MCP4921 datasheet(26/40 Pages) MICROCHIP

26 / 40 page

MCP4921/4922

DS21897A-page 26

 2004 Microchip Technology Inc.

6.8

Building A Programmable Current

Source

Example 6-6 illustrates a variation on a voltage follower

design where a sense resistor is used to convert the

DAC’s 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.

6.9

Using Multiplier Mode

The MCP492X is ideally suited for use as a multiplier/

divider in a signal chain. Common applications include:

precision programmable gain/attenuator amplifiers and

loop controls (motor feedback). The wide input range

(0V – VDD) is an Unbuffered mode and near R-R range

in Buffered mode: the > 400 kHz bandwidth, selectible

1x/2x gain and its low power consumption give

maximum flexibility to meet the application's needs.

To configure the MCP492X in Multiplier mode, connect

the input signal to VREF and serially configure the

DAC’s input buffer, gain and output value. The DAC’s

output can utilize any of Examples 6-1 to 6-6, depend-

ing on the application requirements. Example 6-7 is an

illustration of how the DAC can operate in a motor

control feedback loop.

If the Gain Select bit is configured for 1x mode (GA =1),

the resulting input signal will be attenuated by D/4096.

If the Gain Select bit is configured for 2x mode (GA =0),

codes < 2048 attenuate the signal, while codes > 2048

gain the signal. VOUT = VIN (D/2048).

A 12-bit DAC provides significantly more gain/attenua-

tion resolution when compared to typical Programmable

Gain Amplifiers. Adding an op amp to buffer the output,

as illustrated in Examples 6-2 to 6-6, extends the

output range and power to meet the precise needs of

the application.

EXAMPLE 6-7:

Multiplier Mode.

VREF

MCP492X

Rsense

LOAD

VDD

SPI™

VCC+

VCC–

VOUT

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

OUT

se nse

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

β 1

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

OUT

REFG

-------

----

VCC+

VCC–

VREF

MCP492X

VRPM

–

VDD

SPI™

VOUT

Rsense

VRPM_SET

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