Microchip Technology ADM00317 Data Sheet

DS22272C-page 68

© 2011-2012 Microchip Technology Inc.

 shows an example design of a single-supply

voltage output capable of up to 24-bit resolution. This
requires two 12-bit DACs. This design is simply a
voltage divider with a buffered output.

As an example, if a similar application to the one
developed in 

 required a resolution of 1 µV instead

of 1 mV, and a range of 0V to 4.1V, then 12-bit
resolution would not be adequate.

Step 1: Calculate the resolution needed:

4.1V/1 µV = 4.1 x 10

6

. Since 2

22

= 4.2 x 10

6

,

22-bit resolution is desired. Since
DNL = ±0.75 LSb, this design can be attempted
with the 12-bit DAC. 

Step 2: Since DAC

B

’s V

OUTB

 has a resolution of 1 mV,

its output only needs to be “pulled” 1/1000 to meet
the 1 µV target. Dividing V

OUTA

 by 1000 would

allow the application to compensate for DAC

B

’s

DNL error.

Step 3: If R

2

 is 100

Ω, then R

1

 needs to be 100 k

Ω.

Step 4: The resulting transfer function is shown in the

equation of 

.

          

 shows an example of building a

programmable current source using a voltage follower.
The current sensor resistor is used to convert the DAC
voltage output into a digitally-selectable current source.

 

The smaller R

SENSE 

is, the less power dissipated

across it. However, this also reduces the resolution that
the current can be controlled. 

      

R

1

V

CC

+

V

CC

–

V

OUT

I

2

C™

2-wire

V

REF

Optional

V

DD

I

2

C™

2-wire

V

REF

Optional

V

DD

R

2

0.1 µF

V

OA

V

OB

G = Selected Op Amp Gain

V

OA

 =  (V

REF

 * G * DAC A Register Value)/4096  

V

OB

 =  (V

REF

 * G * DAC B Register Value)/4096 

Where:

R

SENSE

I

b

Load

I

L

V

CC

+

V

CC

–

V

OUT

β

β

×

β

β =

 Common-Emitter Current Gain. 

where

V

DD

 

I

2

C™

2-wire

V

REF

Optional

V

DD

(or V

REF

)