP4901/MCP4902

OUT

REF

------



= Digital value of DAC (0-1023) for MCP4911/MCP4912

= Digital value of DAC (0-4095) for MCP4921/MCP4922

N = DAC Bit Resolution

Common-Emitter Current Gain

where

VREF

VDD or VREF

MCP4901

MCP4911

MCP4921

(a) Single Output DAC:

(b) Dual Output DAC:

MCP4902

MCP4912

MCP4922

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