Rainbow Electronics MAX1494 - Manual (Page 10)

MAX1492/MAX1494
Detailed Description
The MAX1492/MAX1494 low-power, highly integrated
ADCs with LCD drivers convert a ±2V differential input
voltage (one count is equal to 100µV for the MAX1494
and 1mV for the MAX1492) with a sigma-delta ADC and
output the result to an LCD or µC. An additional
±200mV input range (one count is equal to 10µV for the
MAX1494 and 100µV for the MAX1492) is available to
measure small signals with increased resolution.
The devices operate from a single 2.7V to 5.25V power
supply and offer 3.5-digit (MAX1492) or 4.5-digit
(MAX1494) conversion results. An internal 2.048V refer-
ence, an internal charge pump, and a high-accuracy
on-chip oscillator eliminate external components.
The MAX1492 and MAX1494 interface with a µC using
an SPI/QSPI/MICROWIRE-compatible serial interface.
Data can either be sent directly to the display or to the
µC first for processing before being displayed.
The devices also feature on-chip buffers for the differen-
tial input signal and external reference inputs, allowing
direct interface with high-impedance signal sources. In
addition, they use continuous internal-offset calibration
and offer >100dB of 50Hz and 60Hz line noise rejec-
tion. Other features include data hold and peak hold,
overrange and underrange detection, and a low-battery
monitor.
Analog Input Protection
Internal protection diodes limit the analog input range
from V
NEG to (AV DD + 0.3V). If the analog input exceeds
this range, limit the input current to 10mA.
Internal Analog Input/Reference Buffers
The MAX1492/MAX1494 analog input/reference buffers
allow the use of high-impedance signal sources. The
input buffer’s common-mode input range allows the ana-
log inputs and the reference to range from -2.2V to +2.2V.
Modulator
The MAX1492/MAX1494 perform analog-to-digital con-
versions using a single-bit, 3rd-order, sigma-delta mod-
ulator. 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 MAX1492/MAX1494 modulator provides 3rd-order
frequency shaping of the quantization noise resulting
from the single-bit quantizer. The modulator is fully dif-
ferential for maximum signal-to-noise ratio and mini-
mum 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 MAX1492/MAX1494 contain an on-chip digital low-
pass filter that processes the data stream from the
modulator using a SINC
4 ((sinx/x) 4 ) response. The
SINC 4 filter has a settling time of four output data peri-
ods (4 x 200ms).
The MAX1492/MAX1494 have 25% overrange capability
built into the modulator and digital filter.
The digital filter is optimized for f
CLK
equal to 4.9152MHz.
Lower clock frequencies can be used; however,
50Hz/60Hz noise rejection decreases. The frequency
response of the SINC
4 filter is measured as follows:
where N is the oversampling ratio, and fm = N ✕ output
data rate = 5Hz.
Filter Characteristics
Figure 2 shows the filter frequency response. The
SINC
4 characteristic -3dB cutoff frequency is 0.228
times the first-notch frequency (5Hz).
The output data rate for the digital filter corresponds
with the positioning of the first notch of the filter’s fre-
quency response. The notches of the SINC
4 filter are
repeated at multiples of the first-notch frequency. The
SINC
4 filter provides an attenuation of better than
100dB at these notches. For example, 50Hz is equal to
H z
N
z
z
H f
N
N
f
fm
f
fm
N
( )
( )
( )
( )
sin
sin
=
−
−








=

 


 













−
−
1 1
1
1
1
4
4
π
π
3.5- and 4.5-Digit, Single-Chip ADCs
with LCD Drivers
10 ___________________________________________________________________________________________________
FREQUENCY (Hz)
GAIN (dB)
50 40 30 20 10
-160
-120
-80
-40
0
-200
0 60
Figure 2. Frequency Response of the SINC 4 Filter (Notch at 60Hz)