±18-Bit ADC with Serial Interface
MAX132
____________Functional Description
The MAX132 integrates the input voltage for a fixed
period of time, then deintegrates a known reference
voltage and measures the time required to reach zero.
Good line rejection is achieved by setting the (input)
integration time equal to one 50Hz or 60Hz period. The
MAX132 has a 50Hz/60Hz mode selection bit that sets
the integration time to 655/545 clock periods, respec-
tively, so that 50Hz/60Hz rejection is obtained with a
32,768Hz crystal. The MAX132 is tested and guaran-
teed at a 16 conv/sec throughput rate. Figure 1 shows
the basic MAX132 application circuit, with component
values selected for 16 conv/sec .
For applications that don’t require 50Hz/60Hz rejection,
the MAX132 will operate up to 100 conv/sec at reduced
accuracy (typically 0.012% FSR nonlinearity, or ±13
bits). In these applications, the 50Hz mode is recom-
mended because of its longer (655 count) integration
time. See
Increased Speed
section.
__________Analog Design Procedure
Input Voltage Range
and Input Protection
The recommended analog full-scale input range is
±512mV. Performance is tested and guaranteed at
±512mV full scale, corresponding to a 2µV/LSB resolu-
tion at 18 bits. Resolution is defined as follows:
Re solution Volts / LSB
[
]
=
V
IN
(FS) / 262,144
which corresponds to 2µV/LSB resolution at 18 bits.
Consult the
Typical Operating Characteristics
for Noise
vs. Number of Samples Averaged and other important
operating parameters. Note how accuracy depends on
common-mode input voltage (common mode is defined
here as
|
V
IN
LO - AGND
|
). For optimum performance,
set the analog input full-scale between ±470mV and
t
5
CS
t
3
t
1
SCLK
t
6
DIN
t
8
DOUT
MSB OUT
B6–B1
LSB OUT
MSB IN
t
7
B6–B1
LSB IN
t
9
t
10
t
4
t
2
P0–P3
t
11
, t
12
Figure 2. Serial-Mode Timing
+5V
+5V
3k
DOUT
3k
DGND
C
L
DOUT
C
L
DGND
DOUT
3k
DGND
10pF
DOUT
3k
10pF
DGND
a. High-Z to V
OH
(t
8
)
b. High-Z to V
OL
(t
8
)
a. V
OH
to High-Z (t
10
)
b. V
OL
to High-Z (t
10
)
Figure 3. Load Circuits for Access Time
6
Figure 4. Load Circuits for Disable Time to Three-State
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
