Datasheet

MAX1447/MAX1496/MAX1498
3.5- and 4.5-Digit, Single-Chip ADCs
with LED Drivers
______________________________________________________________________________________ 11
MAX1447
MAX1498
BINARY-TO-
BCD
CONVERTERS
ADC
LED
DRIVER
INPUT
BUFFERS
-2.5V
AIN+
AIN-
REF+
REF-
+2.5V
AV
DD
DV
DD
GLED
2.048V
BANDGAP
REFERENCE
OSCILLATOR
CLOCK
SEG1
SEGF
SEGDP
DIG0
DIG4
CLK
TIMING
+2.5V
GND
A = 1.22
CHARGE
PUMP
-2.5V
V
NEG
LED_EN
DPON
DPSET1
DPSET2
RANGE
PEAK
HOLD
VLED
I
SET
Figure 1. MAX1447/MAX1498 Functional Diagram
Modulator
The MAX1447/MAX1496/MAX1498 perform analog-to-
digital conversions using a single-bit, 3rd-order, sigma-
delta modulator. The sigma-delta modulator converts
the input signal into a digital pulse train whose average
duty cycle represents the digitized signal information.
The modulator quantizes the input signal at a much
higher sample rate than the bandwidth of the input.
The MAX1447/MAX1496/MAX1498 modulator provides
3rd-order frequency shaping of the quantization noise
resulting from the single-bit quantizer. The modulator is
fully differential for maximum signal-to-noise ratio and
minimum susceptibility to power-supply noise. A single-
bit data stream is then presented to the digital filter to
remove the frequency-shaped quantization noise.
Digital Filtering
The MAX1447/MAX1496/MAX1498 contain an on-chip
digital lowpass filter that processes the data stream
from the modulator using a SINC
4
response:
The SINC
4
filter has a settling time of four output data
periods (4 x 200ms).
The MAX1447/MAX1496/MAX1498 have 25% over-
range capability built into the modulator and digital fil-
ter. The digital filter is optimized for the f
CLK
equal to
4.9152MHz. The frequency response of the SINC
4
filter is
calculated as follows:
where N is the oversampling ratio, and f
m
= N x output
data rate = 5Hz.
Hz
N
Z
Z
Hf
N
N
f
f
f
f
N
m
m
()
()
()
()
sin
sin
=








=


























1
1
1
1
1
4
4
-
-
-
-
π
π
sin( )x
x






4