Digital Calibration and

Threshold Selection

Figure 13 shows the MAX5158/MAX5159 in a digital

calibration application. With a bright light value applied

to the photodiode (on), the DAC is digitally ramped until

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 read-

ers, 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, 10-Bit, Voltage-Output DACs

with Serial Interface

______________________________________________________________________________________

15

AGND

DGND

MAX5158

MAX5159

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

1024

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

1024

R4

R3

)(

)( )

[

] [

]

SHIFT

REGISTER

Figure 14. Digital Control of Gain and Offset