MCP4921 datasheet(23/40 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21897A-page 23

MCP4921/4922

6.5

Bipolar Operation

Bipolar operation is achievable using the MCP492X by

using an external operational amplifier (op amp). This

configuration is desirable due to the wide variety and

availability of op amps. This allows a general purpose

DAC, with its cost and availability advantages, to meet

almost any desired output voltage range, power and

noise performance.

Example 6-3 illustrates a simple bipolar voltage source

configuration. R1 and R2 allow the gain to be selected,

while R3 and R4 shift the DAC's output to a selected

offset. Note that R4 can be tied to VREF, instead of

AVSS, if a higher offset is desired. Note that a pull-up to

VREF could be used, instead of R4, if a higher offset is

desired.

EXAMPLE 6-3:

Digitally-Controlled Bipolar Voltage Source.

6.5.1

DESIGN A BIPOLAR DAC USING

EXAMPLE 6-3

An output step magnitude of 1 mV with an output range

of ±2.05V is desired for a particular application.

Calculate the range: +2.05V – (-2.05V) = 4.1V.

Calculate the resolution needed:

4.1V/1 mV = 4100

Since 212 = 4096, 12-bit resolution is desired.

The amplifier gain (R2/R1), multiplied by VREF,

must be equal to the desired minimum output to

achieve bipolar operation. Since any gain can

be realized by choosing resistor values (R1+R2),

the VREF source needs to be determined first. If

a VREF of 4.1V is used, solve for the gain by

setting the DAC to 0, knowing that the output

needs to be -2.05V.

The equation can be

simplified to:

Next, solve for R3 and R4 by setting the DAC to

4096, knowing that the output needs to be

+2.05V.

VREF

MCP492X

VREF

VDD

SPI™

VOUT

VIN+

G = Gain select (1x or 2x)

D = Digital value of DAC (0 – 4096)

0.1 µF

VCC+

VCC–

OUT

REFG

-------

IN+

OUTR4

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

IN+ 1

------





REF

------





–

---------

2.05

–

REF

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

2.05

–

4.1

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

If R1 = 20 kΩ and R2 = 10 kΩ, the gain will be 0.5.

------

---

()

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

2.05V

0.5V

REF

1.5V

REF

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

---

If R4 = 20 kΩ, then R3 = 10 kΩ

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