Rainbow Electronics MAX132 - Manual (Page 8)

MAX132
±18-Bit ADC with Serial Interface
8 _______________________________________________________________________________________
Differential Reference Inputs
and Rollover Error
The main source of rollover voltage error is due to
common-mode voltages. This error is caused by the
reference capacitor losing or gaining charge to stray
capacitance. A positive signal with a large common-
mode voltage can cause the reference capacitor to
gain charge (increase voltage). In contrast, the refer-
ence capacitor will lose charge (decrease voltage)
when deintegrating a negative input signal. Rollover
error is a direct result of the difference in reference to
positive or negative input voltages. With the recom-
mended reference capacitor types, the worst-case
rollover error is 0.01% of full-scale. Connect REF- to
AGND to minimize rollover error. As outlined in the ref-
erence section, reference voltages below 500mV also
contribute to rollover errors.
Oscillator Circuit
The internal oscillator is typically driven by a crystal, as
shown in Figure 8, or by an external clock. If an exter-
nal clock is used, connect the clock to OSC1 and leave
OSC2 floating. The duty-cycle can vary from 20% to
80%. The typical threshold voltage is approximately 2V.
For proper start-up, a full +5V CMOS-logic swing is
required.
The oscillator frequency sets the conversion rate. Use
32,768Hz for applications that require 50Hz or 60Hz
line rejection. This frequency yields 16 conv/sec. The
same clock frequency can be used to reject both line
frequencies because the MAX132 integrates for a dif-
ferent number of clock cycles in its 50Hz and 60Hz
modes. In each case, the MAX132 integrates for a sin-
gle complete line cycle (20ms for the 50Hz mode,
16.67ms for the 60Hz mode). Refer to the
Increased
Speed section for operation at higher conversion rates.
External Components
The MAX132 requires an integrator resistor (R INT ) and
capacitor (C INT ), a reference capacitor (C REF ), and a
crystal. All MAX132 tests are performed with a
32,768Hz crystal frequency. The crystal frequency, ref-
erence voltage, and integrator current determine the
values of R
INT and C INT .
Crystal
Figure 8 shows the internal oscillator drive circuitry used
with external crystals. The two external capacitors provide
DC bias at start-up. The 15pF capacitors shown are typical
values. The actual capacitance will vary, depending on the
crystal manufacturer’s recommendation and board layout.
150k
15pF 15pF
5pF 5pF 1M
+5V
6 5
OSC1 OSC2
MAX132
Figure 8. MAX132 Internal Oscillator Drive Circuitry
MAX132
V+
+5V
-5V
IN HI
IN LO
15
14
24
13
+545mV
100k
0.1 µ F ±512mV
16
18 17
AGND
REF- REF+
V-
Figure 6. MAX132 Input Circuit
REF+
REF-
1 µ F
120k
100k
40.2k
2.5V
MAX872
+5V
Figure 7. Dividing MAX872 to Generate the MAX132’s
Reference Voltage