it trips the comparator. The microprocessor (µP) stores

this “high” calibration value. Repeat the process with a

dim light (off) to obtain the dark current calibration.

The µP then programs the DAC to set an output voltage

at the midpoint of the two calibrated values.

Applications include tachometers, motion sensing,

automatic readers, and liquid clarity analysis.

Digital Control of Gain and Offset

The two DACs can be used to control the offset and

gain for curve-fitting nonlinear functions, such as trans-

ducer linearization or analog compression/expansion

applications. The input signal is used as the reference

for the gain-adjust DAC, whose output is summed with

the output from the offset-adjust DAC. The relative

weight of each DAC output is adjusted by R1, R2, R3,

and R4 (Figure 14).

Power-Supply Considerations

On power-up, the input and DAC registers clear (set to

4.7µF capacitor in parallel with a 0.1µF capacitor to

AGND. Minimize lead lengths to reduce lead inductance.

Grounding and Layout Considerations

Digital and AC transient signals on AGND can create

noise at the output. Connect AGND to the highest quality

ground available. Use proper grounding techniques,

such as a multilayer board with a low-inductance ground

plane. Carefully lay out the traces between channels to

reduce AC cross-coupling and crosstalk. Wire-wrapped

boards and sockets are not recommended. If noise

becomes an issue, shielding may be required.

Low-Power, Dual, 12-Bit Voltage-Output DACs

with Serial Interface

______________________________________________________________________________________

15

AGND

DGND

MAX5154

MAX5155

DACA

REFA

CS

SCLK

DIN

REFB

R1

R3

R

R

R

R

R4

R2

OUTB

OSB

OUTA

OSA

DACB

INPUT

REG A

INPUT

REG B

DAC

REG A

DAC

REG B

– OFFSET

[ ]

=

GAIN

[

]

2NA

4096

NA IS THE NUMERIC VALUE OF THE INPUT CODE FOR DACA.

NB IS THE NUMERIC VALUE OF THE INPUT CODE FOR DACB.

R2

R1+R2

R4

R3

2NB

4096

R4

R3

)(

)( )

[

] [

]

SHIFT

REGISTER

Figure 14. Digital Control of Gain and Offset