MCP4902-ESLASHST datasheet(30/48 Pages) MICROCHIP

30 / 48 page

MCP4902/4912/4922

DS22250A-page 30

 2010 Microchip Technology Inc.

6.5

Bipolar Operation

Bipolar operation is achievable using the MCP4902/

4912/4922 family of devices 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 VSS,

if a higher offset is desired. Also 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 EXAMPLE: DESIGN A BIPOLAR

DAC USING EXAMPLE 6-3 WITH 12-BIT

MCP4922 OR MCP4921

An output step magnitude of 1 mV with an output range

of ±2.05V is desired for a particular application.

The following steps show the details:

Step 1: Calculate the range: +2.05V – (-2.05V) = 4.1V.

Step 2: Calculate the resolution needed:

4.1V/1 mV = 4100

Since 212 = 4096, 12-bit resolution is

desired.

Step 3: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 deter-

mined 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:

Step 4: Next, solve for R3 and R4 by setting the DAC to

4096, knowing that the output needs to be

+2.05V.

VREF

VDD

SPI

VOUT

VIN+

0.1 µF

VCC+

VCC–

VIN+

VOUTR4

R3 R4

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

VIN+ 1

------



 V

------





–

G = Gain selection (1x or 2x)

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

VOUT

VREF G

------



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

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

N = DAC Bit Resolution

DAC

MCP4901

MCP4911

MCP4921

(a) Single Output DAC:

(b) Dual Output DAC:

MCP4902

MCP4912

MCP4922

–

---------

2.05

–

VREF

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

2.05

–

4.1

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

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

------

---

R3 R4



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

2.05V 0.5VREF

1.5VREF

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

---

If R4 = 20 k, then R3 = 10 k

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