Rainbow Electronics MAX5159 - Manual (Page 15)

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
zero code). For rated performance, V
REF_ should be at
least 1.4V below V DD . Bypass the power supply with a
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.
MAX5158/MAX5159
Low-Power, Dual, 10-Bit, Voltage-Output DACs
with Serial Interface
______________________________________________________________________________________ 15
AGND DGND
MAX5158
MAX5159
DACA
V DD
REFA
V IN
V REF
CS
SCLK
DIN
REFB
R1
R3
R
R
R
R
R4
R2
OUTB
OSB
OUTA
OSA
V OUT DACB
INPUT
REG A
INPUT
REG B
DAC
REG A
DAC
REG B
– OFFSET
[ ]
V OUT =
=
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 (
V IN
)(
)(
1+
) (
V REF
)( ) [ ] [ ]
SHIFT
REGISTER
Figure 14. Digital Control of Gain and Offset