Rainbow Electronics MAX132 User Manual
Page 7
MAX132
±18-Bit ADC with Serial Interface
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±660mV for 60Hz mode operation or between ±390mV
and ±550mV for 50Hz mode operation. The pseudo-
differential input voltage is applied across pins 14 and
15 (IN HI, IN LO), and can range to within 2V of either
supply rail.
The inputs IN HI and IN LO lead directly to CMOS tran-
sistor gates, yielding extremely high input impedances
that are useful when converting signals from a high
input source impedance, such as a sensor. Input cur-
rents are only 2pA typical at +25°C. Figure 6 shows an
RC filter at the input to optimize noise performance.
Fault protection is accomplished by the 100k
Ω
series
resistance. Internal protection diodes, which clamp the
analog inputs from V+ to V-, allow the channel input
pins to swing from (V- - 0.3V) to (V+ + 0.3V) without
damage. However, if the analog input voltage at the
pins IN HI or IN LO exceed the supplies, limit the cur-
rent into the device to less than 1mA, as excessive cur-
rent will damage the device.
Reference Voltage Selection
The reference voltage sets the analog input voltage
range. For the nominal ±512mV full-scale input range, a
545mV reference voltage is used for the 60Hz mode
and a 655mV reference voltage is used in the 50Hz mode.
The reference voltage can be calculated as follows:
The recommended reference voltage range is 500mV
to 700mV. The MAX132 is tested with the nominal
545mV reference voltage in 60Hz mode. Use amplifiers
or attenuators (resistor dividers) to scale other full-scale
input signal ranges to the recommended ±512mV full-
scale range.
References outside the recommended range may be
used with a degradation of linearity. A reference volt-
age from 200mV to 500mV will result in a lower signal-
to-noise ratio; a reference voltage from 700mV to 2V will
increase the rollover error.
The MAX872 2.50V reference, with its 10µA supply cur-
rent, is ideally suited for the MAX132. Figure 7 shows
how 2.50V can be divided to obtain the desired refer-
ence voltage. The reference input accepts voltages
anywhere within the converter’s power-supply range;
however, for best performance, neither REF+ nor REF-
should come within 2V of the supplies.
MAX132
IN LO
AGND
IN HI
DE+
DE+
REF+
CREF+
C
REF
REF-
CREF-
INT IN
INT OUT
INTEGRATOR
COMPARATOR 1
COMPARATOR 2
DE-
BUFFER
8pF
64pF
Z1+ x 8
C
INT
R
INT
BUFFER
DE-
TO
DIGITAL
SECTION
INT
REST
INT
INT
X8
DE
DE
Figure 5. Analog Section Block Diagram
60
545
512
262 144
50
655
512
262 144
Hz Mode
V
counts
V
or
Hz Mode
V
counts
V
REF
IN FS
REF
IN FS
:
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,
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,
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=
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